# Minimum load voltage with SSRs

• posted

I've a question about solid state relays:

Why is there a MINIMUM output or load voltage? I mean I understand why there's a maximum load voltage, any more and you'll fry the thing. And I understand why there's a minimum INPUT voltage, any less and it won't trigger the relay.

But why is there a minimum voltage on the output? Will they not work below this minimum voltage? Because I'm looking to switch a 12V DC load, and I don't get why there's a minimum that can be switched.

But why is there a minimum voltage on the output? Will they not work below this minimum voltage? Because I'm looking to switch a 12V DC load, and I don't get why there's a minimum that can be switched.

If you look at these on Farnell's website, you'll see what I mean:

Most of them have a minimum load voltage of at least 19V.

Does what I'm asking make sense?

Thanks, James.

• posted

On a sunny day (Mon, 23 Jul 2007 06:33:06 -0700) it happened " snipped-for-privacy@gmail.com" wrote in :

It is probably needed for the trigger opto triac inside. If you just want 12V DC switched perhaps consider a MOSFET with some sort of opto if it needs to be isolated. Solid state relais (triac based) only work with AC (else they stay on...)

• posted

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Ah, I think I may have figured it out. So if I want to switch a DC load (in this case it is a small 15W heater and possibly a fan) then I need to get an SSR that is designed to switch DC loads, like the Opto22 DC60S5.

• posted

On a sunny day (Mon, 23 Jul 2007 07:32:35 -0700) it happened " snipped-for-privacy@gmail.com" wrote in :

Yes the Opto22 DC will work, but drop about 1.5V at 5A, looking at the specs.

Do you realy need opto isolation? If not, then a power MOSFET is cheaper (then 20 \$) and drops much less. Even a power transisor only drops about 200mV when on.

If you have no electronics design experience maybe too difficult. You could also consider a simple relay.

• posted

I'm using a PID controller to switch the SSR directly. Maybe I'll up the voltage on the load side to 24V DC.

No, I have little design experience. I'm a chemist by trade, trying to build an oven to warm some samples up and keep them at a fixed temperature.

This won't work because it's being driven by the PID controller. I've been told to get a solid state relay since it will be switching on and off very rapidly many times.

• posted

Some output devices, especially those intended for AC loads, have minimum operating voltages to trigger them. Select mosfet output device on that search, and the minimum voltage disappears.

• posted

Ah, that does the trick. Thanks John and Jan, I think I have a clearer idea now (even if I don't really understand the electronics, I've a better idea what I'm looking for anyway).

• posted

Ok, what is your idea of rapidly and very many times? It may not be the same as that of some of the posters here.

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JosephKK
Gegen dummheit kampfen die Gotter Selbst, vergebens.  ```

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