Bed heating failes

For a long time I've been printing on a Velleman K8200 with an Ramps 1.4 board. After some trouble with the heatend I installed a new PSU with enough power for the Alu heatbed MK3. After that the heatbed won't heat up to 60C°. The temperature it can reach is 40C°.
When it is heating the transistor for the heatbed is making a whining noise and that stops when it stops heating. This didn't happen with the old PSU. The PSU can provide 12V 15A on 2 separate 12V lines. The transistor is a P55NF06 with a heatsink. The ramps is a 1.4 from China.

It sound - no pun intended - like you are using "PID" control for the bed heater : switching the transistor on and off very fast.

It usually is better for bed heating to use "bit-bang" only - with a longer period of being on or off, around a second.

You should be able to change that setting in your firmware configuration. It will certainly remove the noise, and hopefully it could help with your not-heating-enough problem.

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Most of my technical comments should be correct, but is THIS one ?
Anyway, as a rule of thumb, always double check what people write.

Thanks for the fast answer. I checked in the firmware if it uses PID but it not active. PIDTEMPBED is not enabled "//#define PIDTEMPBED"
Max_bed_power is also 255. I included my configuration in case it is needed. The firmware I use is Marlin RCBugfix downloaded on 11/29

Edited 1 time(s). Last edit at 12/20/2016 10:48AM by Natsuki-chan.

Try looking for a loose wire somewhere. If the transistor isn't switching it shouldn't make any noise at all, so either itself or a connection somewhere is breaking down.

There is no loose wire somewhere and the resistance between the plus and min on the board is really low.

After some time messing around I did get the bed to 60C° but it was a tedious process of going from 30 to 60 with intervals of 10 degrees. This did work a few times but it is not really something I want to do.

Is it better to change the transistor to something else, if so. What part do I need to use for reliability?

Its a MOSFET

best is IRLB 8743, should run cool

other options can be found on [reprap.org]
(yes different board but same part doing same job)