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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Block for regulating large rectified currents. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Regulators of current, voltage, power The time-tested circuit for regulating the current of powerful consumers is easy to set up, reliable in operation and has wide consumer capabilities. It is well suited for welding mode control, for starters and chargers and for powerful automation units. When supplying powerful loads with direct current, a rectifier circuit with four power valves is often used (Fig. 1).
An alternating voltage is supplied to one diagonal of the "bridge", the output DC (pulsating) voltage is taken from the other diagonal. In each half-cycle, one pair of diodes (VD1-VD4 or VD2-VD3) works. This property of the rectifier "bridge" is significant: the total amount of rectified current can reach twice the maximum current for each diode. The limit voltage of the diode must not be lower than the peak input voltage. Since the voltage class of the power valves reaches the fourteenth (1400 V), there is no problem with this for a household electrical network. The existing reserve for reverse voltage allows the use of valves with some overheating, with small radiators (do not abuse it!). Attention! Power diodes marked "B" conduct current, "like" diodes D226 (from the flexible output to the body), diodes marked "VL" - from the body to the flexible output. The use of valves of different conductivities allows installation on just two double radiators. If, however, the “housings” of the “VL” valves (output “minus”) are connected to the body of the device, then it remains to isolate only one radiator, on which diodes marked “B” are installed. Such a circuit is easy to install and "adjust", but difficulties arise if you have to regulate the load current. If everything is clear with the welding process (attach "ballast"), then there are huge problems with the starting device. After starting the engine, a huge current is unnecessary and harmful, so it is necessary to turn it off quickly, since each delay shortens the battery life (it is not uncommon for batteries to explode!). The circuit shown in Fig. 2 is very convenient for practical implementation, in which thyristors VS1, VS2 perform the functions of current regulation, power valves VD1, VD2 are included in the same rectifier bridge.
Installation is facilitated by the fact that each pair of "diode-thyristor" is mounted on its own radiator. Radiators can be used standard (industrial production). Another way is to independently manufacture radiators from copper, aluminum with a thickness of over 10 mm. To select the size of the radiators, it is necessary to assemble the layout of the device and "drive" it in heavy mode. Not bad if, after a 15-minute load, the cases of thyristors and diodes do not “burn” the hand (turn off the voltage at this moment!). The body of the device must be designed in such a way that good circulation of the air heated by the device is ensured. It does not hurt to install a fan that "helps" drive air from the bottom up. Fans installed in racks with computer boards or in "Soviet" gaming machines are convenient. It is possible to perform the circuit of an adjustable rectifier entirely on thyristors (Fig. 3). The lower (according to the scheme) pair of thyristors VS3, VS4 is triggered by pulses from the control unit.
Pulses arrive simultaneously at the control electrodes of both thyristors. Such a construction of the circuit is "dissonant" with the principles of reliability, but time has confirmed the operability of the circuit ("burn" thyristors household electrical network can not, because they withstand a pulsed current of 1600 A). Thyristor VS1 (VS2) is connected as a diode - with a positive voltage at the anode of the thyristor, a triggering current will be applied to the control electrode of the thyristor through the diode VD1 (or VD2) and the resistor R1 (or R2). Already at a voltage of several volts, the thyristor will open and will conduct current until the end of the current half-wave. The second thyristor, on the anode of which there was a negative voltage, will not start (this is not necessary). A current pulse comes to thyristors VS3 and VS4 from the control circuit. The value of the average current in the load depends on the opening moments of the thyristors - the earlier the opening pulse arrives, the greater part of the period the corresponding thyristor will be open. Opening thyristors VS1, VS2 through resistors somewhat "dulls" the circuit: at low input voltages, the thyristor open angle turns out to be small - noticeably less current flows into the load than in a circuit with diodes (Fig. 2). Thus, this scheme is quite suitable for adjusting the welding current according to the "secondary" and rectifying the mains voltage, where the loss of a few volts is insignificant. The circuit shown in Fig. 4 makes it possible to effectively use the thyristor bridge to regulate the current in a wide range of supply voltages.
