Mechanical relay death in stuck in closed postion bed at 120c

claustro · 28 Oct 2018, 15:50

I have a dbot.
Dual 24 volts power supply with one reserved for a 24v 300x300 mk2 heated bed.
The heated bed is driven by a mechanical relay.
This evening the relay was in closed position even if no signal from duet. The bed reached 120 degree Celsius.
I have smoke detector and fire extinguisher but I would like to improve the safety of the printer.
I read about the ssr on wiki but I can't find the raccomanded brand MGR-1DD80D100 or Crydom DC100D4 at reasonable price.
Any idea where I can find some reliable low drop ssr in Europe?

I would like to add a thermal fuse can someone point me in the best suited for heated bed?
What max temp should it have ? I am thinking about 110c° seems reasonable?
This is the only one I was able to find managing more than 10Amp
https://bit.ly/2OY5gXp

Also when I run pid controll I always receive warning message regarding the max temp the heater block can reach if runs uncontrolled, is there a way to limit that risk? lowering a little the 24v of the power supply? running at 22?

Thx
Andrea

Stephen6309 · 28 Oct 2018, 17:26

If you print with a bed temperature near 110C, it may not be high enough. I use a 167C thermal fuse on my bed.

claustro · 28 Oct 2018, 17:31

Are you using a 24 volts driven bed? can you point me to the fuse are you using?

Dougal1957 · 28 Oct 2018, 17:43

I use one from this range manually resettable thermal cutout

HTH

Doug

claustro · 28 Oct 2018, 17:47

Thank you
Did you mount it with screw or silicone glue under the bed?
Should this works?
https://www.ebay.co.uk/itm/150-C-Nc-Termostato-Temperatura-Interruttore-Termico-Normalmente-Chiuso/263953401277?hash=item3d74d945bd:g:KNsAAOSwBahU-3-6:rk:34:pf:0

Stephen6309 · 28 Oct 2018, 19:38

@claustro My bed is a 110vac silicon heater from McMaster-Carr stuck to a 1/4" MIC6 plate. The thermal fuse is similar to these:
https://www.ebay.com/itm/10pcs-167-C-TF-Cutoff-250V-10A-12-4mm-L-D-Thermal-Fuse-Electric-Cooker-Fuses/183188846355

For the mounting, I used kapton tape to hold in place on the heater.

dragonn · 28 Oct 2018, 19:40

That is exactly why you should use a mechanical relay for driving you bed... bad idea overall.
You have 24V, why you don't connect it directly too you Duet?

claustro · 28 Oct 2018, 20:23

First of all thank you all for answering me.
some questions.

@ dragonn I used a mechanical relay but it failed in on position form what you are writing i can't understand if you judge a mechanical relay reliable or not.
I drive the bed by relay because it has a dedicate power supply separated from duet. ( 30Amp + 20Amps)
Also I was afraid it drain to much current for my duet that is 1.2 version and the bed is 300x300

The great majority of thermal switch are rated 10A at 240 will be this specs sufficient for the current drain from a kk3 heated bed?
Tomorrow I'll measure the exact current drained from the bed

dragonn · 28 Oct 2018, 21:26

@claustro mechanical relay are just not designed for this use-case. They will fail fast when doing a lot of on-off cycles.
You should be an external mosfet module like for example https://www.aliexpress.com/item/3Dprinter-heat-control-MKS-MOSFET-for-heated-bed-printer-head-MOS-30A/32405884519.html they are much easier to get then SSR for DC and are reliable too.
And this will work with separated power supply too, only the ground of the power supplies need to be connected together (with this mosfet module probably does already).

AndreS · 29 Oct 2018, 07:34

Those MOSFET-modules have an opto-coupler at the input. No need to connect minus of the power-supplies together. 2 pins from the power-supply, 2 pins to the heatbed, and 2 pins from the duet heater output. The last two can be thin cables, because the are only a few miliamps for the opto-coupler needed.

claustro · 29 Oct 2018, 08:23

Is this connection scheme correct?
I bought an overrated thermal sensor but I prefer be ready in case of upgrade.

AndreS · 29 Oct 2018, 10:13

From Duet to MOSFET-module use the upper terminal (bed+ and bed-). From power-supply to MOSFET Power- is at bottom, then Power+.

Mangy_Dog · 30 Oct 2018, 11:13

Im using a mains heated Silicon matt on mine.... Ive put three levels of protection and control. A mechanical relay, that would fail open (ie off)
This is the the first layer, this is triggered by PS_ON, PS_ON will also kill power when thermal runaway is detected in software. This could happen if a therm has failed or if the SSR fails and is stuck on, The actual PWM power control is done by the SSR from the heat bed pin.

Using an SSR for the PWM protects the mechanical from wear, as these are designed to have fast power cycles, its basically a power transistor...

After that, a thermal fuse on the bed, if the software crashes and for some strange reason PS_ON is stuck on and heat signal is also stuck on (very unlikely but in the realms of possibility) the fuse will trigger when the temp hits around 140 with the fuse ive got.

So in this setup its incredibly unlikely a mains heat bed will cause a fire. Im also thinking of just adding a power fuse in the line as well to protect against possible shorts if anything damaged the power line in there... Though there is going to be 2 fuses, one on the kettle socket and one in the plug socket...

I was just thinking a lower rated fuse just for the bed. maybe 4 or 5 amps... My bed is 3 amps max... So anything above that would be a short or something else failing...

dc42 · 30 Oct 2018, 11:37

@mangy_dog said in Mechanical relay death in stuck in closed postion bed at 120c:

Im using a mains heated Silicon matt on mine.... Ive put three levels of protection and control. A mechanical relay, that would fail open (ie off)

Mechanical relays can also fail short circuit, due to either a mechanical problem (rare) or the contacts getting welded together, which is probably what happened to the relay that @claustro was using to control his DC bed heater.

...Using an SSR for the PWM protects the mechanical from wear, as these are designed to have fast power cycles, its basically a power transistor...

A DC-AC SSR uses a triac, not a power transistor. Triacs are very reliable, unless they are subjected to a prolonged current overload (causing them to overheat) or frequent current surges.

After that, a thermal fuse on the bed, if the software crashes and for some strange reason PS_ON is stuck on and heat signal is also stuck on (very unlikely but in the realms of possibility) the fuse will trigger when the temp hits around 140 with the fuse ive got.

A agree, a thermal fuse is a good protection mechanism to have.

Mangy_Dog · 30 Oct 2018, 13:29

1 out of three isnt that bad

I would say though a mechanical welding closed is incredibly unlikely... The mains fuse will pop before a short could weld a relay im sure...

Triacs, ok lack of a better word.... But yes Triac not a transistor...

dc42 · 30 Oct 2018, 13:35

@mangy_dog said in Mechanical relay death in stuck in closed postion bed at 120c:

I would say though a mechanical welding closed is incredibly unlikely

It's much less likely when switching an AC mains heater than when switching a low voltage DC heater, because the current is much lower, and because the arc that is formed when the contacts start to separate gets extinguished as the mains voltage passes through zero. That's why the rated current of a relay or switch is typically higher when switching AC than it is for DC.