Dual output for one heater?
-
@chriss said in Dual output for one heater?:
So there should be no risk if one of the outputs fail with "permanent on".
I did something similar except that the second ssr is a N.O. mechanical relay and it uses the PS_ON output. If the first SSR is stuck at on, the CPU detect abnormal PID condition and shuts off the relay.
To test it, short the SSR and see what happens. As for the relay, it clicks each time I turn the printer on so I know it's not permanently stuck.
-
Yes, I know what you mean, that would be a workaround. You can enable the relay via the start.g or some start gcode in the slicer to avoid having one phase at the bed.
But all of that is not very elegant, two outputs at one heater is far more flexible. I play with a very high temp hotend concept since a while, two heater cartridges are very helpful there too.
-
@zapta quick word of warning - I had a similar setup with a 10A relay switching the two PSUs on my printer (one for the bed, one for everything else). Unfortunately the inrush current was higher than this and the relay failed closed. However, it still 'clicked' when cycled, just never cut the power...I've now got a pair of 30A relays for either PSU (switching the mains in and 24V out for both) and not had any issues.
@Chriss if you're going for additional safety considerations, it's worth considering the impact of silent failures. As you describe your system, either mosfet or SSR could fail open and you'd never know unless you get your multimeter out. The advantage of the PS_ON method is that it keeps the power shut off separate from the actual load.
If safety is a concern at all, then a thermal cutout fuse should also be the first consideration before anything else. It's a passive component so is inherently more reliable than any controller solution. I don't think it's unreasonable to say that any 'event' (thermal, electrical, etc) that causes a mosfet or SSR to fail might have caused the Duet to misbehave so there's no guarantee it will be able to protect you from disaster. The 'event' might also blow both SSRs/mosfet in one go. That's saying nothing for human error or any mechanical failure that causes the bed to overheat.Having said all that, I don't have a TCO on my bed, but probably should install one!
-
@engikeneer
I was never a big fan of the TCO, I think that is a bit of over-engineering. My 2nd Voron build is changing my mind at the moment.It is clear that you can have two mosfets in "permanent pass tough" mode, nothing can protect you from that. The usual idea is that you want to protect the bed from overheating. I have a other idea here, I want to protect my heart from getting shocked by a one phase 50hz defibrillator in the shape of a 3d printer.
I experienced that from by v0.0 some days ago. I had the build plate in one hand and the other touched the "hot" phase. All of my other printers cut of both phases. I'm to well trained....
Anyhow, I fully understand all of you points and we can work around all of that. But I still think that the most elegant way would be "two outputs for one heater". That can help you for other use cases when you need a 2nd heater with more power (build camper heater) by simply adding 2 or more outputs in parallel.
Cheers, Chriss
-
@chriss said in Dual output for one heater?:
Is that possible? Is there a way to map two heaters to one "M140"? Or two outputs to one "M950"?
In RRF 3.4 that is possible.
I have a other idea here, I want to protect my heart from getting shocked by a one phase 50hz defibrillator in the shape of a 3d printer.
Use a GFCI (also called a RCD in the UK) for that.
-
@dc42 said in Dual output for one heater?:
In RRF 3.4 that is possible.
Thanks David, you made my day.
Use a GFCI (also called a RCD in the UK) for that.
We call them "FI" in Germany, the short for "Fehlerstromschutzschalter". I had the same idea, but this is a "workaround" only, too. That is indeed the reason to ask for a "dual output per heater". I do not want a thermal fuse, a GFCI and a relay. This is a 3D printer and not atomic power plant. And the room on a V0 is very, very limited.
Cheers, Chriss
-
@chriss, using dual heater outputs and two SSRs only protects against failure of one SSR, or failure of one mosfet on the Duet. It does not protect against firmware errors, or over-voltage conditions that could cause the MCU to fail and drive both outputs.
A TCO is a better option.
-
@chriss said in Dual output for one heater?:
And the room on a V0 is very, very limited.
A TCO is about the size of a thermistor. I know the V0 is small... but, really?
-
I'm sorry... it seams to me that we have a problem with my English here. Let me try it again:
The TCO protects you for overheating. The TCO does not protects you for touching the phase which was not cut of by the SSR.
And you want to cut of both phases, not for failures, only for the concept of not having 240V where you do not need it.Cheers, Chriss
-
Thank you for clarification. Yes that makes sense as well.
Keeping the mains away from where you do not want it will be a function of proper cabling, proper routing, physical protection of the cabling, and proper grounding of any metal you might touch. A relay, even two, can still fail open, so can't really replace the other factors.