SSR before or after contactor
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I have a single contactor that the main power comes into before it goes to anything else. Guaranteed that I can kill power to everything that way. For the elements I have a traditional relay first that is controlled by a switch, then I go to the SSR, then the outlet for the element. So I know I can kill just the element if the SSR fails on.
I don't think it matters. Either way, you can turn off the element.
https://www.homebrewtalk.com/showthread.php?t=373408
https://www.homebrewtalk.com/showthread.php?t=373408
WrQth
Well-Known Member
The SSR should be after the contactor, in such a circuit the contactor passes through the power which is to be switched by the SSR and ultimately delivered to the element.
Putting the SSR before the contactor would cause the contact to be cycled quickly greatly reducing the lifespan of the contactor.
Putting the SSR before the contactor would cause the contact to be cycled quickly greatly reducing the lifespan of the contactor.
hio3791
Well-Known Member
In this particular case, I don't think it matters since current is pulled not pushed. Since there's no load, it doesn't matter where the circuit is broken.
I'm not an electrician!
I'm not an electrician!
Remember the latter before you restate the former!
Anyway, put the contactor before ("upstream") the SSR.
hio3791
Well-Known Member
Remember the latter before you restate the former!
Anyway, put the contactor before ("upstream") the SSR.
That's fair. Can you help me understand the logic? I built my controller almost 4 years ago, following PJ's diagrams and Kal's site. They put the contactor downstream from the SSR.
Personally, I like the design since the contactor is the last line of defense before the kettle. Has the consensus on this changed?
Thank you.
Bad engineering principles cannot be morphed into good engineering principles by consensus.
Well, like you said its part of a whole circuit, so if any part breaks, current won't flow. Honestly doing this way is not breaking any laws of the universe or anything. It is just that SSRs tend to fail, and fail closed. So if this happens, you have it on, applying power and generating heat when it isn't called for. Of course it wouldn't get past the contactor (aka relay) to the downstream circuitry, but its still a live part of your circuit. If the SSR is after the contactor, and it gets stuck on, it will be powering your element or pump or whatever and you will be aware as your contactor is closed.
hio3791
Well-Known Member
I didn't say that.
hio3791
Well-Known Member
Thank you for the explanation.
So we could agree that either approach is safe. Placing the SSR downstream from the contactor may increase its life. That gain, I would think, is negligible compared to the characteristics of the entire system and its effects on the life of the SSR.
Not trying to argue your point. Rather, I want to emphasize that there's no significant advantage to either approach necessitating me, or anyone who has the contactor downstream, to open their controller and rewire it.
srice
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Not sure where that came from.
If you want to muddy it up a little more, when current is actually "flowing", it is Alternating directions every 8.3 ms. So technically the contactor alternates from being upstream and downstream from the SSR.
Back to real world construction, as an electrical engineer, I would put the contactor as close to the incoming power as possible. I would want the power to the entire control panel to be cut when the contactor opens.
hio3791
Well-Known Member
It came from nowhere.
It was a deduction that if the SSR is unnecessarily put under stress by placing it before the contactor then putting it after may effectively extend its life.
hio3791
Well-Known Member
Thank you.
With the contactor off, there is no voltage (and thus no current) downstream of the contactor. At the element outlet, there will be no voltage when the contactor is off whether the contactor is upstream or downstream of the SSR. Either way you get a positive disconnect before the element outlet.
My preference is the contactor upstream of the SSR for the reason stated earlier: fewer things with voltage on them inside the control panel the further upstream the contactor. Translates to fewer places where things could go wrong under fault conditions. May not be significant in practice, but it doesn't cost anything to put the contactor upstream, so why not do it? Can't hurt, and it might actually help under some conditions.
Brew on
My preference is the contactor upstream of the SSR for the reason stated earlier: fewer things with voltage on them inside the control panel the further upstream the contactor. Translates to fewer places where things could go wrong under fault conditions. May not be significant in practice, but it doesn't cost anything to put the contactor upstream, so why not do it? Can't hurt, and it might actually help under some conditions.
Brew on
