Help required to diagnoise and repair 12vdc batt charger/supply

Discussion in 'The Projects Forum' started by DanW8KMX, Dec 15, 2014.

  1. DanW8KMX

    Thread Starter Member

    Oct 21, 2009
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    1
    When operating FIELD DAY our small group powers all radios from an automotive type 12VDC battery. One of our group a couple years ago made up three battery charger/power supplies to no only keep the battery charged but to supply some minimal current to power the radio if needed. These 115VAC power supplies were powered by a 300 watt small gas generator. That member has since passed away.
    During the last FIELD DAY, everything was going swimmingly until a couple batteries went below the threshold where the radios would no longer work. The decision by those operating was that the "battery/charger" was not working.
    The battery charger/supplies have now arrived at my work bench to determine what was wrong and fix it.
    Well, I can't find anything wrong with the units. Also, in looking at the schematic, I am not sure just exactly how the unit are intended to operate (assuming they were designed correctly initially and that they actually did work).
    Is there someone out there that can help me with this? Send me an email at w8kmx at bellsouth dot net and I can send you a scanned schematic.
    Thanks
    Dan
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    Just post it here as a pdf or PNG or whatever, and you'll get much more and better feedback.

    One thing I ALWAYS start with when things are delivered to me is to reproduce the claims of the device "not working". Make sure it really isn't working, and that the real problem isn't somewhere else.
     
  3. DanW8KMX

    Thread Starter Member

    Oct 21, 2009
    21
    1
    Schematic uploaded. Dan
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Looks a simple enough circuit, but not the best for a battery charger, what voltage do you get at the output, both unloaded and with a battery?
    Max.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    This is roughly what I would do.
    1. Measure output voltage, ±load as Max says, to confirm it is inadequate to charge a 12v battery, ie. below 13V.
    2. Check the fuse.
    3. Look for an AC voltage feeding the bridge rectifier. Should be at least 10V or so, but not more than 20V.
    4. Look for a DC voltage after the rectifier. Should exceed the AC voltage in #3. DC Out = 1.4 x AC in, minus 1.4. Example 12V AC * 1.4 = 16.8, minus 1.4 = 15.4V DC. You'll note that 1.4 x AC - 1.4 is the same as 1.4(AC - 1).
    5. Look for heat on the rectifier diodes, Q1, and the LM317.

    There can be more but I'd be surprised if you don't find something with these first 5 steps.
     
  6. DanW8KMX

    Thread Starter Member

    Oct 21, 2009
    21
    1
    Yes, it is a simple circuit. That is why I said I am not sure it will actually do what he intended it to do. I don't know what the "the best for a battery charger" is.
    The generic tests were done and all seems to be OK. I measured again so as to answer the post. Fuse is good and 115 VAC is getting to the transformer. 17 VAC is on the input of the bridge rectifier. I have good DC voltage after the rectifier. Voltage out without a battery attached is 13.75 DC. The voltage out with a battery attached is 13.75 and I measured a charging current of 1 amp (I do not have a 12 volt battery that needs charging). When the battery is removed from the charger, the battery measures 13.25 volts. Dan
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Sounds right to me. An automotive alternator usually puts out 13.75v maximun.
    Max.
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,791
    1,103
    Hopefully Q1 is doing its job, but if Q1 has failed you would still get some charging current via IC1.
    Does Q1 get warm/hot in use?
    What voltage is there between emitter and collector of Q1 in use?
     
  9. DanW8KMX

    Thread Starter Member

    Oct 21, 2009
    21
    1
    Q1 does not get hot. Voltage is 9.5VDC between E and C. Huntron Tracker says the transistor is good.
    Max said it is not the best circuit for this. How about a schematic of the best circuit for this application.
    Dan
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,103
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    So far we have not reproduced the original complaint, right? There is no evidence yet that there is anything wrong with this charger?

    Since the old device apparently was acceptable at one time, I'd either continue to pursue what really happened, or just buy a commercial battery charger. They're not expensive.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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  12. DanW8KMX

    Thread Starter Member

    Oct 21, 2009
    21
    1
    Thanks Max. On the battery charger/supply that I have, I am convinced now that there is nothing wrong with the unit. If anything that caused it to "not work" is that it is an improper application. I can see that if the unit will only produce a max of 5 or 7 amps and that the battery supplies a 100 watt transceiver, although low duty cycle on CW, the battery would run down over about 6 - 8 hours and the radio stop working. The operator would then think that the charger did not work etc.
    Thanks for the help guys. I consider this thread closed.
    Dan
     
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