Re: RLC meters

On Sun, 17 Sep 2006 01:37:13 GMT, ehsjr <ehsjr@xxxxxxxxxxxxxxxx>
wrote:



Glad you like it! I wish the display had a little more contrast.
That hasn't bugged me enough to do anything about it, so it is
a minor annoyance. (Yes, I adjusted it to max.)

What the hell, grab a flashlight. Luxeon -based flashlight, of
course! I have a flashlight next to my microscope, another at the
lathe, and always one in my pocket. Luxeon-based, of course...

BTW, an immediate app for this little LC meter will be to measure the
inductance of the four 17-foot 8-gage wires going from the batteries
in the stern of my boat to the trolling motor socket in the bow.

I made a dirt-simple 50% dutycycle PWM to reduce 24VDC down to 12VDC
(average, no filtering) to drive my 12V boatlift motor off the 24V TM
socket. Current drain steadystate is about 43 amps, probably not a
lot more starting surge because of the nature of the beast. I used
120-amp MOSFETS.

http://users.goldengate.net/~dforeman/PWM_50/

The box didn't even get warm. The boat went up, the boat went down...
but then it wouldn't go up again. Kaputski. KIA. WTF, over?

Then I calculated the inductance of 17 feet of 8 gage wire. Oh my!
About 8 uH per wire and there are four wires. There's surely some
field cancellation in the parallel wires, but who knows how close
together they are in the hull. Mechanical switch on loadside opens,
no prob on output side -- but I bet there was one hell of a spike on
the supply side. Pfffft! Post mortem shows that the halfbridge FET's
are dead, no charring but clearly dead. They could probably handle
the avalanch energy but the 24 V supply was there to finish the job
once a spike had punched a path. Oopsie!

So now I want to measure that inductance to see just how bad that
problem is and what it might take to fix it in terms of capacitance
and/or transient suppression. It may take more than I'll want to
bother with.

What I'll probably do is fuggedaboudit and continue to run the lift
winch off of just one battery and maybe rewire the plug twice a season
to distribute the load history between the two batteries. (There are
lines from both batteries to the TM socket) Takes 5 minutes to
rewire the plug and I can do that when it's too windy to go fishin'.
.

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