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SHO Problem

Posted: 26 Apr 2013, 18:56
by guitarist1977
Hi All,

I am a newbie here and have not worked with schematics for a long time. So i decided to build SHO since schematic looks quite easy. I took veroboard layout and schematic from here https://hotbottles.wordpress.com/2012/03 ... m-scratch/

The only difference is I used IRF510 instead of BS170.

I build it and it works ok. But then I decided to measure voltage at the MOSFET and it reads
D to ground- ~7.5V
G to ground - ~0.7V
S to ground- ~1.7V

And it is very strange I know it should be something about D -8V,G-3Vand S-1V respectively. But anyway how it even works with gate having lower voltage than source?

Any ideas?

Thanks,
Alex.

Re: SHO Problem

Posted: 26 Apr 2013, 21:14
by Duckman
Hi, guit... welcome on board!
Have you searched in the forum?
Various threads about SHO and related "erection" problems! :thumbsup
The search function is your friend...

Re: SHO Problem

Posted: 26 Apr 2013, 22:18
by guitarist1977
Hi Duckman,

Thanks for the tip. And I actually used the search before posted my problem. But I found nothing related to the issue I have.

Thanks.

Alex.

Re: SHO Problem

Posted: 27 Apr 2013, 03:12
by guitarist1977
I guess I found the problem. There is nothing wrong with the schematic. I just can't measure voltage at the gate with the ordinary multimeter because I use 10M resistors. Basically in this case multimeter gives me wrong readings.

EDIT: Just checked multimeter specs, it has 1M input impedance. When measuring gate voltage I basically put 1M resistor in parallel with the 10M gate to ground resistor which moves the bias. I simulated it in LTSpice and got the same results as from multimeter.

Re: SHO Problem

Posted: 29 Apr 2013, 07:43
by Duckman
An obvious point, but... BS170 pinout is DGS and IRF510 is GDS... did you check that?

Re: SHO Problem

Posted: 30 Apr 2013, 20:41
by guitarist1977
Thanks for suggestions. In fact my schematic worked in a first place. I was just concerned about multimeter's readings. But now that solved.