[Open_electroporator] HV board reading 'shorted' between 20V and GND

Nathan McCorkle nmz787 at gmail.com
Sun Apr 2 20:44:42 UTC 2017


On Mon, Mar 27, 2017 at 6:19 PM, John Griessen <john at cibolo.com> wrote:
> I'd verify transistor function by seeing pulses after boththe transformer
> driving transistors.

Hmm, couldn't get that far since the power was shorting out.
I got new transistors today, along with some schottky diodes (1N5819).

Swapped the transistors and added the diodes between the drain and 20V.

But the high-side is much weaker (2-3 times) and I get 60Hz ripple
that looks very significant... so I am pretty confused.
I was probing with the same setup, and the same amplification as before.
I can only guess that something else on the high-side is damaged and
acting like a really nice antenna for the mains frequency... or maybe
something in the coil got damaged from arcing (what is the
arc-rating)?

I will try removing the Schottky diodes too, and see if anything changes.

An odd thing I noticed, scoping on PA1 (the microcontroller pin going
to the gate of one MOSFET) was that after the pulse, there was a
capacitive-discharge tail/bleed-down... as if the pull-down was too
weak or something. I checked the pull-down and it was about 3kOhm...
not matched to the other transistor's slightly-weaker ~4.5kOhm
pulldown (which was not showing the odd non-square falling edge).

Maybe this replacement MOSFET is bad, I have a bunch more. I should
have thought out my last Mouser order and grabbed all the HV side
components too, guess I will place another order tonight.

As I said, I'll try removing my diode additions and see if things change.

pics here: http://imgur.com/a/il2vy


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