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Minimum load voltage with SSRs

Discussion in 'Electronic Design' started by [email protected], Jul 23, 2007.

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  1. Guest

    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:

    http://uk.farnell.com/jsp/search/br...13879+113881+113863&Ns=PRICE_PLS_006_PRICE1|0

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

    Does what I'm asking make sense?

    Thanks, James.
     
  2. 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...)
     
  3. Guest

    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.
     
  4. 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.
     
  5. Guest

    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.
     
  6. 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.
     
  7. Guest

    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).
     
  8. joseph2k

    joseph2k Guest

    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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