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Dgonza9 10-31-2010 01:43 PM

Pid Wiring Question...
I have read a number of threads on PID wiring but I remain confused on one thing, where should I put a switch? Basically, I want to be able to confirm that the element is not firing when I don't have the pump running, but still leave the PID on to monitor temps when I want to.

So does a switch go between the SSR and the element or between the PID and the SSR to achieve this purpose?


snail 10-31-2010 02:50 PM

I would put the switch between the PID and SSR so you aren't switching the SSR for nothing. The PID would send a signal to the SSR but it wouldn't get to it until you flicked the switch on.

Dgonza9 10-31-2010 03:08 PM

Yeah, that makes sense. Thanks for the input.

jkarp 10-31-2010 03:25 PM

And for an opposing opinion...

Physical switch on the element, not the SSR. When an SSR fails, it normally fails closed, meaning it will pass current regardless of PID input. Physical switch should always trump any electronics.

SweetSounds 10-31-2010 03:37 PM


The switch on the PID side of the SSR is the easiest way to control when the element can fire. But SSRs can fail closed, so I prefer to also have a contactor between the SSR and the element. Just for safety.

snail 10-31-2010 04:29 PM


I'm confused how if the SSR fails closed it would still power the element if there was a switch between the PID and the SSR. Doesn't the SSR need the PID to tell it when to power the element? I'm confused

SweetSounds 10-31-2010 04:36 PM

The SSR is literally a solid state switch. The "Lever" is the low voltage signal from the PID that tells the SSR to "Close" - Conducting the current to the element.

But SSRs can fail. And when they do, they can fail in the "On" position, if you will.

It may never matter. But I don't like even the possibility that there is current going to something I think is off, when I stick my hand in there...

Dgonza9 10-31-2010 04:38 PM

Does the SSR receive power thru the PID as a pass through or does it merely receive a signal to switch power on?

I believe it's the latter. This would mean that the SSR can turn on your element without the PID, correct?

jkarp 10-31-2010 11:43 PM


Originally Posted by Dgonza9 (Post 2374184)
Does the SSR receive power thru the PID as a pass through or does it merely receive a signal to switch power on?

Both. There are 4 screws. 2 provide the low-voltage signal and 2 are the switched high voltage "hot" leg. More often than not, when an SSR fails, it will continue to pass the high voltage regardless of input from the PID. This is why you want a mechanical switch after the SSR and before the element.

stevehaun 11-01-2010 12:00 AM

You really could put the contactor before or after the SSR. What matters is that you have a mechanical off to the element.

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