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