solid state relay parallel operation


Thread Starter

cem ayday

is it possible to wire in parallel two or more SSRs to have more current for ac switching application?

Mike Johnson

No. Whichever solid state relay latches first will always end up carrying the full AC load.

Therefore, you must buy a single SSR that is the right ampacity for the application.

Power-io Solid State Relays
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Michael Griffin

If you parallel any two arbitrary SSRs, they will not necessarily share the load equally (or proportionally to their individual ratings). There is no inherent means of preventing one SSR from taking more of the load than it is rated for.

On July 30, 2004 14:27, cem ayday wrote: <clip>
> is it possible to wire in parallel two or more SSRs to have more current
> for ac switching application?


Michael Griffin
London, Ont. Canada
Not practical - you cannot assure even current distribution among SSRs - unless you insert a fixed resistor in series with each - that causes energy loss.


mathhew hyatt

Simple answer - NO!

Get a single SSR rated for our application. If the current is too high for using a SSR, use a contactor.

There is no way to insure they will share the load equally.

Of course, it might be fun to see which one fails first if you use three of four SSR and try to switch 150 amps using zero crossing 50 amp SSRs.
I had tried to use 2 40ADC SSR (Cryden) to control 80ADC. But I don't know it can be used in AC load.
First use phase controllable SSRs, not zero crossing turn on SSRs. Then try to ensure SSRs are matched, i.e. same batch of silicon, at a minimum, check that the on-state voltage drop at rated current is the same.

Could you not then place reactors between each SSR output terminal and the AC load to share the current? eg. if you want to power a 120A_rms load with three 50A SSRs in parallel, you place one reactor on each of the SSR output terminals, with all three reactors connected together at the load terminal. There will of course be some power loss depending on the inductance of the reactors and frequency of the AC load...

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Curt Wuollet

Or find VMOS output devices which being resistive, inherently share current better than regeneratively latched devices.