Safety device, 12 volt 1 amp

Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network, uninterruptible power supplies

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A distinctive feature of the proposed device is a low voltage drop in the nominal mode. In addition, after the elimination of the emergency, it automatically restores its performance.

The device is designed to protect against short circuit in the load and overcurrent. It is connected between the power supply and the load. The advantage of the proposed device compared to that described, for example, in [1] is a small voltage drop in the nominal mode, as well as an automatic return to the working state after the elimination of the cause of the accident. The latter is especially important for short-term overloads.

Main technical parameters

• Supply voltage, V ...... 12
• Rated current, A ...... 1
• Protection operation current, A......1,2
• Voltage drop at rated current, no more than, V......0,6

The device contains a transistor switch, protection and start units. The main element is a switch made on a VT5 transistor (Fig. 1). The protection unit is assembled on a transistor VT3, diodes VD1, VD2 and resistors R6. R7. R9. The start node consists of a short pulse shaper on the integral timer DA1 [2]. current amplifier based on transistors VT1, VT2, VT4 and a threshold element (transistor VT6, zener diode VD4 and resistor R10).

After connecting the device to a constant voltage source, the shaper begins to generate short pulses of negative (relative to the +12 V circuit) polarity, which, after amplification, are fed to the base of the VT4 transistor. When it is turned on, the diode VD1 is closed by a voltage of reverse polarity. Transistor VT3 also closes, and transistor VT5 opens, voltage from its input passes to the output of the device.

A threshold element is connected to the output of the device. The collector-emitter section of the transistor VT6 is connected in parallel with the time-setting capacitor C1 of the shaper. When the output voltage is close to the nominal voltage, the VD4 zener diode opens and. respectively, transistor VT6. which closes capacitor C1 to a common wire. The work of the pulse shaper is stopped. At its output (pin 3 of the DA1 timer), a high-level voltage appears, which opens the transistor VT1. Transistors VT2 and VT4 are closed.

In case of overload or short circuit in the load, the voltage drop across the resistor R6 increases. Transistor VT3 opens and closes transistor VT5. by shunting its emitter junction. The on state of the transistor VT3 supports the circuit VD1, R9. Diode VD2 - closed.

During an overload, the voltage at the output of the device becomes less than the nominal and the threshold element "releases" the shaper. The load receives short pulses from the transistor VT4. It must be powerful, capable of withstanding a pulsed short circuit or overload current.

When the emergency mode is eliminated, the device, as already mentioned, automatically restores its working capacity. This happens immediately after the first pulse arrives from the collector of the transistor VT4 at the output.

Diode VD3 protects the device from voltage pulses of reverse polarity in the case of an inductive load.

Transistors are applicable in the device: VT1-VT3. any silicon, VT4, VT5 are powerful pnp structures. The maximum allowable collector current of the transistor VT2 is not less than 200 mA. Diodes KD510A can be replaced with KD522B. Resistor R6 -C5-16V. C5-16MV. the rest - OMLT. Capacitors - K10-17 or KM series.

A properly assembled device starts working immediately. The protection operation current is set by selecting the resistor R6. Its approximate resistance is chosen according to the graph in Fig. 2.

If it is possible to select a VT5 transistor with a low collector-emitter voltage drop, the protection operation current can be increased. Reducing the resistance of the resistor R8 is undesirable, as this entails an increase in the current consumed by the device and an increase in the heat generated on this resistor.

The device is assembled on a printed circuit board, the drawing of which is shown in fig. 3. Transistors VT4 and VT5 are attached to the board with screws. With the specified technical parameters of the device, the VT5 transistor does not require a heat sink. In the case of an increase in the load current between the transistors VT4, VT5 and the board, it is necessary to place a heat sink - an aluminum or copper plate.

For more reliable operation of the threshold element, it is recommended to shunt the emitter junction of the transistor VT6 with a resistor with a resistance of 10 ... 51 kOhm.

Literature

1. Esaulov N. Adjustable electronic fuse - Radio. 1988. No. 5. p. 31. 32.
2. Petsyukh E., Kazarets A. Integral timer KR1006VI1. - Radio. 1986, No. 7. p. 57.

Author: O. Sidorovich, Lviv, Ukraine

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