SSR caused thermal runaway?

Hey everyone, it’s been a while since I’ve worked on my printer, I finally found the time to pick up a new power supply last night to get the heated bed I installed to work.
After wiring everything up the heated bed would engage, then shut off stating temps were rising too slow, my next step was PID tuning the bed using M303, after an hour and a bit it finally completed, stated that the bed was inconsistent I believe the message said, and gave me a line of code to replace M703 with.
I replaced the line with the perimeters given, and went to test the bed, the bed heat up, and was initially cycling on and off as it should, but then I noticed the temps kept rising even though the Solid state relay was off, I let it climb 10° above my set limit to see if maybe the gantry heat was adding to it but it kept going up, so I pulled the plug. Hitting the E stop didn’t do anything it kept rising.

Did I experience and catch a mechanical thermal runaway? How can I make this machine more safe in the event it happens again?

I have to do some further investigation tonight after work but I figured if I got some community feedback before then I might be able to get parts and whatnot before the store closes.

Thanks everyone!!

3D Systems CubePro Duo retrofit
FSYTEC Duet clone running 3.2 (I beleive)
Creality 300x300 heated bed powered by SSR to PS
New PS Creality 350w

Hi Jake

I would agree with your conclusion, It sounds like the SSR stayed closed. I would double-check the wiring to ensure it’s ok and nothing is shorting out, Generally, on the trigger side it’s a ground loop circuit, Meaning 24 Volt is constant and it’s only the gnd that completes to trigger the SSR, Ensure you have a good solid ground and it’s not accidentally grounding out on something else.

I just reread that again, Are you using an SSR on a DC bed? I would recommend if that’s the case to switch over to a Mosfet, Generally, SSR is for AC/DC controlling AC circuits, and Mosfets are for DC controlling DC circuits.

Interesting, maybe I read something wrong back in May when I wired the bed up with the SSR, it was saying to use one to power the bed from the PS instead of the duet board.

I guess I’ll have to stop by after work and pick up a mosfet!

Thanks Jason

Before you buy anything, could you send a picture of your printer and wiring?

An SSR, with a DC load will stay closed as per: Solid-state Relay: DC Load and AC Load | FAQ | Australia | Omron IA

Which may be what you’re seeing.

But, before you start swapping components could I suggest that you explain with pictures how things are wired and exactly what parts you’re using?


Hi Myke!

Just following up on the topic, it was 100% a failure in the SSR, as soon as I got home and plugged the printer back in the temps started rising without any command. I removed the SSR and replaced it with the DC MOSFET that Jason recommended and the bed heater is working properly and stable. I’ll attach a photo and a schematic as to how I had the SSR wired for future reference.

Hi Jake,

What is the manufacturer/part number of the SSR you are using and did it work before you replaced the power supply? I’m asking because the internal schematic of the SSR you give is unlike any I’ve ever seen.

SSRs are actually pretty complex devices and you need to have a good understanding of the application and the requirements before selecting one to use. Here’s a reasonably comprehensive introduction to SSRs and the different applications they are best suited for:

A “true” SSR is really an electronic device which uses a TRIAC as the AC switch. Once it is turned on for current in one direction, it will stay that way until the current changes direction (ie in alternating current).

The problem with using an AC SSR (ie one with a TRIAC as the switching element) with DC is that once the TRIAC starts conducting electricity in one direction, it cannot be turned off until the load current changes direction which, if DC is the load current, is literally never.

This matches the behaviour you were seeing when you originally posted.

What is referred to as a “DC/DC SSR” is really a MOSFET switch, which you seem to have replaced the device you were using for.

I’m confused as to what happened based on your original post. I’m not sure what you mean by the heated bed you “installed” - did you just add it and did you put in the SSR at the same time? Did the heated bed ever work (from your post, I’m guessing it’s new and never worked).

Anyway, I suspect that your SSR is fine - it’s just that you were using it inappropriately.

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