Usage of Varistor/Transient Voltage Suppressor ??

Posted on 2009-02-19
Last Modified: 2013-11-10
I have a 220V to 12V 500mA transformer+rectifier plugged into my wall socket, that drives a pair of relays. I've put protection diodes on the relays. However I still have a 500v kickback into the mains when I turn off the power.

How am I supposed to fix this? Using a varistor/TVS or something? Do I connect it across the mains before the transformer? Can someone suggest a specific part that I should use?
Question by:kurian2z5
    LVL 63

    Accepted Solution

    any TVS rated for the volatage ( 220) and the amperage  should work.

    I would check that the diodes have not burned out as well as have some indicator LED as to the status of the TVS since they also can burn out with too many transients.

    Lastly look into a better relay, or perhaps 2 sets a small one that activates a larger one or an opto-coupler.

    I hope this helps !
    LVL 4

    Author Comment

    I put a 275v MKP capacitor before the transformer, as I assume the kickback will be DC. However it doesn't seem to do anything. It doesn't seem to absorb any current at all.
    LVL 63

    Expert Comment

    You really need to use one of my suggestions.  simple capacitor is not sufficient.
    LVL 4

    Author Comment

    I've put a bidirectional TVS and it works now.

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