Subject: [Shop-talk] Which diodes for a battery charger?

From: "Karl Vacek" <[email-masked]>
Date: Tue, 26 Feb 2013 21:39:50 -0600

My old Schumacher battery charger has been dead for years.  Used to be able
to go to the factory for new diodes, fixed inexpensively while I waited, but
they moved away.  I'd like to repair it with better diodes this time.  I'll
also put a muffin fan into the case to help cool the diodes and the huge
transformer.

The present diodes are little buttons that appear to just be clamped between
a brass contact plate and an indentation in the aluminum flat plate heat
sink.  They're Motorola diodes, MR2502 on one transformer lead (a pair) and
W9312 on the other transformer lead.  The charger is 10 amps continuous, 30
amps fast boost (duty cycle 10 minutes on, 12 minutes off), and 50 amps
motor starting (duty cycle 2 minutes on, 5 minutes off)

I look at diodes and see all sorts of ratings, and don't know what's better
and what's overkill.  May I please have a recommendation as to what rating,
etc.?  A part number to order from XXX company would be even better.

Also - are there positive and negative orientations to the diodes in there
bow?  They're not marked, and if their polarity is related to label
orientation, then one pair is mounted with both oriented the same direction,
while the pair on the other lead are oriented opposite each other.

Thanks!

Karl
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From: [email-masked]
Date: Wed, 27 Feb 2013 05:14:18 +0000 (UTC)

Any diode tough enough to be used in a battery charger will not be damaged by an ohm-meter, so you can figure out the polarity that way.

Phil Ethier West Side Saint Paul Minnesota USA
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> From: "Karl Vacek" <[email-masked]>
> To: [email-masked]
> Sent: Tuesday, February 26, 2013 9:39:50 PM
> Subject: [Shop-talk] Which diodes for a battery charger?
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From: "Randall" <[email-masked]>
Date: Tue, 26 Feb 2013 23:01:03 -0800

> The charger is 10 amps 
> continuous, 30 amps fast boost (duty cycle 10 minutes on, 12 
> minutes off), and 50 amps motor starting (duty cycle 2 
> minutes on, 5 minutes off)

That's interesting.  I have an old Schumacher charger with very similar, if
not identical, ratings; but it only uses two diodes.  I guess they decided
that two more diodes were cheaper than the extra transformer winding.

Anyway, when the original diodes gave out (and it was given to me), I
repaired it with the diode plate from an old car alternator.  Car
alternators are 3 phase and usually only one diode fails, so you can still
get a full-wave (single phase) rectifier out of a bad alternator.  And they
are some seriously heavy duty diodes, especially if you arrange for some air
flow across the mounting plate.  I just hung it on some standoffs, and let
the air convect over it.  That had to be over 20 years ago now, and it's
still going strong.  In fact, I used it to start the TR3 yesterday (looks
like maybe my battery is going bad).

But a MR2502 is apparently only rated at 25 amps, which seems way low for a
50 amp charger.
http://goo.gl/5OBz8

Maybe that's why they didn't last very long?

If you don't want to mess with alternator diodes, here's some that should be
good enough.  60 amps continuous, over 1000 amps peak. $3 each.
http://goo.gl/MJHBO
Here's a datasheet
http://goo.gl/3iYq0

Be sure to use insulators where appropriate, and a little heat sink compound
wouldn't hurt either.

-- Randall 

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