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NiCad Battery Charger repair

Discussion in 'Electronic Repair' started by [email protected], May 6, 2007.

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  1. Guest

    Have an OEM battery charger for mid-1980's equipment that does not
    appear to be operating correctly. It has been longer than that since
    my high school Physics' classes and I have exhausted my personal
    "knowledge base" as to trying to diagnose and correct this "problem."

    Any suggestions as to how to proceed will be appreciated.

    Background:

    The battery charger is marked as Input AC 120V, 50/60Hz, 5W
    and Output of DC 10.43V 180 mA.

    The Red LED for "operating" is not illuminating and the 3A fuse for
    the equipment is blown.

    Checked the battery charger secondary output at the connector and it
    is reading 15.34 VDC with no load.

    Main's current was 121.4 VAC

    Opened the battery charger case. Transformer is marked as "206082" and
    label logo appears to read as "SDB."

    Secondary output side has three post. I will refer to them as L, C,
    and R.

    Voltage at L and C or at C and R is 13.74 VAC.

    C feeds the lead to the equipment connector jack, the negative pole.

    L and R each go to their individual diodes (can not read the values)
    and then connect together. The voltage at either L or R to C, after
    being rectified is 15.39 VDC.

    The combined L & R then connects the lead to a 68 ohm resistor (which
    feeds one side of the Red LED) and also into what I assume to be a
    Ceramic Tubular Capacitor (light bluish colored, "dog-boned" shaped
    with end leads and color banded Gold/blank/Gold/Gold/White(or
    silver?))

    Voltage across the two leads for the LED reads 0.000VDC.

    The output lead of the "capacitor"? then feeds both the other lead of
    the Red LED and continues as the Positive pole of the equipment
    connector jack.

    Questions:

    1) Is the 15.37 VDC "no load" output at the connector representative
    of a battery charger malfunction?

    2) Is this a reason that the LED is not illuminating?

    3) The OEM feels that this "simple" battery charger is worth US
    $120.00. Any suggestions?


    Again, thanks for any suggestions.
     
  2. Probably not.
    Repair the one you have.
     
  3. Jim Yanik

    Jim Yanik Guest

    wrote in
    It MIGHT help if you identified WHAT the "mid-1980's equipment"
    IS;make,model,serial number.
    Somebody here may have a manual or link to one.
     
  4. Guest


    Hello Jim and thanks for the input. I was afraid that I had already
    given too much information!

    The equipment is a Topcon GTS-2B Geodetic Total Station, used for land
    surveying. Most of the device is mechanical or optical/mechanical, but
    the distance measuring function uses an 8.4 volt battery pack.

    The battery charger is a Topcon Battery Charger BC-10B, which is used
    for standard rate charging. When using the standard battery pack, the
    BT-5Q (DC 8.4V output and 1.2AH capacity) the recommended charge time
    is 8 hours.

    Again, any ideas or suggestions will be appreciated.
     
  5. mike

    mike Guest

    What's the history?
    Did it work yesterday and not today?
    Did you pull it out of a dumpster?
    If the latter, idle NiCd batteries go shorted and burn up stuff
    when you turn on the charger.

    A significant clue is that the fuse is blown, but you gloss
    over that fact and don't describe where in the circuit it is.
    It ain't likely to work with a blown fuse.
    The problem is likely downstream of the fuse.

    If you've got 15V on one side of the resistor and nothing
    on the other you've got no current, which is consistent with a
    blown fuse. Also consistent with open resistor or dogbone.

    It's likely that the led illuminates only when there's sufficient
    current to generate some volts across the dogbone.

    Start with the blown fuse. Check for shorted batteries.
    mike
     
  6. Guest

    ---------------------
    the LED should lite loaded or not
    u mentioned no load output
    what is the load u r putting on it?
    is it a switching power supply 100-260Vac 50~60Hz or a linear PS
    if bottom regulation voltage can be seen to ring when u lower ac
    input on your scope then it is linear
     
  7. Guest

    Mike. Thanks for the questions. Let me expound on the situation so
    that you and others may have more to ruminate on as to my battery
    charger situation.
    Battery charger (Topcon BC-10B) was properly charging the battery pack
    on a GTS-2B "Total Station" (used for surveying). I had miswired the
    connector to another port on the GTS-2B. Noticed a problem when saw
    that the Red LED "charging" light was out. Checked battery charger's
    output at the plug - saw more than 15 VDC. Then also noticed the blown
    fuse on the GTS-2B.

