Case short circuit protection. 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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The vast majority of household electrical appliances are not grounded. At first glance, there is no need for it: the cases of the devices are reliably isolated from the mains, and they work with them, as a rule, in dry rooms with a non-conductive floor. However, one cannot rely entirely on technology. In the event of a breakdown or damage to the insulation, a faulty device becomes a source of serious danger. And the fuses are useless here: they will not blow until a short circuit occurs.
An automatic device will help you avoid electrical injury, which will disconnect the electrical appliance from the mains as soon as voltage appears on the case.
Rice. 1. Schematic diagram of the protective shutdown device (click to enlarge)
The automaton is assembled according to the trigger scheme with one stable state (Fig. 1). In the event of a "zero" or phase of the network shorting to the body of the electrical appliance, the rectified voltage through the chain V2, R1 or V3, R2 is supplied to the transistor V4, and it closes. The increased voltage on its collector opens the transistor V5: relay K1 is blocked by its contacts K1.1, and K 1.2 disconnects the appliance from the network.
A half-wave rectifier for powering the protective device is assembled on a diode V1. With the help of a variable resistor R3, the trigger threshold is set, the capacitor C1 eliminates the vibration of the contact plates of the relay K1.
In standby mode, the machine practically does not consume current.
MP26B transistors can be replaced with MP41, MP42, MP21, MP25, diodes can be of any type for a voltage of at least 100 V and a current of 0,1 A.
Fixed resistors - ULM, ML T-0,25, R7 - wire resistance 8-10 Ohm; variable resistor R3 - SP-01. Capacitors: C1 - MBM for 160 V, C2 -K50-ZA.
K1-relay RES-22 (passport RF4.500.129) or MKU-48 for 24 V. For the latter, it is necessary to loosen the armature spring.
To protect powerful electrical appliances, it is necessary to install a magnetic starter designed for the appropriate load.
T1 - transformer TVK-70/110 from TVs. The voltage on the secondary winding is 12-20 V, the power is 3-5 watts.
The wiring diagram of the automatic device is shown in Figure 2.
Rice. 2. Wiring diagram of the machine (click to enlarge)
After checking the correct installation, an alternating voltage of 2-3 V is applied to the diodes V35, V40 and the common wire ("+"), and with the help of a tuning resistor R3, the relay K1 is activated. Then the electrical appliance is connected to the circuit breaker and plugged into the network (Fig. 3).
Rice. 3. Scheme of connecting the device with an electrical appliance
When working together with large electrical appliances (refrigerator, washing machine), it is recommended to install a protective device inside their housing in any free place.
Author: V.Konovalov
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