safety relay for ssr heated bed.
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I am running an AC bed heater with an ssr and i would like to add a relay in series with the ssr to cut power in a thermal fault or bed over temp.
i have everything all wired except the relay to the duet 2 wifi or duex board.
i am not sure how to turn on the relay when the bed is active or switch it off in a fault.
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@Jayson The best way is probably a thermal fuse or thermal switch.
The fuse would typically be wired in series to your ssr.
The switch could be wired to the coil of a "main" contactor or relay to cut power.
Either should give redundant safety in the event of an SSR failure.
More care in wiring & insulation is required for a fuse as it's carrying mains voltage on a possibly moving object (the bed) -
@OwenD I wanted to avoid a fuse because replacing it wound mean a major disassembly and hours of down time as my bed is mounted fixed to the frame.
I never thought of a bimetal switch but that might be the way to go here. thanks.
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@Jayson
Whats about a independent 2nd system? Add a second temperature sensor to the bed and add a 3rd or 4th SSR to the bed power connection. Add an arduino to cut off the added SSRs in the case that the temperature of the new sensor is to high.That would add a complete 2nd control system which is independent from the Duet hard or software.
Cheers, Chriss
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@Chriss said in safety relay for ssr heated bed.:
That would add a complete 2nd control system which is independent from the Duet hard or software.
If you can't make it 100% reliable, make it 100% redundant.
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@Chriss the only reason i want a fail safe is that an ssr will fail closed circuit. with two ssr its still possible to have two failures although highly unlikely its not 100% fail safe.
i currently have an Arduino relay thingy that I wanted to use as an on/off control, controlled by the duet but i cant just run the dc side in parallel with the dc side of the ssr because it won't function with the pwm signal. or maybe it will... the board for the relay has a bunch of other components on it. might try it anyway. if it doesn't work i think i will just put a simple reliable bimetal switch to cut the power if the bed gets higher than its rated for.hooking up a separate Arduino is a little further than i want to go.
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Here's a description about how to use the PS_ON signal to kill power to an SSR using a relay in case of a heater fault.
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@alankilian thanks, that got the ball rolling!
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@alankilian one wire and 5 minutes later, my creation won't try to kill me as I sleep.
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@alankilian do you know what comand i need to enter into my config.g?
if i send M80 the relay turns on and the bed has power.
If i send m81 the relay shuts off and cuts the power to the bed, just what i want.if i bypass the ssr then set;
"M143 H0 S40 ; set temperature limit for heater 0 to 40C" and send M80 when the bed passes 40 it sends a fault code but does not trigger m81 so the bed keeps getting hotter.
I assume i just need to add a line to the config but i cant find any information on it. -
@Jayson, I am using a similar arrangement with a relay connected in series with the SSR. The relay's coil is connected between PS_ON and +24V (remember to add a reverse diode across the relay coil with cathode to +24V and anode to PS_ON).
My config is here https://github.com/zapta/misc/blob/master/hevo/duet3/sys/config.g
I tested that the relay is indeed turned off when the temperature of the bed or nozzle are too high by forcing power but this is only when printing. If you just heat any of them manually, the Duet's protection doesn't work, at least not last time I tested it.
My bed heater is 24V so I am using a low voltage high current relay https://www.amazon.com/gp/product/B0117FB276 . In your case you will need a high voltage, lower current one, but this is the relay's contacts. The circuit for the relay's coil is the same.
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@zapta Thanks alot. that did the trick.
the relay i'm using is here amazon relay
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In RRF3 daemon.g could perhaps, in theory, test the temperature reading and trigger M81 or M999 .. or smoke signals to 999.
But it might be handy to have user varables if you wanna allow some deadband.
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@Jayson said in safety relay for ssr heated bed.:
the relay i'm using is here amazon relay
"AC250V 10A ; DC30V 10A". A trivia question for everybody, how come the relay can be switch 250V AC but only 30V DC?
Somebody once explained it to me and thought the reason is fascinating.
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How are you going to protect the protection relay from some crud preventing its armature from moving, or from the contacts welding shut?
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@mrehorstdmd Those are 1:100000 occurrences for properly-sized relays.
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@zapta I tried switching 120 Volts DC in my electric scooter decades ago using a household light switch with exciting results.
Basically, because AC current "goes to zero" 120 times per second, any spark generated as the contacts open also "goes out" and the circuit is broken.
Since DC current doesn't do that, any spark created can be maintained all the way through full opening of the contacts and you've got a problem.
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@alankilian, yes, that's the explanation I heard. I found it to be counter intuitive because 120VAC actually gets to about 170V at it peaks but still it's easier to break than low level DC.
I don't envy the engineers that need to design switches for this https://en.wikipedia.org/wiki/High-voltage_direct_current
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@mrehorstdmd said in safety relay for ssr heated bed.:
How are you going to protect the protection relay from some crud preventing its armature from moving, or from the contacts welding shut?
My relay is noisy so I hear it clicking on each time I turn the printer on. Kind of a self test.
"perfect is the enemy of good" .
It not a perfect solution but it reduces the risk of fire due to a short in the duet's mosfet or the SSR.