Deliberately slowing down heater warm-up

aidar

Those E3D high temp 300 x 300 beds are 1100W. If it heats up to 60C in 10 secs, then sounds like 110V bed is used with 240V. In that case bed will draw more then 20A and power will be about 5KW

jens55

OMG, that would be perfect if you wanted to have a reliable way to burn down your house .....

deckingman

@bearer I was thinking more of what the temperature might get up to after a few minutes if the bed Mosfet failed. Hope the OP has thermal cut-outs installed but I would still be very nervous about leaving that bed heater unattended.

andywm

I mean... sorry to concern you all. It says right on the heater 240V, 800W. 2400W was a guess based on what the maximum I thought it could even be since it wasn't blowing any fuses on tripping any breakers.

I didn't time that 10 seconds, I just know it heats up to 60C fast.

I'll run it off the mains without the UPS and time it, I used to have a power meter, but that's gone walkies.

dc42

@andywm said in Deliberately slowing down heater warm-up:

It says right on the heater 240V, 800W.

Then its resistance should be 72 ohms. You could check that using a multimeter.

deckingman

E3D give the specifications here https://e3d-online.com/high-temperature-heated-beds

If it's 800 Watts, then it's 300mm x 200mm so about 1.3 Watts/cm^2 - arguable that's roughly over powered by a factor of about 3 times. They also list the maximum operating temperature as being 200 deg C but unless I missed it, I don't see any reference to a thermal cut-out to limit the temperature in the even of a control system failure.

andywm

@dc42 - yeah, 71.2 omhs measured across the bed power.

andywm

This just shows that my time estimation skills are awful. @deckingman

It takes 31 seconds to go from 23C to 60C at full power. If 10 seconds is about 3 times overpowered, then funnily enough.. 31 seconds would be about right.

Okay, what's the proper solution to this if I still want to use the UPS? Wire the relay to a second IEC C14 power connector and power them independently? Or do I need to fork out for an 800W UPS?

jens55

If it was me, I would power them independently. UPS for the electronics, straight line power for the bed heater.
If you want to have the UPS cover the draw of the bed heater then do yourself a favour and go for something with enough head room such as a1200 W UPS.

deckingman

@andywm said in Deliberately slowing down heater warm-up:

I'd do what @jens55 suggested - oh wait, I already have! (although I chose a DC UPS).

https://somei3deas.wordpress.com/2019/10/22/fitting-a-dc-uninterruptible-power-supply-ups-and-separate-5v-supply/

and

https://somei3deas.wordpress.com/2019/10/27/ups-battery-installation/

That is to say, run the bed from the mains and the rest of the printer from the UPS. That way, in the event of a power outage, the UPS will keep the printer running for much longer because the bed has by far the biggest current draw. And it'll take some time for the bed to cool.

I'd also fit a thermal cut-out if the bed doesn't have one (although if it was me, I wouldn't use a bed heater that is so over powered).