Active bed not heating up

Indianagian

![Hello everyone, my name is Giancarlo.
I've received my new board a Duet wifi, following the step by step guide I did set it up to work via wifi on my smartphone, I'm waiting for the lcd to come up).
Wifi firmware 2.01
Wifi server 1.21
The extruder work fine, as soon as I set ACTIVE and choose a target temp it heats up, the bedheat unfortunately does not, I set a target temp on Bed at 55 °C, no heating up and no fault error
Checked with YAT M140 and the resulting message is: Bed heater 0 (slot 0) is currently at 20.6 °C <LF>ok<LF>

Thanks a lot

Indianagian

I've tried to tune the bed as per guide but....

09:38:51 Auto tune cancelled because temperature is not increasing
09:37:40 Auto tune phase 1, heater on
09:37:34 M303 H0 S55
Auto tuning heater 0 using target temperature 55.0°C and PWM 1.00 - do not leave printer unattended

I've double checked the fuse and the 15a fuse did actually blew, the wires are connected correctly, PSU is 24v 20A 480W.
I've set the PSU output at 23, what could have caused the short? Bed wirings are 2.5mm in diameters, how can I troubleshoot this?

dc42

What is the resistance of the bed heater, measured with a multimeter?

Indianagian

Hi dc42, I've checked with the multimeter, and the bed resistance is 01.8 ohm with a sensibility set at 200 on the multimeter.

dc42

@indianagian said in Active bed not heating up:

Hi dc42, I've checked with the multimeter, and the bed resistance is 01.8 ohm with a sensibility set at 200 on the multimeter.

If you short the multimeter probes together, you will probably get a reading of something like 0.3 ohm. Subtract that reading from the reading you got when measuring the bed heater, to get a more accurate value.

If the resistance really is 1.8 ohms, then at 24V it will draw 13.3A, within the rating of the fuse. But if it is really 1.5 ohms then it will draw 16.7A, aboive the fuse rating.

How large is the bed heater? Is it definitely intended for use with 24V?

Indianagian

The multimeter probes read 0.9 ohm subtracting it to the 1.8 it's a net of 0.9 ohm.

The bed is 300x300mm MK3A it came with a 12v psu, but on the bed itself it can be used at 24v, I've attached the bed picture for visual reference.

The reason for 24v is because I'd like to print in ABS instead of PLA only.

Dougal1957

so at 24V it will draw something in the order of 26.6 Amps no wonder the fuse blew

Can you take a good close up of the connections you have made?

Indianagian

The wires were already soldered on the bed, I didn't make any connection, as you can see the Dual power table indicate pin numbers but on the actual pin you can't find any of them, do I have to rewire the bed or should I go with a 12v psu?

dc42

Are you sure that the bed heater you have really is dual voltage? If so then the best option if you have a suitable soldering iron is to rewire the heater for 24V.

Indianagian

I've checked and the bed is dual power, would an external mosfet up to 25a be able to avoid the fuse to blow? I can lower the voltage to 21V on the psu.
Can this solution work?

dc42

If the bed heater is wired for 24V then it's most unlikely that it will draw more than 15A. Are there any more pads or wiring legends on the back of the board, apart from the "12V" one in your photo? What's underneath the blue tape?

EDIT: maybe it's really a 12V-only bed, and the 12V/24V connection instructions on the front are left over from when the original design was cloned? The instructions on the front suggest that there should be 5 pads, but in the photo I see only 4.