Michael A. Terrell wrote:
John Popelish wrote:
Michael A. Terrell wrote:
John Popelish wrote:
RW Salnick wrote:
(snip) You won't need to make any changes to the pulley to
make the
alternator deliver 24 V, but the the diodes and the diode trio will need
to be replaced with 24V versions.
(snip)

Are you seriously suggesting that a 12 volt alternator is
made with a diode trio that cannot handle 24 volts? I was
under the impression that few silicon rectifiers are made
with a breakdown voltage less than 50 volts.

What PIV diodes do you think you need for 24 volts?
Need? 35 volts should be enough to work, but I would like
at least 100 volt.


You are forgetting that when the diode is off, it also has the
battery voltage added to the AC from the alternator, so the bare
minimum would be 24 VDC +(24*1.414=33.936-.6) 33.336, or 57.336 PIV, if
there are no spikes or inductive surges.

I'm not forgetting anything, that is not how it works.
there is a 3 phase bridge rectifier across the AC with the
negative side of the output to ground (so no AC swings can
go more negative than a diode drop below ground). There are
two positive sides to this bridge, connected in parallel to
the 3 phase AC. One set of 3 provides the DC output to the
battery, and the other 3 diodes provide the DC to the field
excitation regulator. But, since the 3 to the battery
prevent the AC from exceeding the battery voltage by more
than a diode drop, None of these 9 diodes sees more reverse
voltage than the battery plus a diode drop.

A load dump can be several
hundred volts, and the diodes have to survive.

This, I agree with, and this is why the main rectifiers as
well as the trio diodes all have PIV ratings in the hundreds
of volts.

What PIV diodes do you think they put in the trio?


The last set I replaced were unmarked. All they had was an OEM part
number. One of the trios opened on the way home from Orlando after
midnight one night. I was laying in wet grass and sand removing the high
output alternator from my stepvan, only to find it was too big to fit my
car. I had to strip both alternators and transplant the diodes to get to
work on time.

What does this failure tell us about the PIV rating of the
trio? Not much, I think. You don't know that thew trio
failed from excessive voltage. Could have been a bond
failure that overheated one of them.

John Popelish wrote:

What does this failure tell us about the PIV rating of the
trio? Not much, I think. You don't know that thew trio
failed from excessive voltage. Could have been a bond
failure that overheated one of them.


All three diodes were open.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Michael A. Terrell wrote:
John Popelish wrote:
What does this failure tell us about the PIV rating of the
trio? Not much, I think. You don't know that thew trio
failed from excessive voltage. Could have been a bond
failure that overheated one of them.


All three diodes were open.

So what do you conclude caused this failure, if anything?

John Popelish wrote:

Michael A. Terrell wrote:
John Popelish wrote:
What does this failure tell us about the PIV rating of the
trio? Not much, I think. You don't know that thew trio
failed from excessive voltage. Could have been a bond
failure that overheated one of them.


All three diodes were open.

So what do you conclude caused this failure, if anything?


I have no idea. All I found was four lugs and burnt plastic ash.
There was nothing left to do failure analysis on.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Michael A. Terrell wrote:
John Popelish wrote:
Michael A. Terrell wrote:
John Popelish wrote:
What does this failure tell us about the PIV rating of the
trio? Not much, I think. You don't know that thew trio
failed from excessive voltage. Could have been a bond
failure that overheated one of them.

All three diodes were open.
So what do you conclude caused this failure, if anything?


I have no idea. All I found was four lugs and burnt plastic ash.
There was nothing left to do failure analysis on.

I would suspect an intermittent short between the rectifier
output and ground. But, I guess, if a high enough voltage
transient occurred, it could have shorted a rectifier and
then that short blew the other two. Of a diode bond failed
enough to overheat one die, causing the same cascade.

But do you agree with my analysis of the normal voltage
applied to these diodes? You clipped it without comment.