Many circuits have been published about the auto-cut arrangement in manual stabilisers. These circuits generally use transistors with zener diodes as the only sensing device. However, these circuits have two main drawbacks.
- Spurious triggering of the transistor
- Fluctuation in zener diode
Circuit Description of Auto cut for manual Stabilisers
The circuit of auto cut for manual stabilisers described here overcomes spurious triggering of transistor and fluctuation in zener diode drawbacks. It uses the most versatile op-amp IC741 a schmitt trigger. The reference voltage is given by a combination of 4.6V, 400mW zener diode D1 and a 1.6V LED. The negative temperature coefficient of forward biased LED cancels the positive temperature coefficient of reverse biased zener diode, thereby eliminating the error due to temperature etc. Resistor R1 and the output transistor T1 eliminate any false triggering. Diode D1 is used to save the transistor from the back emf of the relay coil RL1.
The preset is adjusted such that the transistor turns on at the highest possible voltage on mains. Point A and B are connected to the references transformer. The reference transformer is connected to the sliding contact of the 7-way switch.
The N/O point of the relay goes to the high voltage cutoff indicator which is an ordinary neon bulb. Output is obtained from the N/C point of the relay.
PARTS LIST OF AUTO CUT FOR MANUAL STABILISERS
|Resistor (all ¼-watt, ± 5% Carbon)|
|R1 = 1 KΩ
R2 = 2.2 KΩ
R3, R4 = 4.7 KΩ
R5 = 3.3 KΩ
R6 = 10 KΩ
VR1 = 470 KΩ
|C1 = 0.1 µF|
|IC1 = LM741
T1 = SL100B
D1 = 1N4001
ZD1 = 4.6V, 400Mw
LED1 = 5mm any color LED
|RL1 = 12V, 200Ω SPST Relay|