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Thread: Solid state relay stays on

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    Default Solid state relay stays on



    I have a SS relay purchased from Jaycar which is controlled by 12V DC switching 240V AC.
    On the output side it has a small 5mA forward current as most SS relays do, but the trouble is that this is enough to keep the load on even when I turn the power to the relay off.

    I tried attaching bypass resistors of various sizes (100k, 47k, even 10k) to the mains terminals but that did not help.

    I found that the relay turns off reliably if I attach a lamp in parallel with the load.
    Although this works I don't like this solution because the lamp draws too much power and also because the lamp is likely to burn out eventually.

    I suppose I could go with some low wattage lamp but then I would need something like an opto-coupler or another relay to provide a feedback.
    Otherwise if the lamp fails I will not know that the load is still on when I turn the relay off.

    Does anyone know of a better solution?

    Thank you very much.



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    Can you tell us more about the load - what exactly is it, and what are you trying to achieve by using a SS relay as opposed to a mechanical one?
    [They are good, but they have their inherent limitations, such as the one you are faced with].

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    Well, the load is a small charger, so I guess the small current is enough to keep it on in some circumstances.

    Why did I use the SS relay? Because I had one spare basically
    I also thought that it would be much safer if I wanted to control it using an AVR or USB in the future.

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    Ok, one thing you could try is a snubber network across the output terminals.
    This helps the triac switch off at zero-crossing by limiting the dv/dt voltage rise.

    A 220 ohm 1 watt resistor in series with a 0.1uF cap rated to 400 volts AC should do the trick.

    The values of the components depend a lot on the characteristics of the triac, but those should be a good place to start.

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    Thank you for the suggestions.

    I have tried adding the cap and a few different values of resistors but the darned thing still won't switch off.
    Reading the spec for the relay it mentions a built in internal snubber so maybe that is why it made no difference?

    I guess it is back to a mechanical relay. Oh well...

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    Quote Originally Posted by doktor View Post
    Thank you for the suggestions.

    I have tried adding the cap and a few different values of resistors but the darned thing still won't switch off.
    Reading the spec for the relay it mentions a built in internal snubber so maybe that is why it made no difference?

    I guess it is back to a mechanical relay. Oh well...
    Yeah, well if it has a built-in snubber, adding extra Cs and Rs externally won't make a lot of difference.
    Maybe a re-hash is possible? You mention that the relay controls a charger. What voltage is the charger putting out? Low, I would presume.

    If that is the case, can you axe off the transfomer (or switch mode unit) and simply get your low voltage switching signal to switch a low voltage charging system?

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