Sanity check on home built 3d printer heatbed connections?


#1

Trying to keep from burning something up or worse :smiley: (@Team_3D_Fab might be of help too)

I’ve been getting my Mendel90 working, slowly and my 200x300mm PCB heatbed just doesn’t have the “oomph” to maintain 100 degrees C. I haven’t built an enclosure yet (which can help the bed retain heat) but reading through the specs for my heatbed I found this info:

  • Resistance between 1 and 1.3 ohm for the 12V (110 - 144 Watts)
  • Resistance between 4.5 and 5 ohm for the 24V (115 - 128 Watts)
  • Running 24V on a 12V setting will heatup the heatbed to 100 degree Celsius in only 2 minutes (while consuming 443 - 576 Watts!!!)
    (product: https://www.reprap.me/pcb300.html)
    Which sounds good and all, but my PSU is only 360 watts so I fear it could try to draw too much current and burn up my PSU.

Here’s the relevant pads on the board:
PNG
I had hooked it up in 24v mode to pads 2 and 3. I will re-wire it to +24 on pads 2 and 3 and ground on pad 1.

So my first question is - I should treat this as a parallel resistance problem, right? I have resistances from pad 2 to 1 and from pad 3 to 1.

If that’s the case, I get:

1/Rt=1/1.2 + 1/1.2
giving a total resistance of 0.6 ohms. Which at 24v is 40 amps or 960 watts. If I’m good so far I can adjust my firmware to keep the heatbed driver throttled to about 20% to give me about 200 watts.

Unless I’m doing my math wrong. :smiley:


#2

Visualize this as a resistor with #1 as a center tap.

Those resistance numbers you quoted do not make sense. Did you measure them?


#3

Have you insulated the bottom of your heated bed?


#4

I haven’t measured the 12v resistances (yet) but between pads 2 and 3 it’s 4.8 ohms and other users online report 1.2 ohms using the 12v pads.


#5

I have, and it helped some. It’ll get to about 90-92 degrees (from the thermistor and verified with IR thermometer gun) and then plateau and trigger the thermal runaway shutdown.


#6

The only way that is possible is if there is a diode so that it offers less resistance when hooked up “backward.” Try measuring resistance in both directions to detect this.


#7

ok thanks!


#8

OK doing more research the resistance in “12v Mode” is given as ~1.2 ohms. So they’ve already done the math, I’m just too stupid to understand it.

Still need to throttle it back though - 1.2 ohms @24v is 20 amps / 480 watts. Or I could get a bigger PSU. :smiley:


#9

Reporting back just to confirm, it worked, didn’t burn up my PSU or my house. :smiley: (and in case someone’s ever searching for this stuff and it helps)

The 12v traces once bridged come out as 1.3 ohms, @24v about 443 watts.
I was getting about 110 watts before, so I wanted to try to get about 200 watts to heat it up faster; 200/443 = 45%-ish. On a scale of 255 that’s 115.

To avoid overloading my PSU I adjusted the heatbed max power in the PID settings
#define MAX_BED_POWER 115
and recompiled my Marlin and uploaded to my printer.

Then because I’m cautious I did NOT connect the heatbed but executed a heatup command and measured the voltage on my controller board - 10-11 volts confirmed I didn’t screw anything up.

Bottom line: Heated bed heats up fast and to the temperature I want! Now on to the next step of this build!


#10

Just keep an eye on your PSU.
The printer uses PWM to power the bed so the AVERAGE power will be 200W, but the instantaneous power will still be 443W.
If the PWM pulses are short enough the PSU capacitors can probably handle it, but fundamentally you are still pulling 400W from the PSU, just for very short periods of time.


#11

Good to know! I wasn’t sure how it worked which is why I metered the outputs before connecting my heatbed. The 24v pulses are too quick for my meter to register, which isn’t surprising, because it’s a crappy digital one I got at Wal Mart or somesuch in like 2002.