    Since the voltage read at the connector (15.39 VDC) was well above the
    BC-10B's stated output of 10.43VDC and the battery pack is made up of
    seven ni-cad cells (8.4 VDC) I was concerned that the BC-10B battery
    charger was way out of tolerance.

    As this is a simple, plug in the wall battery charger, I started
    looking for a replacement. Topcon wants $120 for theirs. At that point
    it was worth opening up to try to find the problem.
    No dumpster diving on this one. The battery pack was properly
    functioning and the problem occured only after I miswired the hook-up
    connectors while starting to recharge the battery pack.
    The fuse is on the GTS-2B equipment (actually associated with the
    battery pack and it even has an On/Off switch that controls both
    output (into the GTS-2B from the battery pack for distance measuring
    operations) and input (for charging the battery pack - the switch must
    be On for charging operations.)

    I have +/- 15.4VDC (depending on the line current, it was 15.39 with
    the line current at 121.4 VAC and directly proportional increase as
    the line current changed) coming out of the battery charger, which I
    initially measured at the charger's plug. Now that I have taken the
    battery charger module apart, I measure a similar voltage at any point
    after the current is rectified (the diodes are immediately after the
    secondary side of the transformer.)
    Have +/- 15.4vdc on both sides of the resistor when tapping to the
    secondary side's center output post (supplies the negative for the
    output plug).

    The mention as to the 0.000 VDC was when I check the voltage of the
    two legs going into the Red LCD.

    One leg of the LED terminates into the output side of the 68 ohm
    resistor.

    On the output side of the "dog-bone" is a Tee where one leg runs to
    the output plug as the Positive terminal and the other leg from this
    Tee becomes the "other" leg into the Red LCD.

    I do read +/- 68 ohms across the resistor as it sits, still wired into
    the charger, but the charger's output jack is not hooked up to the
    battery pack (and stating the obvious, not plugged into the line
    current!)

    Believe the "dog-bone" is a capacitor, but was not 100% certain. Is
    what I described as a "dog-bone" a Ceramic Tubular Capacitor, and if
    so, what capacity with the White(Silver?) / Gold / Gold / Gold
    banding?

    The "dog-bone" is still hooked up. Everything is still as it was as I
    have only opened the battery charger case without breaking any
    connections, up to this point.
    The "dog-bone" has +/- 15.4VDC on both sides. Do not have an easy way
    to measure the current at this time. Possibly, I could hook up a small
    automotive lamp to the plug to see if there is current.
    To review, the blown fuse and the batteries (which still have minimal
    charge) are on the GTS-2B equipment. Replaced the fuse and the GTS-2B
    equipment still works.

    It is only because I no longer have an illuminated Red LED and my
    checking of the output voltage (and my belief that this is way to
    high) that I believe that I have a problem.

    Do you think that I do have a problem (of course, I mean with the
    battery charger!) and if so, any ideas of how to correct it/them?

    Again, thanks for your input and attempts to help me.
     
  8. If you managed to introduce a charged battery the wrong way round to the
    charger that might account for a blown LED - they are polarity sensitive.
    Voltage doesn't matter when charging Ni-Cads - within a wide range,
    provided it is higher than the nominal pack voltage. It's the current
    through the pack which is important and in this case (in the main) is set
    by the series resistor.
    It has so few components it should be simple to test them all with a DVM -
    and to see if it is actually producing charging current. The LED can be
    checked by simple substitution - they are very cheap. Check also its
    series resistor.
     
  9. Bill Jeffrey

    Bill Jeffrey Guest

    This is an extremely simple charger. Nicad chargers usually have a bit
    more complexity. However, drawing up the circuit as you have described
    it, I find:
    1. The transformer is center tapped. C is the center tap, L and R are
    the two ends. Given your choice of labels, I think you know this. The
    two diodes provide full-wave rectification.
    2. In this configuration, the voltage that you read beyond the diodes is
    pretty meaningless, since your meter will read the peaks of the
    rectified half-waves, not the average or the DC component.
    3. The "capacitor" probably isn't a capacitor, since it is in series
    with the battery being charged. I am assuming it is a low-value
    resistor, which provides operating voltage for the LED.
    4. You should read the color code from the other end - that is,
    gold-gold-white-blank-gold. But regardless of whether it is cap or
    resistor, that color code doesn't make sense. I am betting that it is
    white-brown-gold, which would make it 9.1 ohms. If the entire battery
    current (180 mA) passed through it, it would drop 1.6 volts, which is
    not out of line for a LED. Its real purpose is to share some of the
    charge current with the LED, so the LED doesn't get overdriven.
    5. The LED is not expected to light unless a battery is being charged.
    That is, if you simply plug the charger into the wall and set it on your
    desk with no battery inserted, the LED won't light. However, if you put
    something like a 47-ohm resistor across the charger terminals for a
    moment, the LED should light. Is this what you observe?
    6. Hooking up a battery backward will NOT damage the LED due to reverse
    conduction. The rectifier diodes will prevent current from flowing.
    7. Having said that, if you a) short the charger output, or b) insert a
    battery backward and plug in the charger, it might damage either the LED
    or the 9.1 ohm resistor or both, due to overcurrent. Assuming that the
    LED does not light in step 5 above, I'm betting that the 9.1 ohm
    resistor overheated (which would account for the difficulty in reading
    the true colors of the bands) and failed open, at which point the LED
    failed from overcurrent.

