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Minimum load voltage with SSRs

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:

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

Does what I'm asking make sense?

Thanks, James.

Reply to
james.padfield
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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...)

Reply to
Jan Panteltje

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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.

Reply to
james.padfield

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.

Reply to
Jan Panteltje

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.

Reply to
james.padfield

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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.

Reply to
John Popelish

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).

Reply to
james.padfield

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.

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller
Reply to
joseph2k

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