Thyristor stabilized power supply with the ability to adjust and protection against overcurrent. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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I bring to the attention of readers a thyristor adjustable voltage regulator with overload protection. This design will be very effective when supplying loads that are not critical to supply voltage ripple, for example, for DC motors and any other devices that consume significant power and require a stable (average value) supply voltage with the ability to adjust.

Its limiting technical characteristics are determined by the characteristics of two circuit fragments - a thyristor and a rectifier bridge. The control system is universal, it is designed and created in such a way as to exclude expensive and / or scarce elements from the design.

The functional diagram is shown in Fig.1.

(click to enlarge)

I want to warn you right away that attempts to power the entire system from a single source have not been successful. The interference of various circuits with each other through the power supply is too great, which greatly degrades the stability of the output voltage. And the creation of a power supply with low output impedance in this design is unjustified in terms of costs and the number of elements.

The circuit diagram is shown in Fig. 2, where R1, R2, R4 are the quenching resistors of the power circuits, and the resistance R4 can be about five times greater than the resistance R1, R2 due to the current being drawn from the capacitor C3, which has the form of a short pulse. The rest of the time C3 is charging.

Resistors can be calculated using Ohm's law for any supply voltage of the unit. In general

R \uXNUMXd (U - Ust) / I,

where R is the required resistance; U is the effective value of the applied voltage; Ust stabilization voltage of the zener diode; I is the current required by the powered circuit and flowing through this resistor.

For large U and small Ust, the value of Ust can be neglected.

When calculating, do not forget about the power dissipated by the resistor, P = UI, where P is the power, W; U - effective value of the applied voltage, V; I current flowing through the resistor, A. I remind you that for the reliable operation of the resistor, the maximum power dissipated on it must be about twenty percent lower than the nominal one.

A single vibrator is assembled on C2 and A1, generating a pulse with a duration of at least 100 ms, which, through the buffer transistor VT2, lights up the optothyristor LED, opening it.

The scheme of node A1 can be performed according to Fig. 3 or 4.

It should be noted that the circuit in Fig. 3 is more stable than the unijunction transistor. The pulse duration should be approximately 10 times greater than the minimum passport pulse duration of the opening thyristor. Diode VD3 provides synchronization of the single vibrator with half-waves of the supply voltage, discharging C2 at the time of zero supply voltage. Node R3, VT1 is a controlled source of charge current C2, which allows you to smoothly adjust the charge time. Resistor R6 determines the pulse width of the single vibrator. It should be noted that its resistance should be less than the resistances R9, R10 of the reference divider. When using the variant of block A1 according to Fig. 3, the resistance R9, R10 can be 10 kOhm without any noticeable deterioration in performance. When using the A1 block option according to Fig. 4, install the outputs of the unijunction transistor in the corresponding holes of the printed circuit board without wiring correction, since the board is universal.

Node R4, VD4, C3 - optothyristor LED power circuit. Excess voltage is "discharged" through the diode VD5. The optothyristor LED had to be supplied with a separate power source due to the large rated supply current, which gives vodka to the rest of the circuit. The inappropriateness of an internal power supply with a low output impedance was mentioned above. Resistor R8 determines the optothyristor LED current. I will not dare to offer a clear method for calculating this resistor due to the fact that I came across optothyristors with a large spread in LED parameters. Just select this item. The limiting passport value of the constant current of the LED of the TO125 optothyristor is 80 mA.

The VD7, C4 node provides the feedback signal integrator with a stable power supply. Resistor R11 straightens the output voltage regulation characteristic. Without it, the output voltage regulation in the low voltage region will be smoother, but sharper in the high frequency region.

Node VT3, R12 is another managed key. Its function is to lock VT1 in the presence of an overload. The degree of influence of feedback signals on the integrator is determined by the resistance of the resistor R12. Node C5, R14 is actually an integrator.

The voltage at the load is integrated, the value of which is determined by the resistor R15. It should be noted that when powering the unit from high voltages, such as a mains voltage of 220 V, it is necessary either to take a wire R15 or increase its resistance by about 10 times. It is easy to verify this by calculating the power dissipated on this resistor using the formula given above for calculating the power of quenching resistors in power circuits. Resistor R13 improves the parameters of the integrator for leakage current C5.

