Well I guess that's what I get for doing these kinds of things late a
night when my mind is restless! D'Oh! Well I thank you for pointing
out the blatantly obvious. Would the slow switching times lead to a
hot inductor? I would assume so from your post.

--- In randrdesulfatorforum@yahoogroups.com, "Bruce Miller"
<bmiller@...> wrote:
>
> Switching a FET for a bipolar transistor will make a huge difference
in the
> circuit operation. Typically FETs are much faster & require less turn on
> energy. Rise & fall times for a FETs can be much faster if proper
gate drive
> is used. Because the switching times for FETs are so much faster - the
> switching losses (heat generated) are much lower.
>
> -Bruce
>
>
>
> -----Original Message-----
> From: randrdesulfatorforum@yahoogroups.com
> [mailto:randrdesulfatorforum@yahoogroups.com] On Behalf Of Rob
> Sent: Thursday, December 11, 2008 5:04 AM
> To: randrdesulfatorforum@yahoogroups.com
> Subject: [randrdesulfatorforum] PIC Low Peak Voltage and Hot Inductor
>
>
>
> 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.
>