Hello everyone, I have bought a few of the 555 timer circuits and a
pic version from courier in the uk and have an issue. I shelved the
pic desulfator for a while and recently came back to it and found it
was dead. Long story short I believe I hooked it up reversed when I
first built it and I replaced the MOSFET and drive transistors and it
seems to work. However I did not have an IRF9640 and so used an
MJE15031 I had laying around. While now it doesn't get scorching hot
immediately when connected the peak voltage (two diode, two cap
circuit) when hooked to a car battery is around 17-18 volts, and
that's with probably 4 inch leads or 12ga wire to some battery clamps.
It looked to be a rather small waveform on the scope and much wider
then the 555 based timer desulfator ones.

After the swap/repair I initially had it attached with twisted
alligator clips to a small electric car battery (the ones for little
kids) that I refilled with distilled water and initially read 0 volts.
There I was reading peak voltages at the battery clips of around
110-140 volts depending on how much I played with the trimpot etc.
Even with the 555's I was getting a good 120 volts or so. If I hook
up a 555 timer desulfator to the same automobile battery I seem to get
around 30-40 volts and a much nicer waveform with a quick rise time on
the leading edge. Since I can't find much in the way of specs on the
MJE15031 is that the cause of these issues? Are they not issues at all
as maybe the PIC does less voltage and more current into the battery?
I see the new MOSFET is only rated at 50W instead of the 125W of the
original but it doesn't give any on resistance or turn off time that I
can see or find. Any good/better substitutions for the IRF9640 if I'm
going to order a replacement? And lastly, perhaps the initial shorting
MOSFET has damaged the inductor, specifically the 1000uh one as that
gets pretty hot, any specs needed on a replacement or way of testing
for a short (suppose I have to unsolder and measure it with my meter)?
I appreciate your time.