@Printrbot @Brook_Drumm Here’s my heated bed setups. The little smalls 150mm pulling 17-18A when hot (it’s more when cold) and the 200mm only pulling 12A when cold.
I have an older green printrbot 150mm bed heater. My meter shows 1.0 ohms through the heating trace and 0.2 when shorted. That would be 0.8 ohms for a pull of 15A.
That is just too low.
Ideally the resistance should be between 4 and 8 ohms.
At 1 ohm you are basically drawing all the current into the bed, and therefore there is not enough power to sustain the main board in a stable state.
The fet is 75 amps I think. We never see them fry 
you have to use an atx and you should plug it in directly, don’t go through the barrel jack. I’ve pig billet connectors on my bot farm
That looks correct- it pulls a lot of current. 1.0 is on the high end bug fine. We spec .7, and do see outliers. We now use thick turnigy Wire to supply power.
Brook
Yes, barrel connectors bad for high current due to minimal surface connection area.
Bullet connectors are better.
I wouldn’t trust a 75A FET to maintain even 20A of continuous DC without a sizable Heatsink
Finished a 5 hour print with no issues.
Power is currently (Ha!) being fed in via 14ga wires, a 20A toggle switch and then into the connector - using all 3 of each black and red wires to carry the current. Nothing gets warm there.
The FET only gets warm during the initial warmup (that was also when I lost the barrel connector) - although I have a binder clamp to help with any heat dissipation. Once the bed is warm, the current is pulsed for maybe 2 seconds every 30 or so seconds.
I’m using a 450W 12V PSU for it and another printer - will have only one running at a time until I get another PSU. (Actually, the 2nd printer died during the barrel melt down - something killed the USB chip - it won’t register anymore).
I have some info on the FETs that Printrbot uses on my laptop (I’ll grab later). If I remember correctly, they shouldn’t have an issue pushing 20A continuous. The PWM is only 10hz so you won’t get any real impedance as a result of high frequency switching.
Be aware- the screw terminals used on be Printrboard are only officially rated to 18-20A, so don’t go too crazy.
(@Brook_Drumm - replacement printrbot board question. https://printrbot.com/shop/printrboard-rev-f6-microusb-connector/ says “Z END STOP NOT COMPATIBLE” - does this mean I have to have a proximity sensor? or can I still use a micro switch? The board that came with my Frankenbot kit died and I need to replace it. But I just have a micro switch for Z since I’m printing onto glass - not metal.)
I’ve got spare boards going all the way back to early versions. The early ones werepre-z sensor. I think the f4/5 are the ones that have a voltage selector for either sensor or switch. F6 dropped the selector since the sensors became so popular.
I’ve been trying to find guys who would rework or reflash old boards to donate to the community… schools, clubs or just enthusiasts. But I can give you a replacement. No prob. Email me w request and address
Brook
