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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Lightweight and powerful RA. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers Introduction This article will focus on a power amplifier (PA) without a power transformer. Such RAs in the amateur radio environment are called "transformerless" (the term, in my opinion, is not entirely accurate - there is only a power transformer, and RF transformers are commonly used), and they are surrounded by persistent prejudices about their electrical danger. These prejudices arose for two real reasons: - according to the principle learned from school: "Everything that has galvanic contact with the network is dangerous!" (Note that this principle is often misunderstood); - the first transformerless RA described in [1] could indeed be dangerous under certain conditions. The prejudices that had grown stronger on this basis could no longer shake the later publications about transformerless RA [2,3,4], in which the problem of decoupling from the network (and, accordingly, safety) was solved. To be honest, I don’t know if this article will be able to dispel the myth about the danger of transformerless RA. There are no technical problems (any unbiased reader who has the patience to read the article to the end will be convinced of this), but psychology remains ... Readers who are confident in the danger of RA without a huge power transformer, please believe (for now, by word of mouth) that the decoupling from the network of such a well-designed power amplifier is no worse (and you can do even better) than that of a conventional transformer. I hope that after reading the article, you will see that this is actually the case. Network isolation First, let's remember that the term "galvanic coupling" means direct current connection: directly, through a resistor, diode, transformer winding, etc. Why is the galvanic connection of the RA case and all its connectors (except for the network, of course) with a 220 V network dangerous? Maybe high voltage? Perhaps 220 V will seem like a very high voltage to someone, but not to a shortwave. Indeed, in lamp RAs with a network transformer, many times greater alternating voltages are used, and the source of this high voltage - the high-voltage anode winding - is connected to the case either directly or through the rectifier bridge diodes. And - no one is afraid of this, because it really does not pose a danger. In fact, the danger of galvanic connection with the network of the device case and all its connectors, paradoxically, is that one of the wires of the network (zero) is connected to the ground. And therefore, through the conductivity of the earth, floor, shoes, etc. - ALWAYS GALVANICALLY CONNECTED TO THE HUMAN BODY. It is easy to understand what will happen with such a circuitry of the RA, when the second wire of the network (phase) can be on the body of the device - a person touches the body of the device closes the circuit (the second wire of the network - earth, do not forget, is already connected to the person). At the very least, an electric shock is guaranteed. The situation will be even worse if the phase wire of the network has a galvanic contact with one of the PA connectors. When a normally grounded device (antenna, transceiver, or computer) is connected to this jack, a short circuit current from the mains will flow through the device connected to this jack. You will be very lucky if the mains fuse blows first, and not the transceiver or computer. Thus, a galvanic connection with the network of the RA housing and all its connectors is not allowed. Even if, as in [1], we use the fact that one of the wires of the network is the ground, and deal with the "polarity" of connecting the PA plug to the network using a starting device, the amplifier [1] is completely safe only as long as everything is working fine. But it is worth disrupting the operation of the starting device (for example, the relay contacts are stuck) and inserting the plug into the socket in the wrong "polarity" - all the troubles described above are guaranteed. But is the situation really so hopelessly bad, and is it better not to have any contacts with the network? Let's try to figure it out. I hope no one is against (in terms of safety) switching power supplies, which are widely used in TVs, computers, etc.? That's great, as long as you don't need more. Therefore, you do not mind that a galvanic contact with the network can have a network noise filter, a rectifier, a high-frequency generator. For example, Fig. 1 shows a simplified diagram of a switching power supply, where thick lines show circuits and nodes that have galvanic contact with the network (and, accordingly, are dangerous), and thin lines show safe circuits isolated from the network.
In the same way, circuits galvanically connected to the network will be shown in all subsequent figures. Let's return to Fig.1. The output circuits of the source are galvanically separated from the network by a ferrite-based RF transformer - the isolation in this circuit is very good. But there is another circuit for communicating with the network (but not galvanic, but capacitive) - these are noise filter capacitors C1, C2 connected to the chassis. I emphasize once again - the connection of the device chassis with the network through these capacitors (or rather, through one of them - the one that is connected to the phase wire of the network) is very weak, and not galvanic, but capacitive! In any well-made transformer RA, noise filter capacitors are also installed on the network wires. For example, Fig. 2 shows a fragment of the circuit of the "Alpha 91 b" amplifier, which is widely used among foreign radio amateurs, where capacitors with a capacity of 0,022 μF are soldered from the terminals of the network connector on the chassis even before the power switch.
