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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Universal AC protection unit. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers The universal speaker protection unit is made on small-sized parts and can be built into any amplifier that does not have such protection. The peculiarity of this block is the use of built-in mains power, reliable electromagnetic relays and LED indication of the appearance of a constant voltage at the amplifier output. The device provides stable delay and protection even after a brief power failure. It is known that when power is applied to the amplifier in the speaker system (AC), a loud click (clap) may occur. To eliminate this phenomenon, it is necessary to connect the load to the UMZCH output with a certain delay sufficient to complete all transient processes (usually 1...3 s) [1]. When the power is turned off, the speaker should turn off until the storage capacitors of the power supply filter of the amplifier are noticeably discharged (by more than 20%). Otherwise, the shutdown process can also create unpleasant overtones or clicks. The presented module implements the functions of silent turning on and off the amplifier (actually the speaker), and also allows you to protect the bass heads of the speaker when a constant voltage appears at the output of the UMZCH, associated with its emergency operation or failure. Technical specifications
With the implementation of the delay and protection of the speakers, there are no questions. But how to implement a quick shutdown of the speakers in the event of a (relatively short-term) mains voltage failure, but sufficient for a transient and a click to occur? There are two reasonable options: using information about the presence of alternating voltage in one of the existing secondary windings of the transformer supplying the UMZCH (as implemented in the μRS1237 microcircuit [2]), or using a separate power transformer (or from an additional winding of the UMZCH transformer) for the protection node. The first option imposes certain restrictions, narrowing the universality of the module. The second one allows you to use a small-capacity smoothing capacitor in the power supply of the device, thanks to which the protection unit is guaranteed to turn off the speakers faster than the capacitors in the UMZCH power supply are discharged. Obviously, the second option is more reliable and easier to implement, allowing you to connect the module to almost any amplifier. The disadvantage of this solution is the higher cost due to the use of an additional power supply, but versatility and reliability prevail here. The scheme of the device is shown in fig. 1. Its inputs must be connected to the outputs of the channels of the stereo UMZCH, and the outputs to the loads (AC) of the corresponding channels. The common wire of the module, speakers (or crossover) is connected directly to the common wire of the amplifier.
When the supply voltage is applied, the capacitor C6 is slowly charged through the resistor R10 to 1,9 V (determined by the ratio of the resistance of the resistors R10 and R11), which is enough to open the transistor VT4. Relays K1, K2 are activated, and the load is connected to the amplifier. If any of the inputs of the device (contacts Х2а, Х3а) has a DC voltage of more than ±0,6 ... U0,7. The illuminated phototransistor of the optocoupler discharges the capacitor C1 through the resistor R2, and the field-effect transistor VT1 closes, de-energizing the relay. The glow of the HL2 LED indicates the AC shutdown and the UMZCH malfunction. Resistor R8 limits the discharge current of capacitor C6, and resistor divider R4R1 provides an artificial midpoint of the supply voltage. Most of these protection devices and AC turn-on delays have an unpleasant drawback - the absence of a restart delay in a short period of time after a power outage. An example of such a situation is a short-term loss of electricity in the network. This disadvantage does not allow obtaining the proper level of protection for the speakers and all equipment in general, where such a node is used. To eliminate this drawback, elements R9, C5, VT3 were introduced. This circuit is triggered for a short time when the supply voltage fails and appears, discharging the capacitor C6, which ensures a normal subsequent start of the protection unit. The use of a field-effect transistor VT4 with a lower opening voltage (about 1,5 V) provides a lower charge voltage for C6, and the restart time is almost equal to the first turn-on time. While maintaining the constant charge-discharge time of the capacitor C6, its capacitance can be significantly reduced by increasing the resistance of the resistors R8-R11 accordingly. It is not recommended to increase the capacitance of capacitor C1 - it determines the speed of turning off the protection unit. At rated mains voltage 230 V and room temperature 25 оWith the stabilizer DA1 heats up to 50...52 оC. When tested at a maximum alternating voltage of 274 V (limited by the capabilities of LATR), the heating of the stabilizer was 64 ... 65 оC - everything is within the normal range. If we exclude the resistor R1, then the lower allowable power supply limit of the unit will drop to 170 V, but at the same time, the heating of DA1 will increase by an average of 10 ... 12 оC. It is clear that this change is advisable only for areas where the voltage in the network is always lower than the nominal one. If we imagine a situation where both UMZCH channels fail, and in the first channel, a voltage of one polarity is formed at the output, and a voltage of reverse polarity is formed in the second channel, equal in absolute value to the voltage at the output of the first channel (with a difference of less than 0,6 ... 0,7 .2 V), then after summing through the resistors R3 and R1, a voltage will be obtained that is not enough to open the transistor VT2 or VT10. That is, the protection system will not work, and this is a disadvantage (it can be overcome by changing the resistance of one of these resistors by ± XNUMX%). But the probability of such an event is negligible and is rather an example of a hypothetical failure simulation. The printed circuit board (Fig. 2), having dimensions of 66x45 mm, is made on foil-coated fiberglass and is designed for the installation of transistors in SOT-23 packages, resistors of size 0805 (except for resistors R1 and R13 - 1206), capacitors C2, C5 of size 0805 and diode VD2 in the SMA package. On the photo of fig. Figure 3 shows the assembled board from the solder side of the surface mount parts.
