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.

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.

You need to wire in a thermal overload switch that will disconnect power regardless of the failure.

Something like this but rated at 20 Amps, this one's only 15 Amps.
http://www.newark.com/jsp/search/productdetail.jsp?SKU=68C2874

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.

Which PID are you using?

'da Kid

Electric kettles contain a temperature switch that turns off the power when the water boils. That sounds like what you need.

I'm using the aberins 2352 pid

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.



'da Kid

Problem with using the alarm output is the relay will open when the alarm trips then it will close and the power will be turned back on when the pot cools. So, until you get home and manually turn the system off your pot will stay at the higher alarm temperature. The contents left over in the pot after the water boiled away will cook off into smoke that will contaminate the entire room or house.

You need to wire in a thermal overload switch that will disconnect power regardless of the failure.

Something like this but rated at 20 Amps, this one's only 15 Amps.
http://www.newark.com/jsp/search/productdetail.jsp?SKU=68C2874

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

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.

'da Kid

So im looking for a 120V coil? That should open up the list of available relays. If I did not make a latch circuit and just stuck with a simple relay would this work? My set up is for 120V (not 240). Sorry I am really bad at finding electrical components

http://www.auberins.com/index.php?main_page=product_info&products_id=250

Thanks for your continued help!

Yep, that one is a winner.

Are you getting the heavy (30A) SSR contactor from them also? They have some good ones listed.

The 30A contactors are the ones that don't like to be cycled too fast.
You will be fine. There should be a long enough delay while the pot cools down, the alarm resets and uncontrolled heating resumes. 5 mins?


You're Welcome,
'da Kid

This is the diagram for my set up
http://www.pjmuth.org/beerstuff/images/Auberin-wiring1-a4-2000w-BIAB-120V-D-s.jpg

Because this has 1NO / 1NC, would it be possible to connect the NO to the alarm and the NC to the contactor? This way when the temp alarm on the PID goes off it turns on the buzzer and turns off the contactor? Assuming this is correct I just need to figure out how to wire the damn thing. I am guessing the following:

Coil - power from PID (#13 in diagram) and neutral.
NO - power to the buzzer
NC - Power to the contactor
COM - Power from main?

ekjohns said:

This is the diagram for my set up
http://www.pjmuth.org/beerstuff/images/Auberin-wiring1-a4-2000w-BIAB-120V-D-s.jpg

Because this has 1NO / 1NC, would it be possible to connect the NO to the alarm and the NC to the contactor? This way when the temp alarm on the PID goes off it turns on the buzzer and turns off the contactor? Assuming this is correct I just need to figure out how to wire the damn thing. I am guessing the following:

Coil - power from PID (#13 in diagram) and neutral.
NO - power to the buzzer
NC - Power to the contactor
COM - Power from main?

Click to expand...

An alternative fail safe design would be to use a separate system that does not depend on the PID. Perhaps, a separate temperature controller that can switch a NO, DPST contactor. Set your high setpoint to, say, 10deg over your PID's setpoint, and if it reaches that it cuts power to the contactor, cutting power to the entire system. This would effectively protect against failures in both the PID and the SSR. It would also be useful to think about a method so you can know the fail safe activated. Just thinking out loud....

ekjohns said:

This is the diagram for my set up
http://www.pjmuth.org/beerstuff/images/Auberin-wiring1-a4-2000w-BIAB-120V-D-s.jpg

Because this has 1NO / 1NC, would it be possible to connect the NO to the alarm and the NC to the contactor? This way when the temp alarm on the PID goes off it turns on the buzzer and turns off the contactor? Assuming this is correct I just need to figure out how to wire the damn thing. I am guessing the following:

Coil - power from PID (#13 in diagram) and neutral.
NO - power to the buzzer
NC - Power to the contactor
COM - Power from main?

Click to expand...

Hey ek, sorry for the delay.
I'm new to P-J's diagrams, so bare with me. I'm having a hard time reading it because it got compressed at some point and I can't zoom in at all.

Let's start with, how exact are you going to wire to the attached diagram?

P-J is already using a contactor, we just have to wire it differently to suit your condition.

I'll try to find a large file of the diagram on HBT somewhere.

Thanks,
"da Kid

No problem I greatly appreciate the help. Here is the original image

http://www.pjmuth.org/beerstuff/images/Auberin-wiring1-a4-2000w-BIAB-120V-D.jpg

What I'm looking to do is use a NC relay that the alarm would trigger the relay to open thus de energizing the contactor and killing power to the element.

jeffmeh said:

An alternative fail safe design would be to use a separate system that does not depend on the PID. Perhaps, a separate temperature controller that can switch a NO, DPST contactor. Set your high setpoint to, say, 10deg over your PID's setpoint, and if it reaches that it cuts power to the contactor, cutting power to the entire system. This would effectively protect against failures in both the PID and the SSR. It would also be useful to think about a method so you can know the fail safe activated. Just thinking out loud....

Click to expand...

I was going to suggest something similar. A separate, stand-alone system that monitors temperature (with it's own probe) and has control of a mechanical relay providing power to the element. Search for an "over temperature relay" to give you an idea. It doesn't need to be as expensive as an industrial system, but you can homebuild something if you're up to it.

OK ek,
Let's start at Switch #4:
You will be connecting the coil side of the mini power relay in parallel with the buzzer. Terminal #21 and neutral.

Next is switch #1:
The wire on term. #23 will be moved from LINE 1 to the N.C. terminal on the mini power relay. The com. terminal will now go to LINE 1.

The the PID alarms, the buzzer comes on and the mini power also pulls in. The N.C. contacts will open breaking the power to the 30A contactor.

Don't hesitate to ask any questions, Happy to help,
'da Kid

I think I follow it. So nothing is connect to the NO? Also when the buzzer goes off it breaks the NC breaking power to the switch thus stopping power to the contactor. Okay got it!

Yep, you got it.

I've been reading the PID manual to get a grasp of the parameter settings to utilize the alarm functions.

Been so used to the other brands, I'm in Auberin 101 class

'da Kid