In this article we discuss how to make a cheap yet effective mains operated AC overload and over-current protector circuit using very ordinary discrete components.
Introduction
I have published a few mains voltage stabilizer circuits in this blog, these units are designed and intended for safeguarding the connected appliances at their outputs.
However these equipment lack one protection which is the overload protection.
The Importance of an Overload Protection Circuit
A particular stabilizer unit may be rated for handling a maximum specified limit of power, beyond which it's effects may start diluting or might become inefficient.
Overloading a voltage stabilizer might also result in heating of the transformer and fire hazards.
A simple circuit shown below may be incorporated with a stabilizer circuit or any such protection circuit for reinforcing the safeguarding capabilities of the units.
How it Works
The diagram shows a very simple and straightforward configuration where only a couple of transistors and few other passive parts are used for forming the intending design.
The mains stabilized AC is derived from the stabilizer outputs and allowed to switch through another RL1, via its N/C contacts.
One of the wires of the AC mains connections is added with a series resistor of a calculated value.
As the load across the mains output increases, a proportionate magnitude of voltage starts developing across this resistor.
The value of the resistor is so selected that the voltage across it becomes just enough to light up a connected LED in response to a load that might be considered as dangerous and over the maximum tolerable limit.
When this happens, the LED just lights up, an LDR positioned and enclosed in front of the LED instantly drops its resistance in response to the illumination generated by the LDR.
The sudden reduction in the resistance of the LDR, switches ON T1 which in turn switches ON T2 and the relay, initiating the latching effect of the circuit and the relay.
The load or the appliance at the output is thus immediately switched off when an overload situation is detected.
The whole action takes place within a fraction of a second, giving no chance for any untoward consequence and the whole system is safeguarded by the inclusion of this simple AC mains overload protection circuit.
Formula for Calculating Current limiting Resistor
R1 = 1.5 / I(intended current limit),
For example if I =15 amps, then R1 = 1.5/15 = 0.1 Ohms, and it's wattage will be 1.5 x 15 = 22.5 watts
Parts List
- All resistors are 1/4 watt 5% except R1 (see text)
- R4 = 56 ohms
- R4, R7 = 1K
- R5 = 10K
- R6 = 47K
- P1 = 100K preset
- Diodes = All are 1N4007
- T1 = BC547
- T2 = BC557
- C2 = 10uF/25V
- LD1 = red LED 20 mA
- Relay = 12 V/200mA 30 amps
The LED/LDR device can be assembled manually as per the following example image
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