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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work.

Selenium Rectifier replacement



 
 
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  #1  
Old July 20th 05, 04:54 PM
Brad H
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Default Selenium Rectifier replacement

I need to replace the selenium rectifier stack in an old battery
charger to bring it back to life. The battery charger is rated at
7.5V-80Amps dc and 14.5V 50Amps DC. It is set up with multiple primary
windings selected by a switch. I verified that the transformer is
good.
Does anyone have a recommendation for a good replacement rectifier
diode set that will handle this current? Will I have to use separate
diodes and build it to get 80Amp capability?
The amp meter is also shot, and I might entertain replacing that if its
cost effective. Any ideas on a small 2" x 2" panel mount 80A dc meter?

Ads
  #2  
Old July 20th 05, 05:43 PM
jim rozen
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Default

In article .com, Brad H
says...

I need to replace the selenium rectifier stack in an old battery
charger to bring it back to life. The battery charger is rated at
7.5V-80Amps dc and 14.5V 50Amps DC. It is set up with multiple primary
windings selected by a switch. I verified that the transformer is
good.
Does anyone have a recommendation for a good replacement rectifier
diode set that will handle this current? Will I have to use separate
diodes and build it to get 80Amp capability?
The amp meter is also shot, and I might entertain replacing that if its
cost effective. Any ideas on a small 2" x 2" panel mount 80A dc meter?


You can replace the Se unit with any silicon bridge, or make one
up from individual diodes - as long as you keep the 80 amp number
in mind. Go to digi-key and search.

You can replace the amp meter with any small milliameter by using
a shunt formed from a short piece of smaller gage copper wire.
Some experimentation required for that. It will be easier to find
a small milliameter like that then a surplus 80 amp meter.

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================
  #3  
Old July 20th 05, 05:55 PM
Jerry Martes
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Default


"Brad H" wrote in message
oups.com...
I need to replace the selenium rectifier stack in an old battery
charger to bring it back to life. The battery charger is rated at
7.5V-80Amps dc and 14.5V 50Amps DC. It is set up with multiple primary
windings selected by a switch. I verified that the transformer is
good.
Does anyone have a recommendation for a good replacement rectifier
diode set that will handle this current? Will I have to use separate
diodes and build it to get 80Amp capability?
The amp meter is also shot, and I might entertain replacing that if its
cost effective. Any ideas on a small 2" x 2" panel mount 80A dc meter?


Brad

Do you have access to broken alternators. Their diodes can be used if
cost is a consideration.
The amp meter from an old truck might be affordable too.

Jerry


  #4  
Old July 20th 05, 05:59 PM
Grant Erwin
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Default

Brad H wrote:
I need to replace the selenium rectifier stack in an old battery
charger to bring it back to life. The battery charger is rated at
7.5V-80Amps dc and 14.5V 50Amps DC. It is set up with multiple primary
windings selected by a switch. I verified that the transformer is
good.
Does anyone have a recommendation for a good replacement rectifier
diode set that will handle this current? Will I have to use separate
diodes and build it to get 80Amp capability?
The amp meter is also shot, and I might entertain replacing that if its
cost effective. Any ideas on a small 2" x 2" panel mount 80A dc meter?


Whatever you do it won't be cheap unless you get incredibly lucky. And you will
also need luck to find a drop-in panel meter.

Other than that, it should be easy to find a rectifier and meter for your
ratings, just watch ebay for awhile. Not too many buyers need those specs. When
you get your rectifier, take the time to find the actual datasheet, download it,
and read it for the manufacturer's suggestions on mounting especially heat
sinking, and use new heat sink grease. If you have to retrofit a new panel meter
with a slightly different footprint it isn't too hard to cut out panels with a
jeweler's saw, or else make a bunch of punches along your profile and then use a
small sanding drum in a Dremel tool (or carbide burr in a die grinder, or...) to
clean up. You can also chain drill, although I hate drilling sheet metal.

GWE
  #5  
Old July 20th 05, 06:37 PM
[email protected]
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Default

I've done a fair amount of selenium replacements in old TVs back in the
stone-age(tubes). You can replace the rectifier part of the function
with any suitably spec'ed silicon replacement, full-wave bridge units
are cheap. Check the usual electronic surplus joints on the web like
All Electronics, Digi-Key and Mouser are two retail outfits that don't
mind onesie orders. Surplus Center out of Lincoln, NE and C&H Sales
also have both rectifiers and amp meters from time to time.

The other thing about seleniums is that they've got quite a voltage
drop, part of the replacement procedure was adding a power resistor in
series to get the proper voltage to match the original selenium setup.
An 80 amp power resistor might be a little spendy. You really don't
want too high a voltage applied to your batteries or they'll boil and
overheat. The selenium rectifier added something resembling voltage
regulation to the circuit, too, with that voltage drop.