So, in well-known professional schemes, the following (proven and safe) solutions are used. 1. Galvanic contact with the network of the noise filter, rectifier, high-frequency generator. 2. Connection of both (including the most dangerous - phase) wires of the network with the chassis through a capacitor with a capacity of 0,01 ... 0,047 microfarads. 3. Decoupling using high-frequency transformers on ferrite. Now let's move on to the next section. Comparative analysis of known transformerless RA Having excluded from consideration the circuit [1], which has a galvanic contact between the chassis and the network, let us turn to those transformerless RAs in which there is decoupling from the network of both the chassis of the amplifier and its input / output circuits that meets all safety regulations. Let's start with the UA1FA design on two 6P45S lamps [2]. An RF transformer is used in the input circuit, which ensures perfect galvanic isolation. The output circuit (already after the P-loop) is also decoupled by an RF transformer, but it is not at all easy to make a high-quality broadband (1,9 ... 30 MHz) transformer for high power. In addition, an expensive ferrite core of considerable size is required. However, ferrites (especially domestic ones) work very poorly for a load with reactivity, and at the edges of the range, any antenna, even a matched one, introduces noticeable reactivity. If you use some kind of LW with an SWR of 7 ... 8, then the output ferrite transformer will work completely inefficiently. In my opinion, in this design it was not worth striving to install an output transformer at all costs, because there are other ways to decouple the output circuit (for more details, see below). Moreover, in the circuit there is still a capacitive connection between the phase wire of the network and the chassis - a surge filter is installed in the design, similar to Fig. 2. It is not very convenient that the parts of the P-circuit also have galvanic contact with the network - this leads to the need to isolate them from the chassis and use isolated axes and tuning knobs. In addition, the 1 W output power specified in [400] without overloading the lamps can only be obtained in short-term, peak mode. With continuous radiation, the lamps will be overloaded, and the reliability of the amplifier will noticeably decrease. Indeed, at Pout=400 W, the input power must be at least 700 W, therefore, Prass=300 W - 150 W at the anode of each lamp. This is more than three times the power overload. In my opinion, in such critical nodes as RA, you should not use elements that exceed their passport parameters. Having saved the reader from calculations, I will say that the anode current overload of the lamps is almost twofold. Let us now turn to a later design - the RV3LE amplifier [3] on a GU-29 lamp. This is a well balanced design for 75...100 watts of power output. As in [2], a ferrite transformer is used at the input. A ferrite transformer is also used at the output (at such a power it is small, and, unlike [2], it is connected between the anodes of the lamps and the P-circuit). This solves two problems at once - it excludes the operation of the transformer for reactivity and allows the use of a conventional P-loop with KPI grounded on the chassis. But this circuit solution, alas, gives rise to another problem - the transformer operates with high resistance values (units of kilo-ohms) and therefore has an inevitable blockage in the frequency response in the high-frequency ranges. As in [2], the lamp is overloaded, but in fairness, we note that it is much less - one and a half times, both in terms of power dissipation at the anode and anode current. In addition, in RA [3] there is no network noise suppression filter, so it is quite possible for RF signals to enter the electrical network. The last construction in our review is RA6LFQ [4]. Three GU50s in a circuit with common grids give about 200 watts of output power. Here, a different principle of decoupling from the network is used than in [2, 3] - the connection of parts of the amplifier galvanically connected to the network with the chassis and input / output connectors through small capacitors. At radio frequencies, these capacitors are practically separating, and for a mains frequency of 50 Hz they represent a very large resistance (see point 2 in the previous section). In this design, in the struggle for the purity of the transformerless idea, there are no transformers at all. Although, in my opinion, a filament transformer could be installed, in any case, the dimensions of the filament transformer are no larger than a paper capacitor 10 μF x 400 V, through which the filament voltage is provided in [4]. At the input of the amplifier, the decoupling from the network is carried out by a capacitor of 1000 pF x 2 kV, at the output - by connecting the common wire of the amplifier to the chassis through a capacitor of 2200 pF x 2 kV. Due to the absence of ferrite transformers, it is possible to avoid some of the problems of matching and transmitting high power. However, if in the output circuit with an anode load resistance of several hundred ohms, a 2200 pF capacitor is used practically as an isolation capacitor (its reactance at a frequency of 1,8 MHz is 40 Ohms - less than 1/10 of the load resistance), then with an input resistance of the amplifier 50 Ohm, the capacitance of the isolation capacitor of 1000 pF is small (at 1,8 MHz, its resistance is 80 Ohms - almost twice as much as the input resistance of RA). It would seem, what a problem - it is enough to increase the capacitance of this capacitor. But not everything is so simple, and more on that in the next section. Again about the decoupling from the network We have already spoken about the galvanic connection with the network. But, in addition to galvanic, there is also capacitive. In the end, it doesn't matter which way the mains voltage enters the RA body. For further discussion, for any device powered by AC, we introduce such a parameter as a leakage current with a frequency of 50 Hz between the ungrounded case of the device and a good electrical ground - IUT50. For measurement IUT50 assemble the circuit shown in Fig.3.
