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Home Brew Forums > Home Brewing Beer > Electric Brewing > SSR Fail Safe
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Old 09-14-2013, 10:44 PM   #1
ekjohns
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Default SSR Fail Safe

Now that I have built my eBIAB pot with PID, I want to get more use out of it including Sous Vide. My concern is leaving the set up running while I am at work. If my SSR fails in the closed position then the the water will quickly begin to boil, and if I am at work this could potentially lead to the pot boiling dry and becoming a fire concern with no one home. Is there a way to wire in a fail safe to kill all power if this happens? I have read someone using the PID high temperature alarm to kill the power to the element. What I think could be done is wiring a normally closed contactor to the high temp alarm which when it goes off it opens the contactor and disconnects the heating element. Would this work, or is there a better option? I am looking to make this the safest I can.

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Old 09-15-2013, 02:41 AM   #2
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Hmm. What if...two SSRs were set up in serial, and so if one fails in the closed position, the other one would still be working and could prevent the circuit from being completed? Kinda like the nuclear key theory, there. Both guys have to turn their keys in order to launch the missile (complete the circuit).

I'm no expert. Just throwing stuff out there.

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Old 09-15-2013, 02:45 AM   #3
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Old 09-15-2013, 04:35 AM   #4
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Your original proposition with a NC contactor could work.

A simpler way is if the PID controller's alarm relays are NO, you can wire the element or main power contactor coil power through the alarm relay and set the process low alarm to be your max temperature. Once the temperature is higher the alarm relay will cut off, thus turning off the contactor.

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Old 09-15-2013, 05:55 AM   #5
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Which PID are you using?

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Old 09-15-2013, 07:06 AM   #6
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Electric kettles contain a temperature switch that turns off the power when the water boils. That sounds like what you need.

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Old 09-15-2013, 05:13 PM   #7
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I'm using the aberins 2352 pid

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Old 09-15-2013, 07:32 PM   #8
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Reading the Aberins Instruction Manual tells me that you have Normally Open alarm outputs.

To keep things simple for ME, I would use a small 'Ice Cube' relay on the PID alarm terminals to fire the heavy contactor feeding the SSR.

Use the Normally Closed set of contacts on the Ice Cube relay.

So . . . . . . .
When a high alarm condition occurs, power is applied to the Ice Cube relay, the N.C contacts open up and the SSR contactor looses power.

There is a condition we sometime see that we call 'cycling on the alarm'.

The SSR fails closed/on and the controller alarm keeps cycling off as the temp drops and back on when overtemp again.
There is 'latching circuit' wiring that you can incorporate with this relay that will require you to intervene before it releases and lets power flow to the SSR contactor again.
I can help you with that too if you think that is something that interests you.

This is an Ice Cube relay:

It can expand your Single Pole N.O. alarm contact by 8. 4-N.O., 4-N.C.



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Old 09-16-2013, 03:18 PM   #9
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Kid that seems like a great solution. I have heard that relays do not like to be cycled on and off but what would the duty cycle of one of those ice cube relays be? The latching circuit seems like a cleaver idea, but if the relay could withstand the on/off cycling for a few hours if there was a failure before I could shut things down and replace the SSR would be nice to keep things simple.

I have been looking for a relay that would be easy to mount in my box. Would something like this work? Flanged Relay

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Old 09-16-2013, 03:59 PM   #10
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The ice cube relays are generally good for 1,000,000 cycles or more.
You can cycle them pretty fast, but of course you'll be eating away at the 1 million cycle life. About the fastest we cycle them for is warning lights on a machine. Maybe a 1sec cycle. They rate them in milliseconds, but no need to test them.

The latch circuit is something we use to indicate we have a problem with overtemp.

ek, the relay you posted will work. There are not enough spare contacts to make a latch circuit though. You do see that it has a 12VDC coil! Are you using 12V control voltage? The PID has no voltage output on the alarm, just a set of 'dry' contacts. You must supply the voltage (pin 13). I'd use the same voltage as the PID since you already have it wired there, the hot on pin 9 or 10.

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