Heated bed woes

I managed to fry my RAMPS board by plugging in a heated bed. A new board is on its way, but if anybody has some tips on how I can avoid a repeat performance and drive a heated bed safely, I should be very grateful.

Here's my setup:

- This Graupner powersupply, set to 12V and 16A, with a split soldered into the positive and ground cables and connected to the 11A and 5A connectors on the RAMPS

- A Ramps 1.3 board, with Marlin firmware and pronterface as host

- This heated bed

After a lot of fiddling with the Pololu trimpots, I managed to get the motors and extruder working properly, and coaxed the firmware into reading the thermistors for hot end and heated bed. During this, the power supply drew at most 3A. Emboldened by my success, I connected the heated bed to the D8 outputs on the RAMPS and set it to 55 degrees in pronterface.

The power supply immediately shot to 16A and the bed started warming up. Before it could get to its target temperature, a thin plume of smoke came out from beneath the Q3 MOSFET. I switched off the power supply right away, dialled it down to 13A and tried again. No smoke this time, but the distinct smell of overheating electronics.

I dialled it down again to 10A, and now while the motors and hot end still work, the bed is no longer responsive. The Q3 MOSFET is visibly discolored and there are (slight, but still) scorch marks on the bottom of the PCB.

What did I do wrong? And more importantly: how can I do better in the future? I really want a heated build platform to print PLA on, and I'd like to power the entire printer from the same power supply if possible.

Replace the burned FET, and fix the heater.. Its pulling to much current.. Test on your power supply/// first.. it should yank around 10A or so ... not 13+

It sounds like you have a short in your bed. What is the resistance of your bed heater? You probably want to stay above 1 ohms. The MOSFET in 1.3 doesn't have a heatsink so it will get quite hot if you run it at 10 amps continuously. This could happen if your thermistor is reading incorrectly low or if you want to go to ABS setting which is about 110C or higher. I would also recommend installing a heatsink but the MOSFETs are crowded in version 1.3.

Thanks for your tips, it turns out that the resistance in my heater was only 0.75 ohms. I built a new HBP with around 1.2 ohms and tested it by plugging it into the power supply by itself - and, sure enough, this one only pulls 10A.

My new RAMPS 1.4 board arrived last Friday. I wrangled a small heatsink onto Q3 and put the board into an enclosure with a fan. So far, I've had it running for two hours at a stretch without issues.