All RA connectors (input, output, control), except for the network, are shorted to the case. A resistor Re = 30 kOhm is connected between the amplifier case and ground (the value is quite arbitrary and approximately corresponds to the resistance of the human body). The current flowing through Re will be IUT50, and the voltage drop across this resistor UUT50 will correspond to the voltage applied to the body of a well-grounded person (For example, standing with wet bare feet on a metal floor, Hi!) when he touches the body of an ungrounded RA. For correct measurements, choose such a position of the power plug in the socket when IUT50 maximum. Of course, during real work on the air, the RA case must be grounded, and not so much for electrical safety reasons, but for the normal operation of the antennas and TVI exclusion. But for a correct definition of IUT50 we deliberately take the worst case - the lack of grounding of the RA case. Let's see through which chains I penetrates the bodyUT50, and compare different designs for this indicator. 1. In a conventional RA with a power transformer, the current lUT50 flows through two parallel circuits - through one of the input capacitors of the noise suppression filter (the one connected to the phase, Fig. 2) and the interwinding capacitance of the power transformer. The latter is usually neglected, and it is not very small. So, for a power transformer with Rgab = 1.6 kW (for powering the RA on GU74B), this capacitance was 1200 pF (tnx EW1EA), for a transformer with Pgab = 500 W (for RA on three GU50) - about 500 pF. For further calculations, it is useful to know that a 1000 pF capacitor connected between the phase and the RA case gives IUT50\u0,06d XNUMX mA and, accordingly, UUT50\u1.8d XNUMX V. So, due to the interwinding capacitance, I flowsUT50\u0,03d 0,08 ... 2 mA, and due to the filter capacitor (Fig. 0,01) with its value of 0,047..0,6 μF - 2,8 ... XNUMX mA. General IUT50\u0,6d 0,29b ... XNUMX mA, which corresponds to UUT50\u19,8d 87..5 V. These are quite large values. However, no one is surprised that the ungrounded case of any device with a noise filter "bites" pretty much. By the way, in the B7-0,1 industrial transformer power supply, XNUMX microfarad line filter capacitors are used! At the same time IUT50=6mA, a UUT50=150V! Those who work with these blocks know what kind of electric shock can be received from its ungrounded case. Conclusion: power amplifiers with a power transformer have a noticeable capacitive coupling with the network, which is primarily determined by the capacitor of the network noise suppression filter, and secondly, by the interwinding capacitance of the power transformer. 2. A device with a switching power supply (a TV set, for example) is also connected to the network through a noise filter capacitor (Fig. 1). Those who wish to verify the existence of such a connection can connect an antenna with an external ground to a TV in a darkened room. The spark that jumps between the antenna connector and the TV jack when connected should convince. Values IUT50 and UUT50are basically the same as in the previous paragraph. The interwinding capacitance of the output high-frequency ferrite transformer is small and can be neglected. 3. Let's turn to PA UA1FA [2]. The interwinding capacitance of the input and output ferrite transformers is very small. UUT50 completely determined by the line filter capacitors with a capacity of 0,022 uF. IUT50=1.3 mA; UUT50\u40d XNUMX V. As you can see, the parameters are no worse than those of a conventional transformer RA. 4. PA RV3LE [3]. Absolutely decoupled from the network, IUT50 practically absent. It was precisely this circuitry that I had in mind when I said in the introduction that the isolation from the network of a transformerless RA can be even better than that of a transformer one. The capacitances of the input and output transformers are very small, and there is no mains noise filter. When installing the filter according to the scheme of Fig. 2 IUT50 will be the same as in [2]. 5. In PA RA6LFQ [4] I flows through two capacitors - input 1000 pF and output 2200 pF. Total 3300 pF, IUT50=0,2 mA and UUT50=6 V. Very good decoupling, but it has already been pointed out that the input capacitance of 1000 pF is small for the isolation in the 50-ohm input path. If it is increased to the required 0,015 ... 0,022 μF, then Iut50 will increase to 1 ... 1.3 mA, and Uut50 to 30 ... 40 V. This, however, is quite acceptable and corresponds to any transformer RA and designs [2,3, 4]. In this RA, a different network noise filter is used (Fig. 1). Due to the presence of chokes L2, L2, RF interference coming from the RA to the network, it suppresses even better than the simplest filter in Fig. 4. A very important advantage of the filter in Fig. XNUMX is the absence of contact with the chassis, so it does not conduct current IUT50.
In transformerless designs of the PA, only such noise suppression filters should be used. Anode circuit power All RA [1, 2, 3, 4] have one common drawback - doubling the mains voltage is used to power the anode. As a result, the resulting voltage of 580 ... 600 V is not enough to power a powerful tube amplifier. It is necessary to "accelerate" the anode current to the limiting passport values (and in most cases far beyond them). The result is reduced lamp life. However, the output powers obtained are not impressive - 100...200 W (meaning that PA[2] works without much overload). In addition, the low anode voltage Ea leads to a low power gain of the amplifier, which, at a constant input power Pin, is directly proportional to Ea. In general, Ea needs to be increased. The conclusion suggests itself - if doubling is not enough, it is necessary to use tripling or quadrupling the mains voltage. But here we are faced with another prejudice that voltage multipliers are suitable only for small currents and have a large internal resistance and, accordingly, a large voltage drop (“drawdown”) under load. The author of this article shared this opinion for a long time, but then, literally on the table, assembling the circuit shown in Fig. 5, he received results that convinced the opposite. Diodes D248B were used, and for the first experiment - six capacitors K50-31 100,0 uF x 350 V.
