Current protection device in a bipolar power supply. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The presence of an effective system of protection against overload and short circuits of the output circuit prevents the failure of both the structure being adjusted and the power supply itself. The system must instantly reduce the voltage to zero on both arms of the power supply, i.e. have a trigger effect. A circuit that satisfies these conditions is shown in Fig. 1.

Schema work. In the initial state, transistors VT1 ... VT4 are closed. In case of overload, short to ground or "minus" of the positive shoulder, the opening of the transistor VT1 will lead to a breakdown of the zener diode VD1, which opens the transistor VT2. After that, the "latch" system is activated, i.e. current flows through two zener diodes, keeping transistors VT1 and VT2 open even after the short circuit is removed. Through diodes VD3 and VD4, voltage is supplied to the bases of transistors VT3 and VT4, which form control signals and close the control elements of the positive and negative arms of the stabilizer. Diodes VD3 and VD4 are threshold elements for transistors VT3 and VT4, respectively.

The circuit can be in this state for a long time. In this case, the voltage at the output of the shoulders is zero, which means that thermal breakdown does not threaten the regulating transistors. With the help of resistors R1 and R2, you can smoothly or stepwise adjust the protection operation current. To return the circuit to its original state, a toggle switch S1 ("Return") is provided, which for a short time opens the power circuit of the zener diodes VD1 and VD2.

The layout on the board is shown in Fig.2. As an independent part of the power supply, it can be connected to almost any bipolar voltage source.

Schema setup. It is necessary to apply voltage to the circuit and check the reliability of the functioning of the current protection within the limits of 0,6 and 1,2 A, then - in the modes of positive and negative short circuits, as well as the positive arm-housing, negative arm-housing. Transistors VT1 and VT2, resistors R1 ... R4, zener diodes VD1 and VD2 are selected with the same parameters.

If the input voltage of 34,5 V is more or less than 10%, then you need to use the formulas:

Ust.stab = Uin / 2,3; (one)

R9 \u1d Uin - Ub.eVT3 - Ust.stab - UVD3 - Ub.eVT0,0011 / 2; (XNUMX)

R7 + R8 \u2d 1Uin - Ub.eVT0,0085 - Ust.stab / (0,0011 - 3); (XNUMX)

R7 = R7 + R8/18, (4)

where Ust.stab - stabilization voltage of zener diodes VD1 and VD2, V; Uin - input voltage relative to the housing; UVD3 - voltage drop across the VD3 diode; Ub.eVT1 voltage base-emitter of the transistor VT1 (0,65 V for silicon); R7-R9 - in ohms.

In the denominator, the number 0,0011 A is the current through the resistor R9, the number 0,0085 A is the current through the resistors R7 + R8.

The circuit has been operating for about 2 years with current protection limits of 0,6 and 1,2 A. There were no failures during multiple short circuits and overloads in the positive and negative arms.

Replacing elements. Instead of those indicated in the diagram, transistors can be replaced with KT361G, KT315E at Uin? 21 V; diodes VD3, VD4 - low-power silicon; resistors R1...R4 - wire; S1, S2 - switches type TP1-2; S2 - current protection limit switch 0,6 / 1,2 A.

Author: V. B. Lovchuk

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