The device consists of three blocks:
Transformer T1 with a power of 20 W provides power to the control unit for thyristors VS3 and VS4 and opens the "diodes" VS1 and VS2. Opening thyristors with an external power supply is effective at low (car) voltage in the power circuit, as well as when powering an inductive load. The opening current pulses from the 5-volt windings of the transformer are fed in antiphase to the control electrodes VS1, VS2. Diodes VD1, VD2 pass only positive half-waves of current to the control electrodes. If the phasing of the opening pulses is "suitable", then the thyristor rectifier bridge will work, otherwise there will be no current in the load. This drawback of the circuit can be easily eliminated: it is enough to turn the power plug T1 in the opposite direction (and mark with paint how to connect the plugs and terminals of the devices to the AC mains). When using the circuit in a starter-charger, an increase in the output current is noticeable compared to the circuit in Fig. 3. The presence of a low-current circuit (network transformer T1) is very advantageous. Breaking the current with switch S1 completely de-energizes the load. Thus, interrupting the starting current can be done with a small limit switch, a circuit breaker, or a low-current relay (by adding an automatic trip unit). This is a very significant point, since it is much more difficult to break high-current circuits that require good contact for the passage of current. It was not by chance that we remembered the phasing of the T1 transformer. If the current regulator were "built-in" in the charger-starter or in the circuit of the welding machine, then the phasing problem would be solved at the time of setting up the main device. Our device is specially made wide-profile (as the use of the starting device is determined by the season of the year, so welding work has to be carried out irregularly). You have to control the operating mode of a powerful electric drill and power nichrome heaters. Figure 5 shows a diagram of the thyristor control unit. The rectifier bridge VD1 supplies the circuit with a pulsating voltage from 0 to 20 V.
This voltage is fed through the diode VD2 to the capacitor C1, a constant supply voltage is provided for a powerful transistor "key" on VT2, VT3. The pulsating voltage through the resistor R1 is supplied to the parallel-connected resistor R2 and the zener diode VD6. The resistor "binds" the potential of point "A" (Fig. 6) to zero, and the zener diode limits the peaks of the pulses at the level of the stabilization threshold. Limited voltage pulses charge the capacitor C2 to power the DD1 chip. The same voltage pulses act on the input of the logic element. At a certain voltage threshold, the logic element switches. Taking into account the inversion of the signal at the output of the logic element (point "B"), the voltage pulses will be short-term near the moment of zero input voltage.
The next logic element inverts the voltage "B", so the voltage pulses "C" have a much longer duration. While the voltage pulse "C" is active, the capacitor C3 is charged through the resistors R4 and R3. The exponentially increasing voltage at point "E", at the moment of transition through the logical threshold, "switches" the logical element. After being inverted by the second logic element, the high input voltage of point "E" corresponds to a high logic voltage at point "F". Two different values of resistance R4 correspond to two oscillograms at point "E":
You should also pay attention to the power supply of the base of the transistor VT1 with the "B" signal, while the input voltage drops to zero, the transistor VT1 opens to saturation, the collector junction of the transistor discharges the capacitor C3 (there is preparation for charging in the next half-cycle voltage). Thus, the logical high level appears at point "F" sooner or later, depending on the resistance R4:
The amplifier on transistors VT2 and VT3 "repeats" the logic signals point "G". Oscillograms at this point repeat F1 and F2, but the voltage reaches 20 V. Through separating diodes VD4, VD5 and limiting resistors R9 R10, current pulses act on the control electrodes of thyristors VS3 VS4 (Fig. 4). One of the thyristors opens, and a rectified voltage pulse passes to the output of the block. The smaller value of the resistance R4 corresponds to the greater part of the half-cycle of the sinusoid - H1, the larger - the smaller part of the half-cycle of the sinusoid - H2 (Fig. 4). At the end of the half cycle, the current stops and all thyristors close. Thus, different values of resistance R4 correspond to different durations of "segments" of the sinusoidal voltage across the load. The output power can be adjusted practically from 0 to 100%. The stability of the device is determined by the use of "logic" - the switching thresholds of the elements are stable. If there are no errors in the installation, then the device works stably. When replacing capacitor C3, the selection of resistors R3 and R4 will be required. Replacing thyristors in the power unit may require the selection of R9, R10 (it happens that even power thyristors of the same type differ sharply in turn-on currents - you have to reject the less sensitive one). You can measure the voltage at the load every time with a "suitable" voltmeter. Based on the mobility and versatility of the control unit, we used an automatic two-limit voltmeter (Fig. 7).