Five 220 V/40 W incandescent lamps connected in series were used as load resistance. Under these conditions, the following parameters were obtained: - open circuit voltage Exx - 1220V; - voltage at the load 200 W En - 1100V; - amplitude of pulsations at a load of 200W Upulse - 50V. Those. The "drawdown" of the voltage is only 10%, and the ripple is 5%. This is better than many transformer power supplies. When the same circuit is loaded with five lamps 220 V / 60 W En \u1050d 80 V and Upulse \u200d 300 V. Also very good parameters. At the same time, a 300 ... XNUMX W power supply had a weight of about XNUMX g! In the next experiment, with the same diodes, six capacitors 220,0 uF x 350 V were used (from television power supplies). The load was also incandescent lamps with a total power of 600 watts. Exx, of course, has not changed, En=1100B, Upulse=65B. Thus, using the circuit in Fig. 5, it is possible to make power supplies for Ea \u1100d 200 V with a power of 300 ... 100,0 W (when using capacitors 350 x 500 V), 600 ... C) and even 220,0 ... 350 W (at 1000 x 1200 V - that is, each of the six capacitors is made up of two 440,0 x 350 V). Such parameters allow the use of such power supplies with many lamps, both in a single connection and in parallel: 3xGU50 at la=0,4...0,5 A and Рout=250... ...300W; 4хГ811 at Ia=0,6...0,65 A and Рout=300... ...350 W; 2(3) GI7B at Ia=0,6...0,7 (0,9...1)A and Pout=400(600)W. In general, you can choose the appropriate option if you wish. By the way, RA [5] uses a 500 V alternating voltage tripler (from the secondary winding of a power transformer) to obtain an anode voltage of 2100 V. So, the use of voltage multipliers is a common practice. The question is often asked: "How is it that polar electrolytic capacitors C1, C2 are connected directly to the alternating current network? An alternating voltage is applied to them, an alternating current flows through them and they will explode!". No, this will not happen. There will be no AC voltage on C1 and C2, because network circuits - VD2-C1 and network - VD3-C2 are ordinary half-wave rectifiers, therefore, reverse polarity voltage is not applied to either C1 or C2. If you connect an oscilloscope directly to C1 (or C2), you can see a constant voltage of 300 V with ripple amplitude of 15 ... 20 V. Alternating current (and significant - up to several amperes) will, of course, flow through C1 and C2, but this is their passport mode. Recall that in many transistorized ULFs, there is a separating electrolytic capacitor of considerable capacity at the output, through which an LF alternating current flows into the loudspeaker, measured in powerful amplifiers by amperes. Transformerless, quadrupled Taking into account all of the above, a transformerless power amplifier with a quadrupling of the mains voltage is proposed, a somewhat simplified diagram of which is shown in Fig. 6. For example, a triode is shown connected according to a common grid circuit, which, however, is not at all important - it can be a tetrode, a pentode, and a common cathode circuit (screen voltage can easily be obtained by a stabilizer connected to the midpoint of the quadruple output capacitors - the voltage at this point is +600 V with respect to the cathode).
The following features are fundamental in the circuit in Fig. 6: - anode voltage - 1200... 1100 V (quadruple mains voltage); - input signal supply - through a broadband ferrite transformer (SHPT); - supplying the output signal to the P-circuit - through two isolation capacitors C1 and C2 of 2000 pF x 2 kV each. It is convenient to apply the input signal through the SPT, because: - in contrast to [4], where a decoupling capacitor is used, the interwinding capacitance of the SHPT is extremely small, and therefore does not contribute to the current IUT50; - ShPT works on a constant load without reactivity - input impedance RA; - ShPT replaces the cathode choke, and also (by changing the number of turns, i.e., the transformation ratio) can be used to match the input impedance of the amplifier with the driver. The RF signal from the lamp to the P-loop is fed through two separating capacitors: C1 separates Ea from the hot end of the P-loop, and C2 provides decoupling over the 50 Hz network, closing the common lamp electrode (grid in this case) with the amplifier chassis. This method of signal transmission (without the ferrite transformer used in [2,3]) allows you to pass any power, work with reactive loads and eliminate blockage in the frequency response of the output circuit. As in all previous figures, in Fig. 6 the circuits galvanically connected to the network are highlighted with thick lines, and those decoupled from the network are shown with a normal thickness. The circuit in Fig. 6 can also be considered as a slightly modified switching power supply. In fact, the rectifier and the high-frequency generator (lamp) are directly connected to the mains voltage. Only in this case it is not a self-oscillator, but a generator with external excitation through the input SPT (in old books on transmitting technology, power amplifiers were called that - generators with external excitation). The output signal of the generator is not taken through a ferrite transformer, as in a switching power supply, but through capacitors C1, C2. This decision is quite logical, because the lowest frequency of the generator (1,8 MHz) is more than 30000 times higher than the mains frequency, and the resistances of the capacitors C1, C2 at these frequencies differ by the same factor. Another difference between the circuit in Fig. 6 and a conventional switching power