HELP! Battery Charger

Discussion in 'General Electronics Chat' started by roadey_carl, Sep 4, 2010.

  1. roadey_carl

    Thread Starter Active Member

    Jun 5, 2009
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    Hello again folks,

    I've got this bosh charging unit that I really need to fix.
    The symptoms are everytime its plugged in it goes "bang" so I know there is a short somewhere
    but I cant seem to find it.
    Lucky I have another bosh charger exactley the same type that works so I have been able to swap compoments round
    and see whats working and whats not.

    Here are some pictures: [​IMG] [​IMG] [​IMG] [​IMG] [​IMG][​IMG]http://img251.imageshack.us/gal.php?g=sdc10228w.jpg

    so far I have tested from the 230v ac supply:

    Capacitor - Working
    NTC 15K @ 25*c - Working
    100UF 400v Capacitor - Working
    Transformer - Working
    P5NK80Z MOSFET - ( the one mounted on the heatsink next the the incoming 230v ac supply ) originaly faulty but has been replaced - Working
    4 x DIODES that you can see I've replaced - All were faulty but now been replaced so are working
    inductor/transformer - ( the one with the silver bar and equal coper windings on each side) - Working
    Yellow inductor - working
    FAST EFFICIENT PLASTIC RECTIFIER - ( the one mounted on the heatsink next to the yellow inductor) Working

    All these's compoments have been tested sepratley or on my other charging unit!

    I really don't know what else to try.

    Thanks in advanced!
     
  2. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    Could you post pictures of the PCB? I'm thinking of an actual short on the PCB. Perhaps even a solder whisker or manufacturing defect. Since you have apparently replaced all the other components.
     
  3. roadey_carl

    Thread Starter Active Member

    Jun 5, 2009
    116
    5
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    You say you tested the transformer...how did you do that? and do you have access to a Hipot resistance tester? It may be a short that doesn't show up at the low voltage levels used by a fluke type meter, but will show up with a Hipot test using 200 to 300 volts.
     
  5. roadey_carl

    Thread Starter Active Member

    Jun 5, 2009
    116
    5

    I tested the transformer on the other charger I had , the transformer thats in there now is actully the one from the working transformer so I know its good!

    I dont have accsess to a Hipot resistance meter.... I've been using a 15v 1a AC transformer and touching it on the live and neutral supply side... and if it sparks I know there is still a problem. I thought id better to that to save damaging any other compoments!

    anymore ideas?
     
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    That may not indicate anything except that a cap is pulling a charging current.

    When you say it goes 'bang', what do you mean? The circuit breaker for the AC wall socket is tripped, or the circuit breaker in the charger is tripped? More description is needed for any chance of successful 2nd hand troubleshooting.
     
  7. roadey_carl

    Thread Starter Active Member

    Jun 5, 2009
    116
    5
    the fuse in the charger goes bang. the fuse in the charger is a 1A and the fuse in the plug top is a 5amp. when I test it with mains voltage I use a 0.5A in the charger just so I don't damage any more components.
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Blows the full 1A fuse or just the 0.5A?

    To verify that the transformer isn't at fault unhook the secondary wires and plug it in, should just sit there. Take an AC voltmeter and verify it's putting out around 16 VAC (or 32 VCT if it's that kind) on the secondary wires.
     
  9. roadey_carl

    Thread Starter Active Member

    Jun 5, 2009
    116
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    It blows both fuses. it originally had a 1a fuse in it but as I have tested it a couple of times with 230v I decided to use a 0.5a just to reduce the risk of damage I'll test the tranny but im sure its working as its from the original charger that was fully working
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Sometimes you've got to start from the beginning again.
     
  11. roadey_carl

    Thread Starter Active Member

    Jun 5, 2009
    116
    5
    would you recomend forgetting everything ive tested and just starting from scratch then? what would be the first componets to test? using a logic approach I guess...
     
  12. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    From the looks of the circuit board, the power supply is a switching supply. Is it blowing both fuses without the battery being connected? I see where the line fuse mounts, near the BLUE and BROWN wires. Where is the output fuse located?
     
  13. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    I would start with how old are the batteries you were charging because chances are it is a bad battery that overloaded the charger and caused the failure.

    If you have it charging on your good charger then it could kill that too.

    It could also be a newer battery that was dropped/damaged or has seen very heavy use.

    Even a battery that has been frozen or overheated. Left in a vehicle parked in the sun it could have been baked at 60 Celcius which might be enough to cause problems.
     
    Last edited: Sep 4, 2010
  14. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
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    Look at the two electrolytics near the output to the battery.

    They are the input and output filters to the switcher that actually feeds current that charges the battery.

    Good chance one or both of them is shorting.

    Do you mean that this is a Bosch battery charger?

    Give us the full model name and somebody might be able to come up with some service documents or schematics.

    Otherwise - have you considered replacement?

    It sounds like you have already put in at least 4 hours work on this.

    What is your time worth? What does a replacement cost?

    Keep the dead one for spare parts.
     