Stan

  #6  
Old July 20th 05, 06:51 PM
Chuck Sherwood
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Default

The other thing about seleniums is that they've got quite a voltage
drop, part of the replacement procedure was adding a power resistor in
series to get the proper voltage to match the original selenium setup.
An 80 amp power resistor might be a little spendy.


How about an extra diode in series with the output after the bridge?
This will drop the voltage about 1v under heavy load. Better be a
big diode with a heat sink though. This also protects the bridge
if someone hooks up the leads backwards. Without this diode, connecting
the leads to the battery backwards will provide a short circuit across
the transformer secondary. Been there, done that, replace the bridge
and added a fuse for the next time. A diode would have been better
than a fuse, but the extra voltage drop was too much for my transformer.

  #7  
Old July 20th 05, 07:36 PM
Tim Wescott
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Default

Chuck Sherwood wrote:
The other thing about seleniums is that they've got quite a voltage
drop, part of the replacement procedure was adding a power resistor in
series to get the proper voltage to match the original selenium setup.
An 80 amp power resistor might be a little spendy.



How about an extra diode in series with the output after the bridge?
This will drop the voltage about 1v under heavy load. Better be a
big diode with a heat sink though. This also protects the bridge
if someone hooks up the leads backwards. Without this diode, connecting
the leads to the battery backwards will provide a short circuit across
the transformer secondary. Been there, done that, replace the bridge
and added a fuse for the next time. A diode would have been better
than a fuse, but the extra voltage drop was too much for my transformer.

The resistance of the selenium rectifier would limit the current of a
charger; using another silicon diode would not. I would add the extra
diode if I could, but before I did that I would make sure that I had
enough resistance is the circuit to equal the equivalent series
resistance of the selenium rectifier.

It may be cheaper, in fact, to get your hands on a bunch of 5A
rectifiers and load them with lower power resistors rather than finding
a single 80A bridge (at the cost of more work, of course). Then connect
them like so:


___
.----|----|___|--.
| |
| ___ |
------o-o--|----|___|--o-o------
| |
| ___ |
'--|----|___|----'
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Just make sure that:

* the parallel equivalent resistance adds up to your desired resistance,
* the current capacity of the diodes adds up to your desired current
capacity,
* the voltage drop across the resistors is at least as much as that
across the diodes,
* the resistor power ratings are observed,
* and that if one of the diodes fails open, the others will follow
in short order.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
  #8  
Old July 20th 05, 08:16 PM
Brad H
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Default


So I can parallel either diodes alone, or rectifier blocks to get
increased current capability? I understand about the cascade failure
if one should go. Since the charger has about 6 taps on the primary,
for increasing rate of charge (simply higher voltages) can't one just
select a lower setting and ignore the voltage drop the old selenium
rectifier had? I'm not quite sure I understand how the old unit acted
as a voltage regulator. Is that just because it had resistance/voltage
drop that modern diodes don't have?
The old amp meter has a shunt of about 1/2" wide copper about
1/16" thick, so it was just a milliamp meter with a calibrated scale.
I suppose I could hook up something like an old toaster with a known dc
resistance to calibrate a surplus one if I wished.
Alternator diodes are a good idea. They might be rated high enough
to do the job individually, without paralleling them. I looked at some
online places and didn't see much over 50A with a quick peek, hence the
first post here.
The old unit has the selenium stack under a fan, so it should be
easy to mount the new rectifier on a surplus heat sink and place it in
the air flow.
This was a curb find, so at a minimum its a good high current/low
voltage ac transformer.

  #9  
Old July 20th 05, 08:21 PM
Lloyd E. Sponenburgh
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Default


"Brad H" wrote in message
ups.com...

So I can parallel either diodes alone, or rectifier blocks to get
increased current capability? I understand about the cascade failure
if one should go.


A tiny amount of resistance in series with each diode will help equalize
currents among them.

But this is getting crazy. Bridges with 500 Amp peak capacity are
inexpensive enough not to have to cludge something together. I don't think
your transformer will deliver that.

LLoyd




  #10  
Old July 20th 05, 08:29 PM
Chuck Sherwood
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Default

So I can parallel either diodes alone, or rectifier blocks to get
increased current capability? I understand about the cascade failure


You cannot parallel diodes directly because they will have a small
voltage mismatch and one diode will hog all the current. You
can only parallel diodes if you force them to share current
by putting a small resister in series with each diode.


if one should go. Since the charger has about 6 taps on the primary,
for increasing rate of charge (simply higher voltages) can't one just
select a lower setting and ignore the voltage drop the old selenium
rectifier had?


Seems very reasonable and worth a try.



 




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