You can experiment with this resistor or eliminate it altogether, but this will not improve the parameters of the circuit. The VD8 zener diode is recommended to be installed when the unit is operating in the high voltage area, but this is a safety element that is not mandatory. Therefore, the installation place for it on the board is not provided.

Node VT4, VT5 - current sensor signal amplifier. The transistors open if the voltage at the base of VT5 is approximately 1,2 V higher than at the emitter of VT4. When experimenting, I do not recommend confusing the loads of the collectors.

When turned on, as shown in the diagram, the base-emitter current of VT5 is almost constant, while VT4 has significant ripples. Now figure out what will happen if you swap the collector loads of these transistors in places.

Node R19, C7 - current sensor signal integrator. If, when using block A2 and small load currents, it is still possible to do without it, then in the absence of A2, the entire current sensor signal conditioner starts to operate in a pulsed mode. Therefore, the operation of the entire system is upset.

Resistor R20 - current sensor (wire resistor). Choose it at your discretion, but keep in mind that if the overcurrent protection system will operate at an average current greater than the allowable average currents of the diode bridge or thyristor, then it makes no sense. The protection operation voltage is 1,2 V and, based on this, calculate the resistance R20 according to Ohm's law: R = 1, 2 / Imax, where R is the resistance of the resistor, Ohm, Imax is the required value of the average current in the load.

Transistor VT6 controls the VD9 LED, which indicates the overcurrent mode. Capacitor C6 eliminates VD9 flicker and softens the operation of the current sensor signal amplifier. Node R1, VD1, C1, VD6 - power supply circuit of the VD9 LED.

If you do not plan to indicate an overload condition, then you can exclude the elements R1, VD1, C1, C6, R16, VT6, R18, VD9, VT4. In this case, connect the VT5 emitter directly to the common wire. In this case, the protection operation voltage taken from R20 will be approximately 0,6 V, which must be taken into account when calculating the resistance of the resistor R20.

Block diagram A2 is shown in Fig.5. It provides the level of the DC component in the load. Choke L1 is used as a ballast. When the thyristor opens, the rectifier bridge diodes operate in the short-circuit current mode, recharging the filter capacitors. At this moment, L1 creates a reactance in the circuit, which saves the bridge diodes and the thyristor from current surges that exceed the permissible limit, and also saves them from overheating and increases the durability of the system.

The diode eliminates self-induction voltage surges, which prevents failures in the control system. Inductor L2 acts as a ballast resistor for the variable component.

Design features

You can replace R18 with either a KS133 zener diode or another LED. It makes sense to do this for more stable operation of the optothyristor and, if necessary, the presence of a second LED, for example, for additional indication. VD6 can also be replaced with a chain of two or three LEDs connected in series. You can replace the LED and the zener diode KS133, connected in series. They will indicate the presence of power in the block circuits.

Instead of VD5, you can install a zener diode with a stabilization voltage of 4 ... 4,7 V between the VD6,2 cathode and the common wire. You can vary these circuits as you like, but do not violate the conditions under which all circuits of the block are powered by a voltage within 4,7. ..6,2 V.

Instead of the R20 current sensor, you can install a variable or trimming resistor, preferably a wire one. This will give you the opportunity to smoothly adjust the level of current protection operation.

About the features of the board

The layout of the printed circuit board from the side of the tracks is shown in Fig.6.

It is designed in such a way that if there is no need for an A2 block, you can simply shorten it. The line to be shortened is indicated by a dash-dotted line. It is possible to install power circuit elements for an additional LED, for example, to indicate the mains voltage or any other high AC voltage. The schematic diagram of this circuit is shown in Fig.7.

Large diameter holes are indicated as a circled dot. All holes, the diameter of which is not indicated in the figure, have a diameter of 2 mm. These holes are strongly recommended to be pierced. This will save you from many minor troubles during installation and operation of the unit.

The board is connected to external circuits by means of the RP10-15 connector. This connector is quite common, allows currents up to 10 A per contact, and, compensating for the minor inconvenience of wired connection of its contacts to the circuit, makes it possible to easily remove any necessary element from the board. For example, install VS1 on a heatsink and remove R20 from the board, making it variable. The connector is attached to the board using two corners, under which two holes are made in the board. It is more reliable and more convenient to put the female part of the connector on the board. Garbage gets into it more often and, of course, it is more convenient to clean it on a removed board, and not on a chassis that is less easy to access.