As T1, a low-power transformer TPK-2 with a secondary winding of 12 V is used. The diode bridge can be any of the DB103S-DB107S or MB2S-MB6S series, for which two seats are provided on the printed circuit board. Diode VD2 - any with a forward current of 1 A and a reverse allowable voltage of at least 200 V. The relay windings should be for a current consumption of not more than 30 mA (high sensitivity) at a voltage of 12 V. It would be possible to use one relay with two pairs of contacts, but the author could not find one for a switched current of more than 8 ... 10 A. The advantage of the indicated on the TRU-12VDC-SB-CL relay diagram in that they have AgCdO (silver-cadmium oxide) coating on the contacts, resistant to mechanical wear, and a maximum switching current of 12 A. You can replace them with more affordable SRD (T73) 12VDC relays -LS-C by SONGLE, allowing switching current up to 10 A. Optocouplers U1, U2 can be used almost any with the appropriate structure, for example, PS2501, PC817. LED HL1 - any, preferably red glow, for example, from the AL307 series or others. Transistors VT1-VT3 can be replaced by any other low-power transistors of the appropriate structure and size. It is possible to use MMBT5551, MMBT4401 (VT1, VT3) and MMBT5401, MMBT4403 (VT2). As a replacement for the n-channel field effect transistor (FET) VT4 with a low gate threshold voltage (Gate Threshold Voltage), NTR4003N, IRLML2502 can be recommended. If such replacements are not available, then it is permissible to use another n-channel FET with an insulated gate, focusing on the open channel resistance of no more than 3 ... 5 Ohm, the maximum drain-source voltage is at least 20 V and the maximum drain current is at least 300 mA . In this case, the following changes will need to be made to the circuit: R8 = 75 ohms, R10 = R11 = 68 k ohms, C6 = 47 uF at 16 V. But it should be remembered that the delay time during a quick restart will decrease slightly. Since the turn-on threshold level for different PTs can vary significantly, it may be necessary to correct the relay turn-on delay by selecting a pair of resistors R10, R11 from the condition of their equality. Fusible insert FU1 can be used for a current of 0,16 or 0,25 A, for example, the domestic VP4-10 0,2 A, which has small dimensions and flexible leads for mounting on a board. Terminal blocks X1-X3 - series DG127, XY304 or similar. As can be seen from the diagram, the center contact in X1 is not used. This is done in order to increase the gap between the mains conductors. The assembled device (its photo in Fig. 4) does not need to be adjusted and works immediately after power is applied. Its design has been repeated many times, and high reliability is confirmed by long-term operation.
On fig. 5 shows a circuit that allows you to eliminate the small-sized transformer. As an example, a simplified diagram of the UMZCH power supply with a voltage of +/-30 V is shown. At the same time, both the circuit and the method of connecting the module to the amplifier are slightly changed.
The module has a bipolar power supply through the quenching resistors R8, R9, so the formation of an artificial midpoint is not required (resistors R4, R5 in Fig. 2). For greater efficiency, the relays are connected in series and a capacitor (C4) has been added as a power filter. On the components VD1, R5, C3, a half-wave rectifier is made, the voltage from which is supplied to the optocoupler U3. In the initial state, due to the resistor R10, the transistor VT3 is in saturation mode, shunting the capacitor C5 until voltage appears on the emitting diode of the optocoupler U3, after which VT3 closes and C5 begins to slowly charge, opening the transistor VT4. In this case, the total delay time for connecting the load reaches 2 ... 2,5 s. When the amplifier is turned off, the capacitor C3 quickly discharges, de-energizing the optocoupler U3. The transistor VT3 opens and discharges the capacitor C5, as a result of which the relays with the load are turned off. Thus, a quick shutdown mechanism is implemented with a total time of no more than 0,3 ... 0,5 s. The subsequent turn-on start occurs with a discharged capacitor C5, therefore, in contrast to the circuit in Fig. 2, its forced discharge is not required. As VT4, you can use an n-channel FET with an opening threshold voltage of 2 ... 5 V and a maximum drain current of at least 1 A, for example, IRF510-IRF540, IRF610-IRF640. Rectifier diode VD1 - any with a reverse voltage of at least 100 V and a direct current of 100 mA: SF12-SF16, 1 N4002-1N4007, etc. When using a relay with windings that consume a current of 50 mA, it is necessary to change the values of resistors R8, R9 to 330 Ohm. Note. To increase the reliability of work between the base and the emitter of the transistor VT3 (Fig. 1), it is necessary to install a resistor with a resistance of 50 ... 100 kOhm. Literature
Author: N. Vashkalyuk
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