Bed Heater Failing on long heating segments

[bold birch]

Been printing at 50-80 degree bed temperatures no problem. Just recently wanted to start doing some ASA and went to heat my bed up to 105.

Started the first print, got a heater error. Looks like at about 100 degrees it stopped getting hotter. Restarted the printer, and with the temperature already fairly high I turned it on again. It got to 103 before it randomly stopped heating again. One more time, starting at 90, and it was able to get to 105 and hold it no problem.

The best I can tell is that something is going wrong with extended 100% power input. That sounds like issues with the power being provided, but I'm not sure how to test that. I have a multimeter and all the tools, I just don't want to probe around without knowing exactly what I should probe.

[dark marsh]

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[urban mason]

What bed heater? Is it the keenovo one ratrig supplies?

[bold birch]

Yes

[urban mason]

Looks like the thermal fuse is faulty and it is activating at 100C instead of the spec value of 150.

Heat it up to 105 and wait for the error, then cut power to the printer and use a multimeter to check continuity between the Heater power wires.

If it's open circuit (no continuity) then the thermal fuse has activated). It resets when it cools down.

[bold birch]

Gotcha

You're talking about the two white wires? I have no continuity along them even when it is heating

[bold birch]

Okay. So long as the bed is turned on I am getting 1600 ohms of resistance over the wires coming directly from the bed. Even when the bed supposedly stops heating I am still getting that resistance. In that point I have also confirmed 120v AC is running through the bed, despite the temperature dropping

[urban mason]

Voltage doesn't run through the bed. It is applied to it. Current runs through. You can have voltage without current.

[urban mason]

Right you are it has a resistance not a short circuit, 😛 I just woke up when I wrote that

[bold birch]

So I should be checking current through the bed wires as it fails?

[urban mason]

But 1600ohms is not right.
What power/size is your bed heater?

[urban mason]

You can do that if you are confident you can avoid getting shocked by the exposed 120v wires while it's powered with a multimeter in series

Otherwise, heating it until it stops heating and then immediately measuring the resistance should tell you of the thermal fuse has triggered

[urban mason]

Not sure how you are measuring resistance of the bed heater with it powered on?

To measure the resistance you have to disconnect it from the SSR and measure between the power wires.

[bold birch]

500mm

[bold birch]

Ah yeah, couldn't do that in series

So heat it, immediately unplug and detach it, then measure resistance over the bed wires

[urban mason]

Resistance should be 12ohms for a 1200W 120V heater pad

[bold birch]

Yep, 12 ohms regularly. Temp starts falling, pulled it off and I'm getting NC across the wires

It's strange that it will work if I step it up

I've had the printer for only about a month now, do I contact RatRig?

[urban mason]

Yea that's the thermal fuse activating at 100C instead of 150,C

So it's faulty.
However, the thermistor could also be the issue if it's faulty or set to the wrong type on your firmware config so ratos thinks it's at 100 but it's really at 150 already.

Only issue that could be your fault is if you changed the thermistor configuration in ratos

If not, then contact ratrig.

[bold birch]

I don't think I did, going to check that now

I'm also fairly sure I did the calibration for the bed as well

Not sure how to check if I did

Ah yeah I can see it, there are PID values for the bed in my printer.cfg

So I did calibrate it

[weak hearth]

I thought the fuse on the bed would blow, not reset after cooling down. Has there been a new kind of fuse installed on newer heatbeds?

[urban mason]

From the keenovo website:

If your heater comes with a built-in thermostat, please kindly note that the thermostat, being mechanical (bi-metal and snap action), should not be considered as a precise temperature control device, but as an auto-regulating and over-heating protection device. Its action temperatures may be subjected to certain tolerances or hysteresis due to factors such as heater power rating, ambient temperatures and heat consumption/loss rate etc.

https://keenovo.store/products/keenovo-square-silicone-heater-3d-printer-build-plate-heatbed-heating-pad

So it's not actually a fuse. It's a bimetal thermostat switch

Maybe there is a fuse as well with a higher blow temperature in case the thermostat switch fails?

[weak hearth]

I think there's an actual fuse, so if the bed shorts it doesn't melt things, then that thermostat as well

[sonic shell]

@waxen thicket can you confirm if it's a resetable fuse?

[waxen thicket]

Both exist, non-resetable fuse, and thermostat (resetable)

We are slowly transitioning our heaters to the latter

But for the past few years we had the fuse type

[weak hearth]

What temp is the thermostat set to cut out at?

[waxen thicket]

150°C on the newer RR stock, cant speak for all keenovo heaters

[bold birch]

Gotcha, so my new one cutting at 90 is definitely not supposed to happen lol

I've sent RR a message through the website, when I get a response I'll update

[bold birch]

Yes, it was broken. New one works fine