The board provides mounting locations for trimming resistors of the SP3-38b type (lying). If you plan to operate the unit outdoors or in an aggressive atmosphere saturated with vapors of acids, alkalis, high humidity or dust, install hermetically sealed resistors. According to the location of their pins, adjust the position of the holes and mounting pads for them. Cover the block itself with varnish such as UR, Sherlak, in extreme cases, with rosin diluted with alcohol.

Do not be lazy to fix the filter capacitors of block A2 on the board with a wire bracket. For this, the corresponding holes are specially left. To improve heat dissipation of the elements R1, R2, R4, R20 during installation, leave them raised above the board by about 5 mm.

The cores of the A2 filter chokes are attached to the board with M4x25 screws through the corresponding holes. In order to prevent the core from cracking, lay a soft washer between it and the screw, it can be textolite.

The power rectifier uses KD213 diodes (when working with voltages below 200 V) or any other sufficiently powerful ones.

Simple to manufacture and quite efficient radiators are shown in Fig. 8.

The design consists of a U-shaped bracket of soft aluminum 2 ... 3 mm thick and a pressure plate made of duralumin of the same thickness with threaded holes.

The pressure plate can also be made of another material, but this will impair heat dissipation. This radiator design is designed for diodes KD213, KD212 or similar. When using other diodes, it may be necessary to adjust the position and dimensions of the mounting holes.

The TO125 optothyristor is attached to the board with two M3 screws through the corresponding holes. The same screws provide electrical contact between the anode and the circuit.

The optothyristor LED is connected to the corresponding contacts on the board using a wire and resistor R8, as a hanging element.

Details

All resistors of the type MLT, MT, VS, C2-XX with powers corresponding to those indicated in the diagram. Electrolytic capacitors type K53-1, K53-4. They have an all-climatic design. You can, of course, take the K50-XX, but I strongly do not advise you to do this. The cost of load and reliability can be much higher.

Zener diodes - for a voltage of 4,7 ... 6,2 V with any letter indices and preferably all of the same type (KS147, KS447, KS156, KS456, KS162). You can replace: KT502 with KT203, KT209, KT3107, KT501 with any letter, KT503 with KT3102 with any letter, KT3102 with KT342, worse if KT503. All with any letter indices. KD522 on KD521 or any other with a constant forward current up to 50 mA and a reverse voltage of at least 15 V. The chokes of the A2 block are wound on armored cores B30 ... B36. L1 contains 10...30 turns of wire PEL 0,8...PEL 1,2, L2 contains 50...100 turns of wire PEL 0,6...PEL 1,0. In these chokes, it is desirable to arrange a non-magnetic gap of 0,1 ... 0,5 mm.

To do this, lightly sand the end of the cup and coat it with any waterproof glue. After that, stick the cup on a sheet of ordinary, and preferably capacitor paper. After the glue dries, remove excess paper so that the coil freely enters the cup. This operation can be done with both cups. It all depends on the thickness of the available paper. To avoid unpleasant buzz of turns or inductor cups at high load currents, dip the assembled and tightened inductor into melted wax, paraffin, stearin for 3 ... 5 s. Let excess filler drain freely.

Setting

A correctly calculated and assembled unit requires the appropriate installation of trimming resistors. First, set the sliders of resistors R3, R12, R15 to the middle position. If the unit does not work, then check the supply voltage. If necessary, select the resistance of the quenching resistors in the power circuits. It is possible that the LED current of the optothyristor is too low. Then pick up R8. Instead, you can solder a circuit of 10 ohm constant and 100 ohm variable resistors connected in series. Do not select extreme LED current values. This whole process is best monitored with an oscilloscope. I remind you that the limiting passport value of the direct current of the LED for TO125 lies within 80 mA.

Finally, I want to express the hope that IC manufacturers will pay attention to this scheme. Then you can seriously think about one, more complex, but more powerful power circuit with a single quenching element and two or three external capacitors for the entire circuit. For us, developers and maintainers, working with one cheap IC in such a block will be much easier. And the market for such a stabilizer can be quite large.

Author: V.B.Efimenko

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