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Registered Member #1107
Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
i rigged my simple 555 mosfet driver up for audio modulation using pin 5 on the 555 and i am having problems with it heating up very fast. I tried it with a irfp250 mosfet rated 30 amps 200 volts and i am barely drawing 3 amps at 12v and it is still heating up like crazy. I am thinking it might have something to do with the modulation because it dident do that when i ran it without audio modulation. My question is there a special kind of mosfet to use or transistor or do i just get a mosfet that is rated for like 70 amps on a big heatsink.
Registered Member #1406
Joined: Thu Mar 20 2008, 12:12PM
Location: Deepest darkest Wales, UK
Posts: 27
Do you have a snubber on the LV side to prevent back EMF? I've noticed with my ignition coil driver that the amount of back EMF I get is dependant on the frequency I'm driving the coil at, so perhaps the audio contains frequencies your flyback doesn't like?
A disposable camera neon indicator in series with a 4K7 across the LV side of your coil is a useful warning that you're getting too much back EMF. In my case I was able to solve this with a .22uF 440V cap across the coil, and this seems to work even with audio modulation. The MOSFET I'm using is a 2SK899, ripped from a modular power supply.
Also - from what source are you feeding the audio? Have you tried a smallish electrolytic cap in series between the audio source and pin 5 of the 555, negative side facing the 555?
Registered Member #1107
Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
Ok the driver still works without a problem except when i decided to audio modulate it it worked but then stopped and i cant get it to audio modulate any more and now it is drawing alot more current then it used to. Also this is the flyback driver i am using
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
teslacoolguy wrote ...
Also this is the flyback driver i am using
You can trash the 2N2222 amplifier and connect the 555 output through 10ohm resistor directly to the base; the 555 can provide enough current to drive the MOSFET.
If it is drawing a lot of current then indeed one or more of your components are dead. Try to change the 555 then the MOSFET. Does it run fine without any modulation? I guess not.
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singing 555 flyback mosfet question
