ADSL Surge Protection

Will it do what i want it to do? (if varistor's and resistor's values are more carefully selected)

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Thread Starter


Joined Dec 7, 2017
I have had a lot of lightning strikes ruin my modem throughout 2017 (adsl line only, as 230VAC in my house already has surge protection that works) and i have decided to make my own surge protection as all the ones i have owned so far did nothing but transferring the overload to the power grid. I have been looking into varistors and i beleve i see a pattern.
1: They die all the time.
2: They are normally used to short circuit the com-lines to allow the potential to discharge which on its own will probably kill the modem and potentially the com-line itself as some points may be very weak due to aging.

I cooked together a circuit based on what i believe i might know so far and i would love some input on the design (take note the symbols may not be correct and are labeled instead):

How the circuit works:Surge protector.PNG
Theoretically, a varistor should have very high resistance when below a certain value. If a very high voltage suddenly is applied, this will make varistor 1 and 2 basically 0ohm's. Varistor1 will short circuit the com - lines in order to limit the strain on the modem because of the slow reaction time of a physical relay. The high voltage on the adsl/com-line and low resistance in varistor2 should provide enough power to activate relay K2. This will in return activate K1, which keeps itself in position until the reset button is pressed. K1 will also disconnect the modem and all circuitry related to varistors to keep them as not-fried as possible. K1 also activates a switch that allows the remaining potential to be discharged through a 100k Resistor. The siren will make noise when the system successfully activates.


Joined Aug 27, 2009
My 2c. Your circuit might alert to the fact the modem is dead again but is unlikely to stop damage.

You need a shunt circuit to provide a low impedance path for the lightning energy before your electronics if at device protection is not working. Energy is inside the building hunting for earth ground destructively via your electronics so provide an alternative path outside of the building if possible with a good surge protector with a good single point earth ground just for surge protection shunting.