Battery Charger parts

[Andy C]

I bought a Marquette 39-100 battery charger at a sale yesterday. It appears the rectifier is not working. I get 12 volts DC output at maybe a few amps but if I switch the voltmeter to AC I have about 6 volts. Took the rectifier off and it looks as if it has been hot. The transformer has aluminum windings but does not appear to have been hot. I didn't pay too much for this thing but would like to learn to troubleshoot. I would also like to understand HOW a rectifier (like the one that has plates in this machine) works. The plates appear to have a soft metal coating. This one also has a center tap which is wired to the one side of the output the other 2 wires go to the transformer. Is this the 6 volt/12 volt part of the system. Thinking I know about just enough to make trouble but gotta try to learn.
Thanks,
Andy
 

[George Marsh]

Andy, A rectifier is a diode. A diode allows electrons to go through it in only one direction and not the other, cathode to anode. All chargers I've seen have 2 diodes and the secondary of the transformer has 3 wires. The middle wire is called the center tap. Google full wave power supply or battery charger if you want to know all the ins and outs of a charger. Diodes can be made using different materials. Edison was the first to invent the tube diode. Today's diodes are mostly solid state diodes.

If you have a smart charger, it has a brain box that turns the charger on and off. Sometimes that goes bad.
 

[Russ from MN]

I have taken bad rectifiers out of chargers that have gotten wet and ruined and replace them with diodes. They work but are not automatic, full charge all the time, so I put a timer in it. At least that's the way I remember doing it, it was 20 years ago!
 

[k6zrx]

A battery charger puts out pulsating DC since there are no filter capacitors in them. Seeing AC is normal and lower than expected DC reading is normal, too. The more modern units will have filtering since they have other circuits that need pure DC. When it is on a battery, the battery acts as the filter.

Maybe you have had an old tube type radio that hummed? That is bad filter capacitors that allow the DC to pulsate a bit.

Try it on a battery and it will probably be fine.

The fins on the rectifier diodes are there to get rid of heat. A diode is like a check valve in this application. It only lets current flow in one direction. The center tap on the transformer allows both the positive and negative excursion of the AC to be used.

Josh
 

[MOFife]

Everytime I come here I am so impressed by everyone knowledge of things.