Multiple appliances have been born to ease human efforts. And in the same run, numerous protective circuits are designed to ensure long lifespan of those devices. We have seen those circuits claiming to protect appliances like refrigerators and air-conditioners from voltage fluctuations and brown-outs. A overvoltage protection circuit have been presented here serving multi-purpose security; under-voltage protection, switch on delay, and regulation.
Description of Overvoltage Protection Circuit
Do not get worried with the overvoltage protection circuit diagram, though it looks complex, the circuit comprises of building blocks we have been familiar with. As seen in the left part of the circuit diagram of overvoltage protection, the ladder resistance in conjunction with modified bridge rectifier circuit has a trim-pot configuration. It guarantees the reliable sequential operations of boosting, low-voltage cut-in, bucking, and high-voltage cut-off.
The maximum limit for input line voltage is 140 V. After that, relay RL2 get energized and the boosted voltage shows up at the N/O contact RL1 (b) of relay RL1. But, under two conditions, relay RL1 lies in de-energized condition. First, when the input line voltage is controlled by trim-pot VR1 and is below 170V threshold voltage level. Second, due to initial shunting effect of capacitor C6 at the base junction of transistor T2. As the charging capacitor raises its base potential to overcome the reverse bias voltage at its emitter, the transistor T2 also gets enabled. This capacitor along with the resistor R6 determines the duration for on delay, (approximately three minutes for given values in figure).
As a result of switching action of transistor, relay RL1 gets energized and boosted voltage appears across the output. The bucking point is controlled by the proper adjustment of VR2 in trim-pot configuration. The output of the system is isolated when the input reaches the prohibitive voltage. The over-voltage sensing is controlled by the trim-pot VR3 in order to saturate transistor. It then cuts off relay RL1 via transistors T5 and T6. Due to this, autotransformer provides no output.
In the overvoltage protection circuit, the resistor R8 is connected so as to discharge the timing capacitor C6 as relay RL1 energizes. It is done such that when capacitor C6 is connected back to the base junction of transistor T2, on continuation after a power failure or an over-voltage condition, repeatability of on-delay is taken care of.
This circuit of overvoltage protecting appliances can be brought in practice by proper selection. Choosing current ratings for relay contacts (5A or 30A) and autotransformer (500VA or 4000VA, this circuit shown in the figure can be implemented to protect and regulate refrigerators and air-conditioners over a range of 200V to 240V, when input voltage varies from 170V to 270V. the circuit offers multi-benefits.
PARTS LIST OF OVERVOLTAGE PROTECTION CIRCUIT
|Resistor (all ¼-watt, ± 5% Carbon)|
|R1, R9, R10, R12, R14, R15, R17 = 2.2 KΩ
R2, R13 = 1 KΩ
R3 = 3.3 KΩ
R4, R5, R7, R16 = 5.6 KΩ
R6 = 8.2 KΩ
R8 = 100 Ω
R11 = 56 KΩ
VR1, VR3 = 2.2 KΩ
VR2 = 4.7 KΩ
|C1 = 1000 µF/40V
C2 = 220 µF/40V
C3 = 100 µF/40V
C4 = 10 µF/25V
C5 = 10 µF/25V
|T1, T2, T4, T5 = BC547
T3, T6 = SK100
D1 – D8 = 1N4007
ZD1 = 7.5V zener diode
LED1, LED2 = Red LED
|X1 = Auto Transformer 1.5A Rating With Taps At 270V, 230V and 190V
RL1 = 6V, 100Ω DPDT Relay
RL1 = 6V, 100Ω, SPST Relay