Repairing a battery charger.

Hello,
I have a few old school 12v automotive battery chargers using the selenium rectifiers. They are rated at 10a yet the aged rectifiers will only produce about 2-3 amps. Being that they are hard to obtain I installed a bridge rectifier which now outputs the full rated amperage.
Being aware of the fact selenium rectifiers will reduce current as the battery becomes fully charged I wish to retain them.
These chargers are set up with 2 taps coming from the transformer feeding each of the selenium rectifiers and then out via the negative battery clip.
My question is can I simply add a switch on the selenium and bridge rectifiers output thus allowing the bridge to provide a high amp setting for faster charging, then switch to the selenium for a slower limiting charge?
My thoughts are making a "Y" in the transformer to rectifier wires and feeding the selenium off one set of "Y" and the bridge off the other then running their output through the switch to the negative battery clamp wire.
Both rectifiers will be fed from the transformer but depending on the switch position, only the bridge or selenium would be active.
Is this a viable endeavour or preposterous

I would imagine it would be fine, however, you simply need to add a series resistor to the silicon rectifier to limit the current and replicate the higher resistance of the selenium rectifier. You will need a suitable value high wattage wire wound resistor.

Nigel Goodwin said:

I would imagine it would be fine, however, you simply need to add a series resistor to the silicon rectifier to limit the current and replicate the higher resistance of the selenium rectifier. You will need a suitable value high wattage wire wound resistor.

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Nigel
Thank you for the reply.
I have a small selection of Ohmite through hole wirewound resistors. Some are vitreous enamel while others are silicone ceramic. What would be an appropriate value to use?
The reason I was substituting the bridge rectifier for the selenium is selenium are difficult to find and quite expensive also I have a few 50A 1000V bridge rectifiers left over from a 13.8V power supply project (used with my Amateur Radio transceivers) as well as several heat sinks from old computer CPU's.
I like the higher charging rate of the bridge rectifiers but the voltage climbs rapidly to over 14.8v before I unplugged it. Not so good for my batteries. The low wattage turn signal bulb is another route yet not my immediate choice.

Gonzo said:

These chargers are set up with 2 taps coming from the transformer feeding each of the selenium rectifiers and then out via the negative battery clip.

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That sounds rather like you are describing a half bridge connected to a centre-tapped winding - where does the positive output come from?

If you replace a half bridge with a full bridge, you will double the output voltage.
(Trying to visualise the setup from the description, so I may be wrong).

rjenkinsgb said:

That sounds rather like you are describing a half bridge connected to a centre-tapped winding - where does the positive output come from?

If you replace a half bridge with a full bridge, you will double the output voltage.
(Trying to visualise the setup from the description, so I may be wrong).

Click to expand...

Not being well versed on the proper terminology, I will attempt to describe it in detail.
A/C is fed into the front of the transformer. One leg goes directly into the transformer while the other goes through the 6V/12V switch then two wires into the transformer and green goes to chassis ground.
Three wires come out the back of the transformer. 2 wires feed the two selenium rectifiers and the third wire is the battery positive which goes through the auto reset circuit breaker, throught the amp meter and out to the (+) battery clamp. The battery (-) comes off the aluminum plate the selenium rectifiers are riveted to and goes out to the battery negative clamp.
The two chargers I am currently working on are both designed the same way and both rated at 10 amps. They both also output roughly max 2-3 amps regardless of what battery i connect them to. As they slowly charge the batteries, the voltage will also very slowly rise to about 13.8v.
I have already taken the 2 spade terminals from the selenium rectifiers and connected them to the bridge rectifier A/C terminals and the wire from the selenium rectifier aluminum plate to the bridge rectifier (-) terminal and it charges 10-11 amps while quickly climbing up to 14.8v according to my Fluke 88 DVOM.
My idea is as stated earlier. To keep the selenium available for a slow charge while adding the bridge for a initial quick charge via a switch to determine high or low charging rates.
Also I picked up a box of five Volt Amp meters with 100a shunt. They are quite accurate and capable of 4-48v and 0-100a. I couldn't resist as they were at a local Amateur Radio swap for $1 each.
These chargers still function even though they are probably 40-50 years old so I though of bringing them more up to speed without going overboard.
I guess I should add the bridge rectifiers I have are KBPC5010 rated at 50a 1000v. A little overkill but they only cost me $2 each new. They are extras from a pair of 13.8v 35a regulated powers supplies I rebuilt.

OK,
that's replacing a half bridge with a half bridge, so it should work fine.

I was just surprised you were getting such a drastically different current output with the battery at full voltage.

You could do exactly what you suggest and leave both sets of rectifiers permanently connected, then switch between them - or even leave the selenium output on all the time and also link in the output from the silicon rec with a straight on-off switch for "fast charge" mode.