230v heatbed thermal runaway

achrn

@poul-erik said in 230v heatbed thermal runaway:

The wiring is pretty standard and worked until yesterday.

SSRs do fail, and permanently-on is the most common failure mode, which would match your symptoms (bed starts heating as soon as mains power is applied, no control over it).

Gixxerfast

Is the bed marked as active in the DWC?

Also have you measured the input of the SSR, does the LED on the SSR (if there's one) light up ?

zapta

@poul-erik , if you disconnect the driving wires between the Duet and the SSR, does the bed heats when you turn the printer on? If so, you may have shorted SSR.

Also, does your SSR have a led, indicating the control signal? I so, it will help you identify if the problem is on the duet/firmware side or the SSR.

And whatever you do, think safety first.

Poul Erik

@achrn
Thank you, much appreciated!

Poul Erik

@gixxerfast said in 230v heatbed thermal runaway:

the input of the SSR, does the LED on the SSR (if there's one) light up ?

HI @gixxerfast The heatbed will turn on even if the bed is not marked as active.
There is no led on the SSR.

Thank you.
Poul Erik

Poul Erik

@zapta said in 230v heatbed thermal runaway:

Duet and the SSR, does the bed heats when you turn the printer on? If so, you may have shorted SSR.
Also, does your SSR have a led, indicating the control signal? I so, it will help you identify if the problem is on the duet/firmware side or the SSR.

Thank you @zapta
What might cause the SSR to short?

//pe

achrn

@poul-erik said in 230v heatbed thermal runaway:

What might cause the SSR to short?

SSRs are not 'bulletproof' and are more finicky than many people believe.

The main problem with SSRs is that most of the ones on ebay, amazon, etc. are fakes and only good for a fraction of their stated rating. Even if you have a genuine one, the rating is that for an ideal situation - no heating, no inrush current, no capacitive load etc. Admittedly, a heater element is much closer to that than many loads.

Personally, my rule of thumb is always use an SSR that has a rating at least twice and preferably three times what the load actually draws.

Poul Erik

Thank you @achrn
Your time is much appreciated.
I will change the SSR and see if this solves my issue.
//pe

dc42

@poul-erik I suggest you disconnect the SSR from the Duet. If the bed heater still turns on as soon as you turn the power on, then the SSR is shorted.

zapta

@poul-erik, I recommend one of the ssrs listed here https://vorondesign.com/sourcing_guide?model=V2.4, for yours mains voltage.

3A sends to be on the low side.

Th0mpy

I'd also recommend looking at a physically bigger model. I have had good luck with this style, plus you can mount it on some aluminum to dissipate heat: https://www.digikey.com/en/products/detail/kacon/KMSR-DS0104/13148008 rated for 10A which should be enough.

Gixxerfast

@th0mpy I agree. This is the one that I use:
https://www.filafarm.de/en/products/solid-state-relais-1?variant=17214068163

They also have the heatsink to match.

No fun burning down the house ... I have a heater mat with a built in thermostat and also an external temp fuse plus dual thermistors.

zapta

@gixxerfast said in 230v heatbed thermal runaway:

No fun burning down the house...

A thermal fuse can be another safety measure. Nero3DP has a video on youtube demonstrating how he builds a mains bed with a silicon pad, thermal fuse, and high temp silicon adhesive. On my own printer I have two 115C thermal fuses in series. I also have a mains relay that is connected to the duet's PS ON output such that if the duet thinks something is funny, it turns off power to bed and 24V DC/DC.

Keeping the house intact is very important because it protect the 3D printer from the elements.