supply is that the generator does not operate in a key, but in a linear (envelope) mode, so the efficiency of converting the mains voltage into an RF signal (in other words, the efficiency of the amplifier) is not 85%...90%, and 55...60%. The output includes a conventional P-loop. The leakage current of the network to the case for the circuit in Fig. 6 (when using the noise filter according to the circuit in Fig. 4) is determined only by the capacitor C2 and is IUT50=0,12 mA, while UUT50= 3,6 V. This is better than many transformer RAs. Some requirements for circuit details. Diodes must be designed for Uobr>600 V and average current not less than 4Ia_max. Permissible impulse overload current of diodes should be 2...3 times more. KD202R, D248B are well suited. Power supply capacitors must be >350 V, their capacitance must be at least 100 uF for every 250 mA of anode current. Capacitors C1 and C2 are chosen such that at the lowest operating frequency their reactance would be less than 1/10 Roe of the P-loop. For Roe>500 Ohm, C1 and C2 of 2000 pF are enough. The voltage on C1 and C2 does not exceed 900 V, but since they provide electrical safety, it makes sense to take them with a large margin - by 2 kV or more. From a safety point of view, the requirements for breakdown voltage C1 and C2 are the same as in a conventional power transformer for breakdown voltage between the mains and secondary windings. The cathode and grid circuits can have a potential of up to 900 V with respect to the chassis (if grounded). Accordingly, the insulation of these circuits, the interwinding insulation of the input FSHT (it is enough to use the MGTF 0,5 wire) and the winding insulation of the incandescent transformer (any unified VT is suitable) should be calculated for this value. We now turn to the description of practical schemes. Transceiver output stage Figure 7 shows a schematic diagram of a terminal amplifier of a transceiver with an output power of 100 ... 200 W. Do not rush to grin skeptically, arguing that transistor PAs have long been used to obtain such power, and a call to return to lamps is printed here. Firstly, the author knows about the existence of transistor RA. He developed them himself and exploited them for a number of years. Secondly, let's compare a typical push-pull transistor RA with an output power of 100 W with a lamp RA of the same power (Fig. 7) in terms of the main parameters.
1. Reliability. Here, the tube RA is beyond competition. Are there often transistors with Ppac = 350 W and resistance to tenfold impulse overloads? And for GI7B, these are typical parameters. There is no need to talk about work on a load with a high SWR and resistance to static charges on the antenna - the tube RA practically does not require any protection systems. 2. Power transfer coefficient. Approximately the same for both schemes - about 10. 3. Coordination with the load. The p-loop at the output of the lamp RA ensures coordination with almost any load. In a transistor RA, for this purpose, after the output low-pass filter, you will have to use a separate matching device. 4. Dimensions. A transistor (even a pair in a push-pull stage) is, of course, smaller than a lamp. But if you install them on a radiator, this difference disappears. The fact is that the lamp radiator can have a temperature of 140 ... 150 ° C, for transistors such a high temperature is unacceptable. In fact, the power given off by the radiator to the environment is directly proportional to both the radiator area and the temperature difference between it and the environment. Therefore, a hotter heatsink of the lamp gives off heat more efficiently, and therefore, in order to dissipate the same power, the heatsink for transistors must be larger than the anode heatsink of the lamp. 5. Efficiency. At first glance, the lamp should lose - the power in the filament circuit is lost uselessly, and for GI7B this is a lot - 25 watts. But let's count. The efficiency of a push-pull transistor RA is 40% at best (both according to [6] and according to practical measurements of the parameters of imported transceivers). For lamp RA, taking into account losses in the P-circuit, the efficiency in the anode circuit is 50 ... 60%, i.e. at Рout=100 W, Рsubv will be 180...200 W. Even if 25 W are added here in the filament circuit, then the overall efficiency will be 45% ... 50%, i.e. higher than that of the transistor RA. 6. Price. Of course, if you buy a lamp and transistors at factory prices, then the lamp will cost more. But if, speaking practically, we turn to the prices of the radio market, then a pair of powerful high-frequency transistors will not be cheaper, but most likely more expensive than a lamp. 7. Weight. As for the amplifier itself, everything that was said in paragraph 4 about the dimensions is true here. The power supply for a transistor RA must provide more than 250 W of output power, the overall power of its power transformer (including losses in the stabilizer) must be at least 300 W. In general, the weight of such a block is more than kg. The weight of the power supply unit (mains filter + quad + incandescent transformer) of the power amplifier shown in Fig. 7 is just over 1 kg. With fully transistorized transceivers (including imported ones, especially old models without a built-in tuner), a rather paradoxical situation is obtained. The transceiver itself is small, light and beautiful. But in order to work on the air on real antennas, it is necessary to put an antenna tuner and a power supply unit nearby (twice as large as the transceiver itself in weight and size). In this