CAUTION: THE CIRCUIT PRESENTED HERE IS AT AC MAINS POTENTIAL, THEREFORE EXTEMELY DANGEROUS TO TOUCH IN SWITCHED ON POSITION. UTMOST CARE AND CAUTION IS ADVISED, USE OF A WOODEN PLANCK UNDER YOUR FEET IS RECOMMENDED. NEWBIES PLEASE KEEP AWAY.
We know that metal iron’s property of conducting electricity is not good compared to some other electrical conductors like aluminum and copper.
Now if we go according to Ohm’s law we find that resistance of a conductor and the current passing through it is directly proportional to the applied voltage, implies that as voltage increases current also increases and in case of iron as the conductor, an increase in current through it would force it to offer a proportionate amount of a rising resistance against it. This opposing resistance of iron would help to blunt-off the dangerous spiking of sudden voltage fluctuations.
This particular property has been primarily exploited here. Let’s understand the circuit functioning in detail.
Referring to the diagram alongside, C1, R1, D1, D2 and D3 together form a solid state transformerless power supply. D1 and D2 effectively remove the input voltage transients thus creating a safe voltage for the preceding electronic components. C2 does the rest by filtering out any of remaining residual AC interferences.
The above voltage is fed to a circuit mainly involving transistors T1, T2, and the Triac TR1 as the active components.
Preset P1 is adjusted such that T1 just starts conducting at a voltage (DC) threshold which may be equivalent to the applied mains AC. For example, suppose at normal voltage the DC input to the T1 is about 9 volts, an increase by 25% of the AC mains would make the DC potential to increase proportionately to about 11.25. So here P1 could be set so that T1 just conducts at this threshold.
Normally as long as T1 is switched OFF, T2 remains switched ON and supplies the required gate voltage to the triac TR1. During this time the mains voltage to the appliances is supplied through TR1 and it receives the full normal voltage input without any restriction, R5 being kept inactive.
If accidentally the input shoots beyond the set threshold, as explained above, T1 conducts, T2 is switched OFF and so is the triac, cutting off the normal unrestricted AC supply to the load or the appliances. However an interesting thing happens at this point – the load now starts receiving the AC through R5 which is a low value resistor made up of iron coil.
The introduction of R5 instantly takes the sting out of the dangerously rising voltage, making sure that no damage is caused to the appliances. Also the transition is smooth, safe and without breaks.
According to the maximum load of a house R5 needs to be dimensioned appropriately. It may be simply done using Ohm’s law.
This simple and low cost design of a mains surge protector circuit is very effective, safe, easy to build and thus should be incorporated in every house.