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MOSFET problem

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HV Boy

Sat Oct 05 2013, 09:39AM

Registered Member #33572 Joined: Wed Aug 28 2013, 09:02PM
Location:
Posts: 9

Hello everyone, I'm new here and sorry for a possible newb question. But anyways let's start. Right now I'm twiddling and fiddling around with FBT's. And as I don't have the parts for a ZVS driver and tired of little purple arcs I decided to make this circuit right here (just a little bit different resistor values and no 0.1uf cap on pin 5) http://i.imgur.com/MIwQscV.png. So here's the problem. The 555 timer outputs a really nice beautiful square wave, but once I try to switch it with a MOSFET I get this http://i.imgur.com/XzkTa50.jpg. And here's the part that confused me, when I disconnected ground from source I got a normal square wave http://i.imgur.com/vG92d8f.jpg. When I plug in the ground in to source and tune down the voltage 0.5V/cm I can see a square wave. Power supply is a computer PSU 12 Volts. The MOSFET is IRF634B. Here's some pictures of the mosfet: http://i.imgur.com/uVu6DQp.jpg http://i.imgur.com/1uAYqlK.jpg. Maybe the long and thick wires of the MOSFET could be causing this, or because 0.1uf cap is not on pin 5? I would appreciate any help!

Dr. Dark Current

Sat Oct 05 2013, 03:00PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

your mosfet is dead

HV Boy

Sat Oct 05 2013, 03:01PM

Registered Member #33572 Joined: Wed Aug 28 2013, 09:02PM
Location:
Posts: 9

Oh, well that explains everything. Thanks.
EDIT: I just tried another MOSFET. Still the same. Could it be I kill them when I solder them out of a TV PCB?

Patrick

Sat Oct 05 2013, 10:07PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Mosfets are kinda sensitive, mostly that gate... theyre real finicky, easy to puch through then your left with a useless device. can you make a test jig? just the the right voltage switched to the gate to light up an LED with current through the source to drain terminals.

PS ill assume you have the gate voltage put to the device across the gate and source pins right?

Dr. Dark Current

Sat Oct 05 2013, 10:12PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Hmm, then maybe you got the pinout wrong? Its GDS when you look at it from the front. I have never damaged a MOSFET by desoldering or handling it.

HV Boy

Sun Oct 06 2013, 07:33AM

Registered Member #33572 Joined: Wed Aug 28 2013, 09:02PM
Location:
Posts: 9

Patrick wrote ...

Mosfets are kinda sensitive, mostly that gate... theyre real finicky, easy to puch through then your left with a useless device. can you make a test jig? just the the right voltage switched to the gate to light up an LED with current through the source to drain terminals.

PS ill assume you have the gate voltage put to the device across the gate and source pins right?

Sure I can try testing it. And by putting the voltage across do you mean putting a resistor between Gate and Source? If yes then no, I haven't put a resistor between them.
Dr. Dark Current I'm pretty sure I didn't mistake the pinout.

P.S. I tried with a normal NPN transistor. Still doesn't work. I tried 2n2222 and KT872. Could it be possibly the power supply which is faulty or maybe the circuit? I also noticed a strange thing that when I push the 0.1uf cap to the board my oscilloscope shows that the frequency is decreasing and when I don't push it the frequency starts slowly increasing again.

Dr. Dark Current

Sun Oct 06 2013, 02:35PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

This circuit can not drive bipolar devices directly, but it must drive the MOSFET well. I think you got the pinout wrong.

Patrick

Sun Oct 06 2013, 03:40PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

what country do you live in? can you goto a radio shack and get a IRF510? theyre expensive but we can be sure your starting with a working one. (And it shows the pinout)

im thinking as Dr Dark Current youve got the pins wrong...

Pics

HV Boy

Sun Oct 06 2013, 05:22PM

Registered Member #33572 Joined: Wed Aug 28 2013, 09:02PM
Location:
Posts: 9

Patrick wrote ...

what country do you live in? can you goto a radio shack and get a IRF510? theyre expensive but we can be sure your starting with a working one. (And it shows the pinout)

im thinking as Dr Dark Current youve got the pins wrong...

Pics

I live in Lithuania and I'm also in a really rural area so I really cannot get an MOSFET. Also as I said, a normal NPN transistor doesn't let through the square wave too.

Dr. Dark Current

Sun Oct 06 2013, 05:30PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

HV Boy wrote ...

Also as I said, a normal NPN transistor doesn't let through the square wave too.

This is expected.

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