    Hope this helps. To answer your question "Is it worth spending $100+ to
    replace this thing", the answer is no. You can replace the LED and the
    resistor for less than a buck each (Radio Shack). Before you do so, I
    would recommened using the ohmmeter part of your VOM or DVM to confirm
    that a) the rectifier diodes are still diodes, b) the 68 ohm resistore
    is good, c) the LED looks like a diode (or doesn't), and d) the 9.1 ohm
    resistor is way out of whack (or isn't).

    Bil Jeffrey
    -----------------------------------
     
  10. Jim Yanik

    Jim Yanik Guest

    wrote in
    that's a slow charge rate;must be a trickle charger.
    NiCds charge at ~1.5x the pack voltage,usual slow charge rate is 1/10
    C,takes several hours,but IMO,NiCds do better with fast charges,1 hr or
    less.("C"= AH rating,IOW,Capacity.)
    But for fast charge,you need a temp sensor or a smart IC to monitor cell
    voltage.

    With most trickle chargers,you need to use a timer so you don't OVERcharge
    the cells,shortening their life.


    1.2AH would be AA-size cells.IIRC,AA NiCds are around 1200mAH.

    AA NiMH cells go even higher,around 2200mAH.

    since your charger outputs a much higher voltage,I suspect the regulating
    circuit has shorted or failed.
     
  11. cmdrdata

    cmdrdata Guest

    Its amazing the various replies you got on this problem. Reading your
    description of the circuit, it sounded like a simple fullwave bridge
    rectifier circuit converting transformer secondary AC voltage via two
    discrete diodes with the no "regulation" whatsoever and a light load /
    power indicator of LED with current limiting resistor. So, if I were
    you, instead of being gouged by the vendor wanting 120 dollars, find
    a similar DC wall power supply with output spec near 10-11 VDC
    ( similar current rating capability) and adapt the power plugs (cut
    off and rewire the old to new one), and your problem is solved.

    Nicads, like someone posted is pretty tolerant to voltage levels, and
    like to be charged at 1/10th the AH capacity or if you're willing to
    tolerate it, at even lower charge rate. As far as the LED not
    lighting up, I would check the LED and the limiting resistor. One of
    them is probably open circuit. Also, if you hook up 100 ohm load on
    the output leads, my guess is that the open voltage measurement you
    got will drop down a few volts closer ot the label voltage rating you
    mentioned. If the voltage with the load on drops real low, then
    perhaps you have a shorted winding in the transformer, thus unable to
    even proved the 100-150 mA charging current.

    The fuse thing was a misleading issue, since you miswired it when it
    happened.
     
  12. Gary Tait

    Gary Tait Guest

    Likely Sub-C cells, considering the date. AA cells would be around 600 mAH.

    $120 for a replacement charger doesn't seem out of line considering the age
    and nature of the equipment.
     
  13. mike

    mike Guest

    This is excellent advice assuming you actually measure the resultant
    charge current. And assuming the power supply does not attempt to
    power the device, and, and. Last thing you want is a fire due to very
    poor choice of power brick. It's trivial if you know what you're doing.
    Not so trivial if you don't.
    That the question was asked implies the latter.

    Better to fix the busted one and remove the risk...assuming you measure
    the resultant charge current, and, and...
    mike
    I claim to know what I'm doing...ask me about my singed eyebrows...
     
  14. Guest

    Hello all and again, thanks for your help. I have replaced the LED on
    the battery charger, hooked it up to the battery pack and the voltage
    now show 9.42 VDC. I'll let it recharge, but I expect everything to
    work correctly now.

    I am a happy camper and thankful for the advise that this group
    provided.
     
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