Problems with industrial battery chargers and optoisolators

Hi I'm a freshman in the electronic workfield. I´ve been assignes to repair a 120 V AC to 12V DC Battery charger. It got a high current problem. I cannot locate the comparator in charge of keep the current level, because there's no circuitry drawing. So I think the problem is in the optoisolators.

A battery charger with optoisolators?

If I'm right they're used for high current trouble handling. Theres this burning marks on the anode input resistance wich means there is more current than is supposed to exist. So please someone send how to test one optoisolator and what are the mos common causes for the high current problems in battery chargers.

Most common - shorted leads or shorted diodes.

Optoisolators are commonly used in switch-mode power supplies and methinks you are in over your head.

Mission of Doom, I.M.O:

This sounds like a switching supply so there will be LETHAL voltages on one side of the Optocouplers, on filter capacitors, probably on heat-sinks too 'cause it costs USD 0.000001 to insulate them e.t.c.

Did they give you a RED shirt by any chance before assigning this mission to you??

Battery chargers are *cheap* - dump and buy new. No more than an hour of your wages should buy a new one, so that is how much time you have to waste on it before it becomes stupid!

Measure across the LED - there should be aaabout 1.2 V if it is On (and within the proper current range). Less than 0.8 and higher than 1.6 is dodgy.

The voltage across the transistor should be "low" if the led is On

The current transfer ratio of optocouplers decrease over time, this may cause the loop to fail.

The current through the LED might be too high because it comes off the output which is way too high anyway because of some other fault, this is causing the burning but is not the real fault.

Depends on what it is - if it is a switcher, the feedback loop is probably shot.

If it is a linear regulator then the series pass regulator is likely short as well as the chip/device driving it.

Where the heck do you get a battery charger for fifteen bucks? ;-)

Thanks, Rich

I got mine for free at the town's recycling center. Then again, I'm retired ;-)

Al

or

I collect bottles ;-)