  15. roadey_carl

    Thread Starter Active Member

    Jun 5, 2009
    116
    5
    There isn't a output fuse... I think it works on the battery matching the slot on the charger.... the batterys are 2.0Ah and the charger can charge up to 2.5Ah... anything about 2.5Ah is a diffrent type of slot.

    I've probably spent about 12 hours on this as im quite slow and not very experanced. I don't mind buying a new one now but Its really anoying, its the 2nd charger i've had that has done this.

    the batterys are all fine. I've tested all the caps now ( apart from the really small ones) and they are fine.

    also if it helps, the 4big diodes you can see get really hot when I connect up my temp 15v ac supply... if I kept it on there to long I think they would breakdown.

    the model is : AL 60 DV 2425
     
    Last edited: Sep 5, 2010
  16. roadey_carl

    Thread Starter Active Member

    Jun 5, 2009
    116
    5
    Any more ideas anyone?:)
     
  17. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    You could cut traces as needed to separate the input supply from the switching circuit. If things still go south, you will know it is in the input section. If things hold up and you develop the high voltage DC OK, then you know it is in the switching or output section.

    Another thing to try is to apply a DC voltage (proper polarity observed), equal to the normal output voltage, to the output terminals with no input power supplied to your charger. It should sit there with no heat build up on any components.

    Switching supplies depend upon every element to do its job properly. Any anomaly in almost any component can result in catastrophic failure.
     
  18. roadey_carl

    Thread Starter Active Member

    Jun 5, 2009
    116
    5

    Ok sounds good i'll give it a go! Thanks for your help :D
     
  19. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Those diodes are there to rectify 230 VAC, they shouldn't be bothered AT ALL by a 15 VAC source. Either you have a bad diode (I know you 'fixed'/checked them), or you have a short just after them which is drawing mucho current through them. Check the input side(output from the transformer) of the diodes to ground. Do this twice and reverse your meter leads for the second check. You should get a very high resistance, or open reading one way, and a very low reading(conducting) the other way. You should probably unsolder them from the input side before checking, to prevent a parallel situation with something else in the circuit.
     
  20. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
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    And from another website -
    Older batteries are likely to be smaller Amp Hours and need lower currents.

    If you happened to be using the charger with smaller batteries than it is meant for then that could cause problems.

    It will be a current sensing switcher. It is likely to have a minimum current that it is meant to provide and the mentioned adaptor could be meant to add a parallel resistance to the smaller batteries to prevent current dropping below that minimum current too soon.

    Dropping below the minimum current will leave the switcher trying to narrow the pulse width towards 0 seconds which causes heat and destructive high voltage noise.

    When a charger is designed for such large range of voltages that really only means they made all of the batteries similar in their charging currents requirements and confirms that this is a current sensing regulator.

    I needed to say all that first, but it is probably too much current instead of too little.

    The coils and transformers were all good.

    The capacitors were all good.

    The Diodes heat up.

    Lets go back to the capacitors. They could be leaky and breaking down under voltage. I will assume you mean that they worked when switched to the working charger. They will tend to "work" even if they are going bad. They heat up with increasing Equivalent Series Resistance and they transition towards decreasing Equivalent Parallel Resistance as they start to short and overload their input circuits.

    You could test them with an Ohm meter and you might want to sit them on top of a heating pad or other warmer that will get them up towards 40 Celcius and check them while they warm.

    Good capacitors will be Open circuit or in the case of Electrolytics they might show MegaOhm resistance. Beware that not every Ohm Meter likes it if you measure a capacitor. You might want to parallel them with a 500k resistor.

    In order to measure leakdown you should charge them up (getting them out of circuit and respecting polarity and voltage ratings) and check them about every 5 to 10 minutes to see if they are holding their charge.

    Another way to test them is to put a large resistor in series after charging the capacitor. Leaving the capacitor and resistor connected to the power supply, after 1 Minute if a 500k resistor has any voltage across it to indicate a current through it then your capacitor is leaking.

    All of the charging of a possibly faulty capacitor has the danger of it exploding, added to the possibility that you might overvoltage it or connect it with reverse polarity. Please wear eye protection. Especially with this last test you should be careful. Connect it with a SMALL resistor, the minimum to limit the current to what you power supply is rated for. If you have a 1 Amp power supply and you are testing a 10 Volt capacitor with the power supply set at 9 Volts ( You will tell if its bad without going to 100% of rating) then use a 10 Ohm resistor as an example. If the resistor is less than 2 Watts and the capacitor is really bad then be prepared for that resistor to be a sacrifice. Leave the capacitor connected. Check it for heating. If it get noticeably warm at all then it is bad and you can stop testing it. It should have no current and if it gets warm it is breaking down. It could even explode - but smoke is more likely.

    But the hot diodes sounds like you had transistor that overheated and melted itself into a nice short. That transistor failure is normally an indicator of an overload and the bad caps are a likely cause but there are other possible reasons so even though I think the output caps are to blame I am not counting on it.
     
    Last edited: Sep 6, 2010
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