Car battery charger repair

Discussion in 'General Electronics Chat' started by Lm741, Sep 17, 2008.

  1. Lm741

    Thread Starter Member

    Sep 2, 2008
    Hi there.

    I managed to blow up my car battery charger earlier ( fuse did its job then :rolleyes: ) ripped it apart and soon realised that the diodes had some 'slight' damage.

    What i'd like to know is whether they can be repaired? The legs have been soldered into the heatsink thingys that the transformer connects to. It appears to be in two parts which have sealed the legs in between.

    I can't see how they can be repaired without pulling them apart to free the diodes. Any advice would be appreciated.

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    I melted a pair down a few years ago. I got a pair of stud mount (might have been 1N1189) diodes to replace them - but I had a much larger and better spaced heat sink to work with. Got any room in the charger case?
  3. mik3

    Senior Member

    Feb 4, 2008
    If the diodes has blown up the you cant repair them. You must replace them with new ones. Are they burned outside (check the diode package)?
  4. Lm741

    Thread Starter Member

    Sep 2, 2008
    Sorry I didn't make myself clear, I have no plans on repairing the diodes themselves, as you can see, they are in two seperate pieces! :eek:

    The diodes are labelled PN BY550-50 and after a quick search I found a data sheet for similar diodes :

    Again it was a quick search, I have not had time to find a supplier yet.

    There is a little bit of space in the charger case, but to be honest I might be tempted to make another case. Would make a good mini project :)

    Either that or I was considering buying another charger and maybe using the transformer out of this broken charger towards making my own power supply, again it'd be an interesting project.

    What do you think?
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    The cheapest way to replace the diodes may just be a bridge. Any 50 volt 35 amp unit will do, although find a good place to sink it may be interesting. That may be less expensive than two big rectifier diodes.
  6. awright

    Well-Known Member

    Jul 5, 2006
    Assuming that your diagnosis that only the diodes failed is correct, repair is trivial. You have many options for repair using stud or axial lead or tab-mounted diodes.

    I recommend repair rather than replacement because it is good training and confidence-building for more advanced repairs on some more valuable, less easily replaced item in the future.

    I couldn't see enough detail in your photos to be able to give specific advice, but if the old diodes were, in fact, soldered to the heat sinks then you can solder replacement axial lead diodes in the same location. If the originals were crimped in place and soldered, you don't have to go to any great trouble to crimp the new ones before soldering. The crimp was probably for convenience in assembly. Solder alone will be adequate.

    You can also simply drill holes in the heat sink and install stud-mount diodes or use a 6-32 screw and nut to clamp one lead of the diode to the heat sink. Just be sure to orient the diode with the correct polarity. This is easily done with axial lead diodes, but for stud mounts, you have to purchase diodes of the correct polarity.

    For rectifier diodes in a battery charger, practically any diode with at least the current and voltage ratings of the original diodes will do. You can use any diodes with higher current and voltage ratings without concern and with some added insurance against failure, but I wouldn't spend much extra money to do that. Since you have specs on the original diodes, look at the "surge current" rating and try to use replacement diodes with the same or higher rating.

    If you mount axial lead diodes with screws and end up with longer leads between the diode body and the heat sink, you will probably have poorer heat dissipation via the leads than the original, soldered connection. In that case, using a little higher current rated diode could be beneficial.

    Consider also using a slightly lower current fuse, since the original obviously didn't protect the diodes. Also, be sure to use a "fast" fuse, not a "slo-blo" or time delay fuse. You can sometimes (but not always) recognize a fast fuse that you can see inside of because it has only a single, thin wire inside the glass tube. Fuses with little springs, ceramic-looking bars, or any other junk inside the glass are normally time delayed. You can also find out what the type code on the fuse means at the manufacturer's website. For fuse types that you can't see inside, just buy "fast."

    Have fun.