regard, the RA shown in Fig. 7 does not require any additional devices - it includes both a power supply and an antenna matching circuit. Let us now turn to the circuit diagram (Fig. 7). Diodes VD1 ... VD4 and electrolytic capacitors C3 ... C8 - mains voltage quadrupler. C1, L1, C2 - network noise filter. The three-position switch S1 and the current-limiting resistor R1 are elements of a two-stage system for turning on and reducing the inrush current when turned on. T1 is a cheeky transformer. C9 - radio frequency blocking of the anode power source. C12, C13 - dividing by HF and decoupling through the network. Ldr - anode choke. VD5 provides the initial lamp offset. C10, C11 - blocking on HF.T2- input isolating transformer. C14, C15, C16, L3, L4 are the usual elements of the output P-loop. Switching RX-TX for the lamp is not provided, the initial current is 5 ... 10 mA, and the power dissipation at the anode in pauses and in the receive mode is small - 6 ... 11 W. If you need to lock the lamp in receive mode, it is enough to connect a 5 kΩ resistor (or a D100 zener diode with any letter index) in series with VD817 and close it with the RX / TX relay contacts when switching to transmission. Details C1, C2 - type K73-17 for a voltage of at least 400 V, C3 ... C8-K50-31.K50.27, K50-29 (capacitors of the K50-35 type are better not to use because of their low reliability); C9, C12, C13 - KSO-11, K15-U1 for a voltage of at least 2 kV, and C12 and C13 - for reactive power of at least PA output power; C10, C11-KM-5 or similar; C15, C17 - K15-U1 for reactive power of at least 10 times the output power of the RA; C16 - built-in KPI from transistor receivers. C14 is made from a standard KPE 2x12/495 pF by thinning the rotor and stator plates through one, followed by centering the stator sections by soldering their attachment to the base of the KPI. L1 - interference filter choke, contains 2x20 turns of a network wire on a 2000NN brand ferrite ring of suitable sizes. The designs of the anode choke L-dr and coils of the P-loop L3, L4 have been described repeatedly in the literature [7,8]. T1 - any with good insulation between the windings, for example from the TN series, will do. The T2 core consists of two adjacent ferrite tubes, each of which is glued together from three rings 400NN K10x5x5. The windings connected to the lamp contain 2x4 turns of MGTF 0,5 wire. The number of turns and the design of the primary winding T2 depend on the type of driver and its output impedance. If the primary winding contains 4 turns, then Rin will be 100 ohms; if 2, then Rin - 25 Ohm. The author's primary winding contains 1 + 1 turns of MGTF 0,5 wire and is connected directly to the collectors of the driver transistors with its outputs, and the driver supply voltage is applied to the middle output. I emphasize once again that the primary winding T2 must be well insulated. If there is a need to introduce ALC, then the signal can be removed from the additional winding by winding it around T2, as is done in the RA3AO transceiver. Design The details of the P-loop are located at the front panel of the transceiver. Behind them is a horizontal lamp. The output compartment (lamp anode, C12, Ldr, U-loop) is separated by a grounded U-shaped screen. The lamp is fixed to the anode radiator with fluoroplastic bosses on self-tapping screws. If it is necessary to replace the lamp, it is unscrewed from the anode radiator, which is fixed "once and for all". In the U-shaped screen, a hole was made with a diameter of 6 ... 8 mm larger than the diameter of the output of the lamp grid (to avoid closing the grid on the body). A duralumin plate measuring 70x70 mm, isolated from the chassis, is put on the grid output. Through four fluoroplastic spacers, the plate is attached to the reverse side of the U-shaped screen. A capacitor C13 is placed between this plate and the screen. Behind the lamp (near the rear panel) is a cheeky T1 transformer. C10, C11 are mounted on the terminals of the lamp and T1. Transformer T2 is located on the bracket under the output of the lamp cathode. All parts of the power supply, including R1 and VD5 (with a small heatsink), are placed on a separate fiberglass board. The board must be positioned so as to exclude heating C3 ... C8 from the VL1 lamp. With a dense layout, it may be necessary to install thermal screens, for example, from thin asbestos glued to fiberglass. The results In this circuit, the lamp easily "swings" up to the current Ia=200...250 mA at Pin=8...12 W (2xKT913V). With a more powerful driver, you can get Ia = 0,38 ... 0,4 A. However, for the transceiver it is recommended to limit the current to Ia = 200 mA and, accordingly, Pout = 100 W. With such a power, the lamp can work without blowing even with continuous radiation (FM, for example) - it turns out a very comfortable transceiver that does not "howl" the fan right in front of the operator. In addition, the power of 100 W is enough to "build up" almost any RA, as well as for everyday work on the air. If you use the RA according to the scheme of Fig. 7 as an external one, then at Pin = 40 W it gives Ia = 0,38 ... 0,4 A and Pout = 190 ... 220 W (of course, when using forced cooling of the anode). RA on three GU50 Widespread among radio amateurs of the CIS RA on three GU50 lamps at Ea = 1100 V, it turns out that it does not need a power transformer at all! The circuit diagram practically coincides with that shown in Fig. 7, it is only necessary to increase the power R1 to 5 ... 10 W, the capacitances C3 ... C8 to 220 microfarads, and the cathode circuit should be made in accordance with Fig. 8.
