120degC MOSFET, huge voltage drop on MK2 heatbed

I'm testing the electronics on my newly constructed Prusa, and I have two major issues: my heatbed MOSFET reaches over 120degC, and my heatbed will not go over 65degC.

I am using an ATX power supply that is rated to provide 16A at 12V. The wiring between the power supply, RAMPS 1.4, and MK2 heatbed is 18AWG speaker wire. The heatbed has a resistance of 1.5 ohms. The heatbed thermistor is kapton taped to the underside (silkscreen on top) with a drop of thermal paste for accuracy. The RAMPS 1.4 was ordered from www.reprapdiscount.com.

If I turn the heatbed on, but disconnect the heatbed wiring, I measure 11.67V from the power supply and across the MOSFET pins (D8). However, if I reconnect the wiring and turn it on, I get 11.13V from the power supply, and only 9.34V across the MOSFET. The MOSFET also becomes extremely hot (measured 120degC with a thermocouple). The heatbed thermistor never records higher than 63degC.

Will you please offer your advice?

- the powersupply neads to be a bit bigger with the capability to supply at least 30 amps,
- i would put a heatsink on the mosfet and/or change the mosfet to somthing like the IRLB8748 or a FDP8878

Did you put a load on the +5V leg of your ATX power supply? A lot of ATX supplies need it or the +12V output will be low. I used a wirewound resistor from Radio Shack (10W, 10 ohm) which should draw 0.5A. I would like to replace it with light (LED) but I haven't found one yet.

Also, your MOSFET need a heatsink if it doesn't have it and it may even need a fan. What electronics are you using? Perhaps you can also change it to a better one with a low Rds.

brnrd Wrote:
-------------------------------------------------------
> Did you put a load on the +5V leg of your ATX
> power supply? A lot of ATX supplies need it or the
> +12V output will be low. I used a wirewound
> resistor from Radio Shack (10W, 10 ohm) which
> should draw 0.5A. I would like to replace it with
> light (LED) but I haven't found one yet.
>
> Also, your MOSFET need a heatsink if it doesn't
> have it and it may even need a fan. What
> electronics are you using? Perhaps you can also
> change it to a better one with a low Rds.

I think it most likely he's suffering from a PSU underpowering due to no +5 load. As you mentioned.

I haven't had issues with heat on my reprapdiscount Ramps MOSFETs. One out of the 3 does have a nice sink on it. Seems to me the metal sink attachment on the other 2 MOSFETs do a decent enough job with a small fan blowing over the board.

This is my setup however. YMMV

Edited 2 time(s). Last edit at 01/24/2013 11:05AM by xclusive585.

I will go to Radioshack and find an appropriately sized resistor. The heatbed MOFSET does have a 80mm fan literally resting on it while blowing air at full speed. I understand this particular MOSFET operates up to 175degC, but I hope I can lower the current temp substantially. Do you think the 5V load will help with this?

I switched to a IRLB8743 and can get away with a 1cm2 heatsink with no fan, and it remains touch cool.

-Rob A>

russd Wrote:
-------------------------------------------------------
>...
> at full speed. I understand this particular MOSFET
> operates up to 175degC, but I hope I can lower the
> current temp substantially. Do you think the 5V
> load will help with this?

The fan alone is probably not enough. You also need to put a heatsink on it. You mention that the Mk2 has a resistance of 1.5 ohm. That's actually a bit high to reach 110 C with 12V. My Mk1 boards are usually between 1.1 and 1.2 at ambient temperature. Did you subtract the resistance reading on your meter when the leads are shorted together? That's usually a few tenths of an ohm.

The load will no help cool your mosfet. If it works, the +12V output will go up and it would actually make both the bed and the mosfet run hotter.

I have put a 20W 20 ohm resistor on the 5V, and I also put a heatsink twice the size of the last one on the heatbed MOSFET. Heatbed temp is still hovering around 60degC, while the MOSTFET remains blistering hot. 8.69V across the actual heatbed, and 9V at D8. I'm leaning towards installing a new MOSFET before spending on a power supply.

whats the current mosfet in there atm the moment?

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