Electromechanical protection of the charger against short circuit. Encyclopedia of radio electronics and electrical engineering

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

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Modern chargers (chargers) and power supplies (PSUs) are usually equipped with an electronic short circuit (SC) protection system at the output. However, in amateur radio practice, there are also simple network sources of electricity, consisting only of a step-down transformer and a rectifier. The necessary components to supplement them with electronic protection are sometimes expensive, far from being for everyone and not always available.

But even in ultra-modern, it would seem, units and devices with a parametric or compensating supply voltage stabilizer, electronic protection sometimes also turns out to be clearly not up to par due to thermal overloads of the regulated transistor. It turns out that the lower the output voltage is set here and the closer to the maximum current consumed by the load, the faster the heating occurs. Such a semiconductor triode can no longer limit the flow of short-circuit current by switching to the design mode of operation. And as a result - a breakdown of the transistor and the failure of the entire power supply.

I offer simple electromechanical protection against short circuits using relays or automatic switches of multiple action (for example, automatic fuses in apartment meters - AVM). Advantages of such protection: simplicity, lack of expensive semiconductor devices, guaranteed galvanic isolation of the load and the supply voltage. The downside is inertia. So, the speed of relay protection is approximately 0,1 s, with the use of AVM - up to 3 s. However, in practice, this is sometimes quite enough.

Consider a circuit protection circuit that can be successfully used in chargers and power supplies with unregulated voltage (Fig. a). By pressing the SB1 button, the K1 relay is activated, which goes into self-locking mode, keeping the K1.1 contacts closed and supplying electricity directly to the load. In the event of a short circuit in the supply circuits, the output voltage decreases sharply, the relay winding is de-energized, which leads to opening of the contacts and disconnection of the load from the source.

Re-enabling the load with the SB1 button is possible only after the fault has been eliminated. In this case, the capacitor C1, charged to the output voltage of the power source, is discharged to the relay winding, causing K1 to work. Resistor R1 limits the discharge current pulse, preventing the destruction of the internal structure of C1 when the load is switched on by mistake, when the short circuit at the output of the power supply has not yet been eliminated. Resistor R2 limits the short circuit current of the rectifier diodes. It can even not be introduced into this circuit if the diodes are designed for pulses that exceed the short-circuit current in their amplitude. Otherwise, the named resistor is required. However, it should be remembered that the output voltage of the source in this embodiment must exceed the voltage drop across R2 at the rated charging current or load current.

Relay for protecting power supply from short circuits with unregulated (a) and regulated (b) output voltage (click to enlarge)

AVM protects against overcurrent, which relay protection cannot do. An automatic fuse (or a multiple, automatically resettable switch) is installed instead of the resistor R2, because the active resistance of the AVM usually does not exceed 0,4 ohms.

Now let's consider a circuit protection circuit that can be used in a power supply with an adjustable output voltage (Fig. b). As with the previous one, the load is turned on by the SB1 button, by pressing which the capacitor C1 is connected (through resistors R2 and R3) to the base of the transistor VT1. If there is no short circuit at the output, then VT1 will open, having received the necessary bias voltage. Relay K1 will work, turning on its contacts K1.1 and the adjustable base stabilizer, and the load. Now the output voltage, whatever it may be, will keep VT1 open.

Well, in the event of a short circuit at the output, the base of the transistor will be grounded through the resistor R2, and the electronic guard - the semiconductor triode will close almost instantly. As a result of this operation, relay K1 will de-energize, turning off both the stabilizer and the load.

The role of the resistor R3 in the second circuit is similar to the purpose of R1 in the first circuit. Capacitor C1 during the operation of the stabilizer performs the function of the capacitance of the low-pass filter. The diode VD1 protects the transistor VT1 from the inductive current that occurs during switching in the winding of the relay K1.

The relay parameters depend on the rated current of the charger or power supply. For example, to charge car batteries, it is necessary to select a relay for a rated voltage of 12 V with a permissible switching current of 20 A (or more). Such conditions are satisfied, in particular, by REN34 (passport KhP4.500.030-01), whose closing contacts should be connected in parallel. You can also use a 12-volt relay with a contact spacing of at least 3 mm and a switching current of 20 A or more.

It is quite acceptable for chargers and power supplies with a rated current of up to 1 A and a RES22 relay (RF4.523.023-05 passport) or similar in terms of switching current and operating voltage. Capacitor C1 in both circuits is oxide, from among K50-12, K50-16 and similar types. As resistors R1-R3, the common MLT-0,5 or MLT-0,125 are suitable. The only exception here is high-current (32 (Fig. a), it must be wire. Transistor VT1 - KT815A, KT817 A or a medium-power semiconductor triode similar to them. VD1 has a wide scope for choice, in place of which KD410 diodes work with equal success, KD503, KD512, KD519, KD521 Button BV1 - any type.

With serviceable parts and correctly performed installation, the performance of both circuits is ensured, as they say, one hundred percent.

Author: D.Ataev

See other articles Section Protection of equipment from emergency operation of the network, uninterruptible power supplies.

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