Transformer T2 has an equal number of turns in the primary and secondary windings. If T2 is constructed as described in the previous section, it should contain three turns in each winding. In this design, T2 can also be performed as follows on a ferrite ring 400 ... 600 NN with an outer diameter of 20 ... 32 mm with a thin coaxial cable to wind 8 ... 12 turns. The central core of the cable forms the secondary winding, and the braid forms the primary. Of course, you can wind T2 with a twisted pair of MGTF wires. In any case, do not forget about the quality of the insulation of the T2 windings. RA on two (three) GI7B The scheme practically coincides with the scheme of Fig.7. The differences are as follows: capacities C3 ... C8 for two lamps should be 330 microfarads (for three - 470 microfarads or 2x220 microfarads); the value of R1 must be reduced to 180 ... 240 Ohms, and its power increased to 10 ... 20 W, instead of VD5, a transistor analog of a powerful zener diode should be turned on (Fig. 9).
VT1 must be installed on a heat sink isolated from the chassis and allow a power dissipation of 15 W (for three lamps - 25 W). T2 has the same number of turns in all windings. When choosing a core for T2, it should be taken into account that the direct component of the lamp cathode current will bias the core. The P-circuit must be designed for Roe = 800..900 Ohm (for three lamps - 500 ... 600 Ohm). For two lamps at Pin=45...50 W, the anode current reaches 0,75...0,8A (Pout=400 W). For three lamps at Pin=70...75 W the anode current reaches 1...1,1 A (Pout=600 W). Design The main grounded chassis is located horizontally approximately 50...60mm from the bottom. A square hole 14x14 cm in size was cut out in the chassis at the place where the lamps were installed. The lamps are installed vertically and fastened with clamps by the grid outlet to a square plate 16x16 cm in size (approximate dimensions, depend on the number of lamps and their layout). This plate with lamps attached to it is installed above the hole in the chassis and is attached to it through insulating fluoroplastic gaskets. C13 is installed between the plate and the chassis. In case of self-excitation or unstable operation, PA C13 is best done as a set of several capacitors (with a total capacity of 2000 pF), placing them around the perimeter of the plate with lamps. The lamps are blown by exhaust air as follows: fans are selected (according to the number of lamps) with a diameter equal to or slightly larger than the diameters of the anode radiators, the fans are attached to the top cover of the RA (holes are cut out under them) exactly opposite the lamps. Cylindrical air ducts are rolled up from 2-3 layers of fiberglass (you will have to stratify a piece of suitable size). In order to avoid unwinding, the ends of the fiberglass are stitched with metal brackets. The upper diameter of the air duct must exactly match the outer diameter of the fan, the lower one must match the diameter of the anode of the lamp (if they differ, then the air duct is made conical). As a result, when the top cover is lowered, the air ducts fit exactly onto the anodes. Conclusion So, transformerless RAs are no more dangerous than amplifiers with a power transformer. To obtain anode voltages of 600 ... 1100 V, a power transformer is not needed at all. The complication when switching to transformerless power is minimal, and the need to isolate some of the parts from the chassis is unlikely to frighten shortwavers - there are more than enough similar parts in a transformer power amplifier with a high anode voltage. Is a transformerless RA really so good that it has no flaws. Of course it has (like any other device). Here are some: - inconvenience of adjustment. If you want to measure the lamp mode or examine the signals in the mains-related circuits with an oscilloscope, you must use a 1:1 mains isolation transformer. However, for a proven, worked-out circuit with sufficient qualifications of a radio amateur, this is not required; - use of electrolytic capacitors. In 10-12 years, they may have to be replaced. In other matters, firms producing RA power amplifiers are not embarrassed - in the vast majority of industrial RAs, it is electrolytic capacitors that are used; - transformerless power amplifier can only be powered by AC mains; - to obtain high output powers (1 kW or more), an anode voltage of 1,1 kV is not enough. However, if you use a lamp that provides Ia> 2 A (GS3B, for example), you can try to create such a device. The author has not tested this option yet. Questions and Answers 1. Does the safety of the circuit depend on the "polarity" of the plug in the network? No, it doesn't. Isolation from the network is provided in any position of the plug. The differences are only in the magnitude of the current IUT50. If the "zero" of the network is connected to the lower wire of the network according to the diagram (Fig. 7 in N2 / 99), then the minus of the rectifier (lamp grid) is under a constant potential of 600 V relative to the housing, and IUT50=0. If there is a "phase" on this wire, then on the minus of the rectifier (lamp grid) there will be a potential that varies from 600 to 900 V with a frequency of 50 Hz. The variable component of this potential through C13 (2000 pF x 2 kV) causes the flow of IUT50 about 120 uA. In this case, UUT50 is only a few volts. 2. What will happen if the RA case is not grounded or is grounded badly? In terms of safety and operation of the RA, nothing will change, but there may be problems with antennas and TVI. (Once again, we remind you of the mandatory presence of a grounding system at an amateur radio station. Note ed.) 3. About the capacitance of the capacitors of the voltage quadruple. The minimum required capacitance of each of the six metering capacitors can be estimated as follows - its capacitance in microfarads should be equal to the output power of the RA in watts. In this case, the "drawdown" of the anode source under load will be approximately 100 ... 