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solid state relay behavior

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wdonovan

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Hi all. This is my first post here. In my position I have to design and build most (all) of my test equipment but my electronic knowledge is poor except for simple circuitry, some data acquisition, etc. I am attempting to build a PID temperature controller that is software based (Labview) and operates the oven through a basic load switch (circuit diag attached). I'm using a standard OAC5 s.s. output module to turn a logic signal into a usable power switch, driving a higher load s.s. power relay. I have chosen an AC output module & AC coil power relay because of no need for a power supply. When I energize this circuit I have power coming from the output receptacle, regardless of input or no input to the OAC5. Is there a peculiarity of these relays (in particular the OAC5) that it does not operate without a resistive load across the output? I do not have access to the circuit right now but I have breadboarded just the OAC5 to light an incandescent lamp and it works OK. It just will not go to low output when wired to the coil of the power relay exclusively. Am I correctly diagnosing the problem? If so could I switch the high side of the indicator lamp (incandescent) from terminal 2 to terminal 3 of the power relay and supply enough resistive load to force the output module to behave?
 

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tcmtech

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Who makes the OAC5. That part number does not mean anything to me.
you may want to check out the other thread on SSR's someone else has had a similar question as you do.
 

wdonovan

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Omega, Grayhill, Omron.... It seems to be a generic number. I searched threads and found nothing that relates. I looked for mfr data sheets but they're really lax about giving useful data for this part.
 

tcmtech

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I have often found that a generic number is used to hide crappy parts or fakes! It most likely shorted out and is now just permanetly on.
Check ou t digikey.com and look in their SSR devices section . They give the actual manufatures specs sheets too. That should help you get better info on SSR's and there intended applictions.
 

MikeMl

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...If there's something in that data sheet that mentions this quirk I sure can't find it.
Look at the "Load Current Range Spec". It says 0.03 to 3.5A. This implies that a minimum load would be about 120/0.03= 4000Ω. Try connecting a 4K 5W resistor as a test load, and I'll bet that the SSR will turn off properly.

Be advised that your SSR is a zero-voltage switch, and once triggered, it delivers a ½ cycle of the the 60Hz power before turning off. This makes it not too useful if you truly want proportional PID type of control.
 

wdonovan

New Member
Thanks Mike. So if I move the output lamp like I proposed, that should cure it (providing it's at least 4kOhms?

Regarding the 1/2 cycle you mention, you mean 1/2 of a 60Hz cycle, i.e. 1/120 sec? This control will cycle minimum 1 cycle / sec to 1 cycle / 10 seconds so 1/2 cycle is no problem. (Unless you mean something else)?
 

MikeMl

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Yes, think of the SSR of turning on for integer multiples of 8.33ms. As long as your control loop can tolerate this quantization, then it should work.
 
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