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
What can I do to reduce the electricity bill?
Use LED bulbs for all the lamps and reduce the fridge setting to minimum, don’t use geyser too often.
Dear sir,
Please sir, help me with an Ac circuit that can protect 220V Ac led bulbs, flat screen Tv etc from over voltage/current condition from the Mains supply.
Godfrey, for this you will have to install a voltage stabilizer unit which can regulate the voltage for the loads. Or you can also build an over voltage cut off circuit as explained in the following article:
https://www.homemade-circuits.com/highly-accurate-mains-high-and-low/
Okay sir
Thank you very much
Operating supply is 12v think, coz not define in schematic.
yes can be 12V or 6V depending on the relay coil voltage
Hello sir , have a nice day ,
Sir I need a circuit diagram for Overload , No load protection for 1 hp submercible Motor . used by current transformer .
thanking you .
with regards ,
Mohamed farook .
Hello Mohamed, if possible I will to design it and post it here:
R1 dissipates a lot of power. Could you suggest a way to reduce this?
Try the following modified diagram using a standard opto”
Calculate R1 in the following manner:
R1 (Ohms) = 1 / Max current limit (Amps)
Wattage = 1 x Max current limit
Thank you!
Dear,
I plan to connect it to 5kva stabilizer, and cutoff watts will 4700watts @240v,
please advise the value and watts for R1 resistor, please tell me what is the alternative option if specified R1 resistor is not available…
You can create R1 by adding many high watt resistors in series/parallel combination, until you reach the correct desired value….
Sir ,
please advise the value and watts for R1 resistor,i am planning to build 5kva stabilizer, and cutoff watts will 4700watts @240v
Arun, please see the formula and the example solution at the end of the post. In your case “I” will be 5000/240 = 21 amps, now please calculate the rest.
sir is can you explain what led/LDR means because i dont have enough knowledge about electronic parts or is it found in chargers.
can i get the part number for the LDR thanks
To make this, you have to pack an LED and LDR face to face inside a light proof box….or you can simply use a ready made opto coupler.
LDRs do not have any number.
Hello sir, is it possible to modify this circuit by adding a delay timer for automatic restarting? If yes, pls throw more light to that. Thanks.
Hello Solomon,
it is possible, you can integrate the 3rd circuit from the following link with the collector of the above circuit, and get the required results
https://www.homemade-circuits.com/simple-delay-timer-circuits-explained/
I hope you will know how to do it
Hello sir, can i use TRIAC instead of relay, if yes how can i connect the pin
yes it is possible, I will to update it soon…
Hi Swag,
Will this also help in short circuit?
also I have both MCT2E (6pins) and PC817 (4 pins). which one should I use for better detection?
I know PC817 is most used in many power supply circuits and MCT2E is absolute i think.
Hi Saqib, yes it will also safeguard against a short circuit.
any opto coupler can be used here, although in the diagram an LED/LDR hand built opto is shown, an LED/transistor opto will also work.
be sure to check and confirm the stages separately while setting up the design.
Thanks. Will update you with experiments.
OK thanks!