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What's Wrong with my Diodes? - Page 16

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Posted by Tim Jackson on May 30, 2009, 4:36 pm
Ulysses wrote:

Don't forget the PIV in a rectifier circuit is the peak AC voltage PLUS
the rectified output voltage, ie roughly the peak-peak AC voltage or
about 3 times the RMS AC voltage.

You might want to check out the open circuit winding AC voltage to give
you a starting point for rectifier design.  While the full-load working
voltage will be a lot lower, usually half that, it is what your
rectifier has to withstand unloaded.

I think you said it was permanent magnet, in which case there is nothing
you can do to keep the volts down except to throttle back the engine,
and to withstand what you get in the worst case.

Changing the windings from star (wye) to delta connection will reduce
the AC voltage at the rectifier.  If that is feasible.

Tim Jackson

Posted by Ulysses on May 31, 2009, 2:59 pm


Time to go back and re-read a lot of stuff that I either forgot or never
knew :-|

So, is the diode breaking down and conductong in both directions when the
voltage exceeds the PIV?  That would explain why it is putting a load on the
alternator.  I would have expected that from a zener diode but I thought a
typical rectifier diode would simply burn out and become open if overloaded.

I'll do that again.  Problem is that I did some of this stuff quite some
time ago and I'm going by memory.  Lately I can't remember why I went into
the garage in the first place.  But I always find something interesting to
do so most of the time it doesn't matter.

While the full-load working

*IF* the voltage was going to end up low enough to use for battery charging
then yes, it's feasible.  The coil wires are pretty easy to get to and
there's enough room for some jumpers etc.  Also, IF I could get the voltage
down to below 144 I could consider using my OutBack charge controller.  I
tried doing something with it a while back but don't remember exactly why I
abandoned the idea.  Probably just too many volts.

Posted by Gordon on May 31, 2009, 11:45 pm

Yes, when the PIV is reached, the diode will conduct in the
reverse direction.  Not completely tho, there will be a
voltage drop on the diode roughly equal to the PIV. It
will be acting like a zener.

If you look at circuit diagrams that have zeners, you
will see that they are placed in the circuit in revere
bias.  The zener voltage is just a more tightly controlled

That's a bit simplified, but you get the idea.  

Posted by Eeyore on June 1, 2009, 11:48 am

Gordon wrote:

Actually usually an avalanche breakdown diode but commonly misunderstood.

Diodes in series is *dumb*.


due to the hugely increased level of spam please make the obvious adjustment
to my email address

Posted by Ulysses on June 1, 2009, 4:50 pm

I'm not sure why you are saying that--the diodes are not really in series in
this 3-phase rectification circuit.  I merely used "series" do describe how
the diodes were connected in a way that most people could easily understand
without seeing a schematic.  It's basically a bridge rectifier circuit plus
another half of a bridge.


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