120 V. Of course, larger capacitors can be used, the "drawdown" will be less. 4. Is it possible to use a higher degree of multiplication of the mains voltage instead of quadrupling? Theoretically yes, practically it doesn't make much sense. The fact is that high-voltage high-capacity electrolytic capacitors are not very common, and if you collect batteries from low-capacity capacitors with an operating voltage of 350 ... 450 V, their number grows disproportionately quickly. AT; for quadrupling - six such capacitors, for gearing - 350, for increasing - 17 (!). With such a number of capacitors, the main advantage of this RA is lost - small weight and dimensions. 5. Some imported alternators give output not 220 V, but 110 ... 120 V, what to do in this case? Of course, if you are making a set of equipment for field work, it is not very practical to carry a 110x220 V autotransformer with you. There are two options. First: leave the RA circuit unchanged and be content with an anode voltage of 600 V. Second, assemble a voltage multiplier by 8, as shown in Fig. 1 of this article. The result is a voltage of 1,1 kV at a load current of 1,2 ... 0,35 A (ЗхGU0,4). I note that if the generator produces 50 V AC voltage, then the capacitors C120 and C1 (each of two K2-50) operate at a voltage close to the limit. The circuit can be easily re-switched to work as a quadruple from a 7 V network. To do this, it is enough to break four circuits with a switch (break points are shown in Fig. 220 with a cross)
6. Why is the RA shown in fig. 7, does not deliver 200 W to the load? Unfortunately, I didn't express myself exactly. The power supply unit RA in the mentioned circuit is designed for only 100 W of output power. 7. How can I get the ALC signal when using a transformerless power supply? Unfortunately, the traditional methods for obtaining an ALC signal (by grid current, by grid voltage amplitude) are not applicable in this case - the lamp is galvanically connected to the network. Only the signal on the winding of the input transformer can be monitored. Well, we should not forget that any RA should not be "pumped over". 8. About lamp operation mode and RX/TX switching. The D7A bias zener diode indicated in Fig. 2 (in N99 / 816) does not provide sufficient initial current on every instance of GI7B, it may be necessary to replace, for example, with D815Zh. The contacts of the RX / TX relay, which switches the lamp operation mode, are (like the entire cathode circuit) under a potential of up to 900 V relative to the case. Switching requires a relay that withstands 900 V between the contact group and the winding, as well as between the contact group and the relay housing. Reed relays are absolutely unsuitable - their contacts "stick" very quickly. Optical isolation cardinally solves this problem. Moreover, it is necessary to use a home-made optocoupler, industrial integrated ones are not suitable, because. their allowable voltage between input and output does not exceed 500 V, and in this case >900 V is required. One of the possible options is shown in Fig.2.
On transistors VT2, VT3, an adjustable analogue of a zener diode is assembled. The stabilization voltage VD2 is used as a reference. This voltage is compared with part of the output taken from the divider R3, RP1, R4. The differential voltage is amplified by VT2 and controls the powerful VT3. When the photoresistor RF1 is illuminated by the LED VD1, the resistance of the photoresistor decreases sharply, and the divider R3, RP1 is shunted, the R4 transistors VT2 and VT3 close. The output voltage rises to the stabilization level VD3 (47V), which ensures reliable closing of the lamp upon reception. When transmitting, VD1 goes out, shunted by an open transistor VT1, the resistance of RF1 increases to several hundred kilo-ohms, and it practically ceases to affect the operation of the circuit. The voltage at the output of the circuit decreases to the level set by RP1 (with the ratings R2, RP3, R1, VD4 indicated in Fig. 2, it is regulated from 11 to 18 V). VD3 - protective zener diode. To reduce the power dissipated by VT3 (it is installed on a small radiator), a powerful resistor is installed in its collector. The output dynamic impedance of the circuit is less than 1 ohm. Photoresistor RF1 and LED VD1 are placed in a black tube (coaxial cable sheath) at a distance of 2 .. 3 mm from each other. The circuit shown in Fig. 2 is designed to operate in the cathode of one lamp (Imax = 0,35 A). If a larger maximum current is required, then it is necessary to install a composite transistor instead of VT3, for example, KT825, and recalculate the value and power of R7 based on the fact that at the maximum stabilization current, about 7% of the total voltage should fall on R75 (in this case, about 10V). 9. About inaccuracies in the publication In Fig. 8 (No. 2/99), the grids of the GU-50 lamp should not be on the body, but of course, on the negative wire of the rectifier. Literature
Author: I. Goncharenko (EU1TT); Publication: N. Bolshakov, rf.atnn.ru
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Comments on the article: Alexander Very interesting stuff. Many thanks to the author for the work! Alexander, US5LCW Goga Yes awesome amp!!! [up] Novel Thanks to the author for posting! I read it with interest! I used to be afraid to use a transformerless PSU. I read and assembled a quadruple amplifier for three GU-50s. Everything works great. Roman, R3WBK. 73!
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