Mains rectifier - voltage and current stabilizer. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Surge Protectors

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One of the main problems in the development of a network rectifier is the limitation of the amplitude of the charging current of the smoothing capacitor at the time of connection to the network. In low-power rectifiers, a current-limiting resistor or thermistor is installed at the input for this. In more powerful devices, to increase the efficiency, the limiting resistor is shunted with a relay contact or a trinistor when the voltage across the smoothing capacitor reaches a value at which the amplitude of the further charging current pulses will no longer exceed the permissible value.

The scheme of the proposed device is shown in fig. 1. The controlled rectifier bridge is assembled on two trinistors VS1, VS2 and two diodes VD2, VD4. Capacitor C5 - smoothing. Resistor R16 - load current sensor. Diodes VD1 and VD3, together with diodes VD2 and VD4, form an uncontrolled rectifier bridge used to power the SCR control unit, which includes the remaining elements. The opening voltage to the control electrodes of the trinistors is supplied alternately through the diodes VD1, VD5 or VD3, VD6, depending on the polarity of the half-wave of the mains voltage, when the analog of the trinistor assembled on transistors VT2 and VT3 is opened by the voltage supplied to the base of the transistor VT3 through the resistor R9 when the transistor is closed VT1.

Capacitor C1 is charged at the tops of the half-waves to the voltage UC1:

Uc1 = Um - Uvd8,

where Um - network voltage amplitude; Uvd8 - stabilization voltage of the zener diode VD8 (about 7,5 V).

In the pauses between pulses of the charging current, the voltage across the capacitor C1 decreases by dUc1 as a result of discharging through the resistor R2. Capacitor C3 is charged to voltage Uvd8 when the instantaneous rectified mains voltage U exceeds Um - (Uvd8 - Uc1). The capacitor C3 is discharged through the VD10 diode when the analog of the trinistor VT2VT3 is opened.

Neglecting the voltage drop across open p-n junctions, we can say that the resistor divider R4-R6 receives the voltage difference U-Uc5. When this difference decreases to the set value dU, the transistor VT1 closes, allowing the switching on of the analog of the trinistor VT2VT3 and the trinistors VS1 and VS2. Adjustment of the value of dU is carried out by changing the position of the trimmer resistor R5.

The resistance of the resistor R2 affects the position of the start of charging the capacitor C3 relative to the beginning of the half-cycle of the mains voltage and, together with the voltage Uvd8, determines the maximum possible opening angle of the trinistors, as well as the maximum level of output voltage ripple.

Capacitor C2 eliminates the possibility of premature opening of the trinistors after the moment they are connected to the network until the required voltage is established on capacitor C1. Resistor R3 discharges capacitor C2 after the device is turned off. The minimum time interval (about 5 s) before switching on again depends on its nominal value.

The cascade on the transistor VT4 provides stabilization of the output voltage and current, reducing, if necessary, the value of dU, determined by the position of the resistor R5 slider. The output voltage is regulated by moving the trimmer resistor R14 in the range from zero to maximum Um - Uvd8 - dUc1 - dU (about 250 V).

When the voltage at the load current sensor - resistor R16 - exceeds 0,6 V, the transistor VT4 opens, causing the output voltage to decrease, which ensures the limitation and stabilization of the load current. If this function is not needed, the resistor R16 is replaced with a jumper.

Most of the elements are mounted on a printed circuit board made of one-sided foil fiberglass, the drawing of which is shown in Fig. 2. The elements of the rectifier bridge (VS1, VS2, VD2, VD4) are selected with a reverse voltage of at least 300 V and at least a double margin of the maximum forward current in relation to the maximum load current. For most powerful diodes, the case is connected to the cathode, and for trinistors - to the anode, so it is convenient to mount the VD2 diode and the VS1 trinistor on the same heat sink (similarly to VD4 and VS2).

Capacitors C1 and C6 - K73 -17, C3 and C4 - any ceramic or film. Oxide capacitor C2 - K50 -29 or similar imported. Smoothing capacitor C5 - K50 -17, its capacitance is chosen, as for a conventional bridge rectifier, so that the output voltage ripple does not exceed the value allowed for the load used.

Zener diodes VD8 and VD13 are micropower, with a stabilization voltage of 7 ... 10 V at a minimum current of 0,1 mA. Zener diodes KS175Ts, KS182Ts, KS191Ts, 2S175Ts, 2S182Ts, 2S191Ts are suitable. In extreme cases, they can be replaced by transistors of the KT315 series with any letter index (the base is turned on as an anode, the emitter - as a cathode, the collector is left free).

First, all elements are mounted on the board, except for the resistor R8 and capacitor C5. A load is connected to the output, for example, an incandescent lamp with a power of 100 ... 200 W. The device is connected to the network through an isolating transformer and an oscilloscope checks for the presence of peaked voltage pulses on the load with a decline coinciding with the end of the half-wave of the mains voltage. They check that the amplitude of the pulses can be adjusted by moving the engine of the tuning resistor R5. Install this engine in the lower position according to the diagram and connect the capacitor C5, connected in series with an additional resistor with a resistance of 10 ... 20 ohms, with a power of at least 10 watts. The voltage on the capacitor C5 should gradually increase in a few seconds to about 290 V with a characteristic jump at the end. If this is the case, the capacitor C5 is connected by directly removing the additional resistor, and the resistor R8 is installed. The resistance of the resistor R16 is selected for the required level of output current limitation.

Since the protection operation threshold and the maximum amplitude of the output voltage ripples determine both voltages dU and dUc1, when the resistance of the resistor R2 decreases, the threshold and "sharpness" of the protection operation increase. Experimentally selecting the resistance of the resistor, you can change the ratio of these voltages and achieve the required load characteristics of the device.

Author: V. Kaplun, Severodonetsk, Luhansk region; Publication: cxem.net

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