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Registered Member #1936
Joined: Sun Jan 25 2009, 11:34AM
Location: USA
Posts: 17
I'm assuming you're full-wave rectifying and filtering the output of the 35v transformer, correct? If not the extra zero-crossings(well, it shouldn't *cross* zero but you know what I mean.) are probably what is heating your mosfet to hell and back. Meaning instead of jumping from an infinite Source-Drain resistance to near zero and staying there, it constantly has to jump back and forth, so it dissipates more power.
Registered Member #1936
Joined: Sun Jan 25 2009, 11:34AM
Location: USA
Posts: 17
Franky wrote ...
The power supply is an old marklin transformer. 30VA max 1.5 amp. AC I have no clue what the original resistance was.
Edit: This power supply
Oh dear. Sounds like you and Killa-X have the same issue. He was blowing up 555s till he rectified it. see this post for more info.
Those extra zero crossings are what's heating up your fet. The Source-Drain voltage is varying, and so is the resistance.
Baaaad!
GOOOOD. Avast ye, nublets! img width=400 is gooood too
Put a bridge rectifier and filter cap on the AC output of your transformer and things should be happier in the thermal department. Alternatively pop it open and replace the internal single diode with a bridge rectifier and have a real DC out. Those "toy" (or is it Model?) train transformers only have a "dc" output so that the motors go in a predetermined direction. Its just one way current, its not filtered or full wave.
Talk to Killa-X, he seems to have been through this and come out for the better.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Also, loading a transformer down with just single diode will result in significantly lower available power output without overheating the transformer, as this causes DC current to flow through the secondary winding, induce DC flux in the core and the current draw on the primary will go up dramatically.
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