Voltage measurement up to 30 V is carried out by the PV1 head of the M269 type with additional resistance R2 (the full-scale deviation is adjustable at 30 V input voltage). Capacitor C1 is necessary to smooth the voltage supplied to the voltmeter. The rest of the circuit serves to "roughen" the scale by a factor of 10. Through the incandescent lamp (barretter) HL3 and the trimmer resistor R3, the incandescent lamp of the optocoupler U1 is powered, the zener diode VD1 protects the input of the optocoupler. A large input voltage leads to a decrease in the resistance of the optocoupler resistor from megaohm to kiloohm, transistor VT1 opens, relay K1 is activated. In this case, the relay contacts perform two functions: they open the tuning resistance R1 - the voltmeter circuit switches to the high-voltage limit; instead of the green LED HL2, the red LED HL1 turns on. Red, a more visible color, is specially chosen for the high voltage scale. Attention! Adjustment of R1 (scale 0...300) is performed after adjustment of R2. The power supply to the voltmeter circuit is taken from the thyristor control unit. Decoupling from the measured voltage is carried out using an optocoupler. The switching threshold of the optocoupler can be set slightly higher than 30 V, which will make it easier to adjust the scales. Diode VD2 is necessary to protect the transistor from voltage surges at the moment the relay is de-energized. Automatic switching of voltmeter scales is justified when using the unit to power various loads. The numbering of the optocoupler pins is not given: using a tester, it is easy to distinguish between input and output pins. The resistance of the optocoupler lamp is hundreds of ohms, and the photoresistor is megaohm (at the time of measurement, the lamp is not powered). Figure 8 shows the top view of the device (cover removed). VS1 and VS2 are installed on a common heatsink, VS3 and VS4 are installed on separate heatsinks. The threads on the radiators had to be cut for thyristors. The flexible outputs of the power thyristors are cut off, the installation is carried out with a thinner wire.
Figure 9 shows a view of the front panel of the device.
On the left is the load current control knob, on the right is the voltmeter scale. LEDs are fixed near the scale, the upper one (red) is located near the inscription "300 V". The terminals of the device are not very powerful, as it is used for welding thin parts, where the accuracy of maintaining the mode is very important. The engine start time is short, so the resource of the terminal connections is sufficient. The top cover is attached to the bottom with a gap of a couple of centimeters to ensure better air circulation. The device is easy to upgrade. So, to automate the mode of starting a car engine, no additional details are needed (Fig. 10).
It is necessary between points "D" and "E" of the control unit to include a normally closed contact group of relay K1 from the circuit of a two-limit voltmeter. If the restructuring of R3 fails to bring the switching threshold of the voltmeter to 12 ... 13 V, then you will have to replace the HL3 lamp with a more powerful one (instead of 10, install 15 W). Starting devices of industrial production are adjusted to the switching threshold of even 9 V. We recommend setting the switching threshold of the device to a higher voltage, since even before the starter is turned on, the battery is supplied with a little current (up to the switching level). Now the start is made with a slightly "recharged" battery, together with an automatic starting device. As the on-board voltage increases, the automation "closes" the current supply from the starting device, with repeated starts at the right moments, the recharge is resumed. The current regulator available in the device (duty cycle of rectified pulses) allows you to limit the amount of inrush current. Authors: N.P. Goreiko, V.S. Stovpets
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