The circuit presented here can be used as overload protector with reset option for inverters or as an electronic fuse in AC mains supply. In this overload protection circuit, the mains supply to the load s like refrigerator is routed via the the N/C (normally closed) contacts of relay RL1. In an inverter, the relay contacts could be used as ‘inverter oscillator’ on/off control. Whenever overload occurs, it inhibits inverter oscillator circuit, which, in turn, stops generation of power. In applications like inverters and UPS, the load must not exceed the rated output power since it can cause excess heating of output transformer windings and active driving devices and thereby damage them.
|Circuit diagram of Overload Protector : Click on image to enlarge|
Working of the overload protector
Resistor R1 is used as the overload sensing element. When the load exceeds the maximum rated value, it draws current in excess of its rated value. This causes the potential drop across resistor R1 to increase. An optocoupler is used to sense this voltage drop. The optocoupler, in addition, isolates the AC mains part from the rest of the circuit physically.
Resistor R1 is selected as 1 ohm for 230V, 500 watts (max.) load capacity. When the load just exceeds 500 watts, the current through R1 is approximately 2.1 amperes, producing a potential difference of 2.1 volts across R1. The inbuilt transistor inside the optocoupler senses this voltage and its collector current increases proportionally. When the current reaches the required designed value, voltage drop across resistor-preset combination R3-VR1 also increases. (Note. The power dissipated in 1-ohm resistor for 500W load is just 2.1 watts, which is negligible compared to the maximum power rating of the load. To use this circuit for 1kW load, select R1 as 0.5-ohm, 10W.)
Overload limiting point can be set by preset VR1. When the potential at wiper of preset VR1 becomes greater than VZ+VBE (where VZ is the breakdown voltage of zener diode ZD1 and VBE the for ward voltage drop at the base-emitter junction of transistor T1), it causes forward biasing of transistor T1. This results in the collector of transistor T1 to be pulled down to ground and trigger IC555, which is connected in bistable mode.
The output of IC1 causes overload indicating LED1 to glow and forward biases transistor T2 to energise relay RL1. Once the output of bistable IC1 goes high, it continues to remain high, unless reset push button S1, which is connected be tween Vcc and threshold terminal (pin 6) of timer 555, is pressed. On pressing S1, a high pulse is applied to the threshold pin that resets the flip-flop output to low state. The circuit can be reset after removing unwanted loads.
Implementation Note: Since the circuit is very sensitive, fluctuations in AC mains can also trigger the circuit undesirably. This effect can be eliminated by using 4.7µF bypass capacitor C1 as shown in the figure.