Help with Solid state relay

Discussion in 'The Projects Forum' started by dionysian, Jan 27, 2014.

  1. dionysian

    Thread Starter New Member

    Jun 26, 2007
    I am trying to control a Opto 22 240D45 DC Control solid state replay with a micro controller. However before I hooked it up to the micro controller I though i would simply apply a DC voltage to the control input. I was expecting that in doing this the output (pins 1,2) would be short together. However when ever I do this no matter what DC voltage I apply the output pings never short together.

    I am misunderstanding what this relay is suppose to do?

    Below is the schemtic of the interal of the relay:

    See attachment

    It is my udnerstanding that just as long as I hook up a 3V or high voltage to out 3,4 then pin 1,2 should short???? is this incorrect.

    In the data sheet it says the Signal Pick-up voltage" is: 3VDC(32V allowed).
    when I adjusted the controll voltage V34 the resistance between 1,2 does change but not much. I was expecting to go short.
    What I am I doing wrong? What I am I not understanding?
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    The output will only "short" if connected to the AC line in series with a load. You cannot see the "short" with an Ohmmeter, because it doesn't put enough voltage across the output pins.
  3. crutschow


    Mar 14, 2008
    A DC type SSR with a MOSFET output will show a short across the output with an ohmmeter when turned on but not an AC type, with an SCR output.

    If you increase the voltage across the AC output to > 2V (with a resistor in series) the output will conduct with somewhat over a 1V drop. Note that the SCR will then continue to conduct, even when the input voltage is removed, until the output current is reduced to zero.
  4. AfdhalAtiffTan

    Active Member

    Nov 20, 2010
    If the SSR contains SCR, with any voltage lower than 1V, it wont able to trigger it on.
  5. MaxHeadRoom


    Jul 18, 2013
    You might see a change if you have a Diode check range on your meter, as this range provides forward biasing voltage in order to see the conduction of the diode/SCR.
  6. dionysian

    Thread Starter New Member

    Jun 26, 2007
    Thanks guys this makes sense. Before I was able to see the short when using a DC relay, but this makes sense with an AC output relay.

    I appreciate the response from all of you.
  7. dionysian

    Thread Starter New Member

    Jun 26, 2007

    I put my meter across the output and used the “diode check function" (thing that looked like a diode on my meter) and I got 1.8... I noticed on the datasheet that there is something called "Maximum output voltage drop" that is 1.6V. Is this what I am measuring? I think so.

    this is the forward voltage drop right?
  8. MaxHeadRoom


    Jul 18, 2013
    Yes that would be F.D., if it worked, it would be the same in either direction due to the back-to-back SCR's.
    In reconsideration, I am surprised it did work due to the resulting low SCR gate current?


    Jan 20, 2014
    At theese extremely low current values Your multimeter is doing it's meassuring, You will not get any reliable reading of the devices actual forward voltage drop.

    Check in the datasheet, and You will see there what voltagedrop at wich current value You can except.