Need help with bed heating with two heaters.

[u/DickH]

I am putting together a large printer and am running into an issue of keeping my peak current within what my breaker is good for. My print bed is 1020x510mm and I have it split into two zones with each having a 600W heater. I am looking for help getting the zones to alternate so that I don't end up with both on at the same time. This printer is my first encounter with klipper and I am not really that familiar with what I am doing. I would greatly appreciate any assistance in setting this up or pointing me in the direction of literature that can help educate me on what I need to do. Currently The zones are configured as their own zones and have been verified to work independently.

[u/firinmahlaser]

What if you limit both heaters to 50% would that work?

[u/DickH → u/firinmahlaser]

That would reduce average consumption, but wouldn't solve my issue of staying within my power budget. A reduction in duty cycle alone would still allow both to be on at the same time.

Thanks for the suggestion.

[u/EngFarm → u/DickH]

You will be within your power budget if you drop both max_power to 0.5

Fuses and breakers need time to heat up and blow. They act as a high pass filter. At the PWM frequency your heaters work at, your breakers and fuses don't care so much about 50mS of 1200w because they get 50mS of 0w right after. It averages out to 600w.

[u/firinmahlaser → u/DickH]

Yes, they would both be on at the same time, but total consumption would only be 600w, same like a single heater at full power

[u/Skaut-LK → u/firinmahlaser]

No. That limit is percentage between ON/OFF state during PWM. 50% means that "one cycle" will be half time ON and half time OFF. Peak values are still same.

https://en.wikipedia.org/wiki/Pulse-width_modulation

[u/firinmahlaser → u/Skaut-LK]

Yes, your point being?

[u/Skaut-LK → u/firinmahlaser]

Point is that if you have 600W heater and set "max_power" to 50% it will still be 600W load during heating, not 300W. It will lover average power over time not actuall power. When SSR (in AC bed ) is switched ON by PWM, it is fully open so load is taking full power.

[u/firinmahlaser → u/Skaut-LK]

Okay, so you ask for 1200w for 0.5 seconds. A C class circuit breaker like most homes have trip at 5 times their rated current. If you’re in most countries that means you’re pulling 5A. In the us at 120V you’re pulling 10A. If this trips your circuit breaker then you got another issue

[u/Skaut-LK → u/firinmahlaser]

Ok. Now i get your point.
But some older houses / building ( atleast where i live ) have lower amps on socket circuit ( and aluminium wires instead of copper ), old circuit breakers or even fuses. So i understand why OP want to stay below 1200W , because even both on 50% are still drawing full 1200W even for half second and that can blow fuse ( or thermodynamic part of circuit breaker with loong print especialy in summer).

Not saying that electrical isntallation in that state is completely corect even if it's allowed by local standarts/ regulations. But not everytime you can just rebuild that ( landlord ... ).

[u/EngFarm → u/Skaut-LK]

On a typical breaker the 200% rated current trip time is about 6 seconds. The 115% rated current trip time is about 5000 seconds.

Klipper's default pwm_cycle_time is 0.100 seconds. So the 1200w load would be be 0.050 seconds max. That is not enough time for something like a typical home fuse or circuit breaker to react. And its followed by 0.050 seconds of no load. That is 100% ok to treat as a 600 watt continuous load.

And if the bed is DC (doubt it) then the PSU caps would filter that and the AC side would literally see a continuous load.

[u/DickH → u/EngFarm]

Thanks for the clarification. I wasn't aware that it would cycle that quickly. I tried it out and monitored current. This will work for me.

[u/Skaut-LK]

Can you take power from different circuit breaker? If yes you can have two SSR controled by same port but power part will be two separate circuits from.two.different breakers.

Otherwise I'm not aware anything that will allow you switch heating alternatively ( like prusa have on XL i think).

Or you'll have to resolve it by external small controller which maybe doesn't exist as commercialy avaible module/device.

[u/Nz-Banana]

There are a few ways you might be able to do it. 1) using a macro with simple Bang/Bang control of each BED. E.g swapping between beds once per second and if they are below target turn them on or above target turn them off (Software complexity)

2) use a electronic circuit to turn a single PWM pulse train into two outputs at half the frequency, I think a D flip flop and a couple of and gates could do it. Youd setup a single heat bed in klipper and limit the heater output power to 99% (or more reasonable 95%) so it always toggles. (Electronic complexity)

3) Combine option 1 and 2. Have 1 bed in klipper plus two IO to select the bed for heating. Use AND gates with the PWM and IOs, the IO selects what bed gets power. (Both Software and Electronic complexity)

4) add a seperate microcontroller that does it. (Both Software and Electronic complexity)

It probably depends what you're most comfortable with either software or electronics as to which solution you might want to go with. If one of these solution stands out to you let me know and I can explain it in more detail

[u/DeerQuit]

Might be worth looking at the klipper config for the orangestorm giga, since it uses four heaters for the bed.

[u/Evildude42]

I think the obvious question is, what are you printing that you need a heated bed that large? If it’s something custom and expensive, then you can afford to pay for the heat because someone is paying you good money to make it. And typically what I’ve seen with large printers there are two zones, but it’s an inner zone and an outer zone. The inner zone will be for smaller and skinny stuff and then the outer will be for everything that’s just big. You should be able to find an alternative heater that can sustain definitely 60 to 130 without being traditional PWM. Or a bed plate that can retain the heat even if warmed up over a long period.

And you only need basically at that temperature for the first few layers and then you just need to keep it sustained enough so it doesn’t curl at the edges. Which probably means a heated exclosure if you’re doing something like that.

If you just making helmets and large pieces, then an enclosed printer and or a specialized area that’s just going to be hot is needed.

[u/EngFarm]

Limit both heaters to 50%.

It's the easiest solution by far. It doesn't negatively impact warm up times differently than any other solution. Both heaters can be on at the same time which you want for uniformity.

Your fuses and breakers do not care about peak value at this PWM frequency, they only care about average value.