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Voltage converter circuit - Bosch / Siemens washing machine

Discussion in 'Electronic Repair' started by [email protected], Jun 4, 2007.

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  1. Guest

    I need your help repairing the voltage conversion circuit of the Bosch
    WFL2061BY washing machine. I have the following questions:

    1) No operating lights etc. are on. Visual inspection shows a damaged
    resistor (first two bands brown and black, next band difficult to tell
    - possibly brown, gold or even yellow). As far as I can tell no fuse
    is fitted - is this normal? A fuse seems to be essential to protect
    against possible overcurrent conditions.

    2) The damaged resistor is directly connected to the mains in series
    with a transformer (which I suspect is used to derive the low voltage
    for driving the relays). I have experimented with
    a 100R resistor (1/4 Watt) connected to 220V, but it is immediately
    burned out.
    Four (1/4 Watt) resistors of 10k in parallel also got hot in a few
    seconds. My initial guess would be that the damaged resistor provides
    current limiting?

    3) If the damaged resistor provides current limiting, why is it placed
    on the high voltage (220V) side?

    4) A number of diodes (likely 1N4001) also seems to be connected to
    the high voltage side, before the transformer. If they form a half-
    wave or full-wave rectifier, why do they connect the diodes before the
    transformer (on the high voltage side)?

    5) It is possible that some other components also got damaged. From
    visual inspection I cannot confirm damage to other components. In
    circuit measurements of the diodes show about 4.5k in one direction
    and infinity in the other. An 8 pin IC TP209P is found close to the
    (small) transformer. The voltage regulator is a BT916 (or BTB16 -
    difficult to tell from visual inspection). Any suggestions on how to
    proceed?

    Best regards,
    Theo van der Merwe
     
  2. James Sweet

    James Sweet Guest


    There's certainly a shorted component which burned up the resistor.
    Resistors don't just burn up on their own.
     
  3. Guest

    Thanks for the email message and feedback. The problem occurred when
    testing the pumping out of water (which wasn't working at all -
    everything else seems OK). However, I have disconnected all the
    external connections to the motor and pump from the circuit board. Any
    suggestions on how to proceed with fault finding (assuming some
    component is shorted on the circuit board)? At present I don't
    understand the power supply design on the circuit board.

    Best regards,
    Theo van der Merwe
     
  4. Dave

    Dave Guest

    there's likely a thermal fuse in the transformer windings. If the motor
    pulls anywhere close to 15A they figure your circuit breaker will do the
    trick. BTW fuses don't protect semiconductors, they're too slow.

    Something's shorted and drawing too much current. A metal film resistor
    tends to be a pretty good fuse in many cases.
    Less current, smaller resistor, cheaper. Also, it ensures that the
    transformer is not hot in the event of a fault.
    The AC cannot be rectified prior to your transformer. Can't can't can't.
    I'm confused as to what you've got here... does the main power all run
    through the transformer, or is the transformer just to step down the voltage
    for powering the electronics with the AC going to power the motor relays? I
    don't think you've got any motor relays if you've got Triacs (see below).
    The BTB16 is a 16A Triac, not a regulator. It's likely used to switch on
    the motor or pump and possibly control the speed depending on the complexity
    of the control circuit.

    First, can you confirm that if you physically disconnect all wiring from the
    pump, that the problem persists? That would effectively rule out the pump.
    I might try powering the pump directly by attaching a line cord and confirm
    that it works. The triac which controls (I am assuming here, the pump MAY
    be controlled by a relay as it only needs to operate at one speed) has
    virtually no resistance when biased by the gate voltage, the only thing
    limiting the current draw is the load which is in this case is the pump. So
    let's rule it out.

    If in fact the pump is fine, and if removing it from the circuit does not
    eliminate the problem of the resistor smoking, I'd try removing the triac
    which controls the pump. When triacs fail they either stop working or fail
    shorted. Neither of these matches your scenario. Check the pump wiring and
    make sure there are no shorts to ground or shorts from hot to neutral lead.

    If it's a relay and not a triac which controls the pump I'd take a good look
    at it... are the contacts burnt? What else is around it in the circuit?

    Can you confirm the writing on the 8-pin DIP? If there are various motor
    speeds it could be a pulse width modulator driving the triac, but I come up
    empty with "TP209P"... TP209 yields a DC-to-DC upconverter, 5VDC in and
    15VDC out, 50mA rating...

    Dave
     
  5. Guest

    Thanks for the feedback, now I understand better.


    Thanks for the feedback, now I understand better.
    I can send you a JPEG of the circuit board if it can help, but below
    is a brief description of the connections:
    damaged resistor of unknown value, probably 2W from the size)
    (and R1). C1 is a large 0.22uF 275V AC
    Red Black Orange Red or other way round). Eventually the connection
    from R2 ends up at a connector at the edge of the circuit board (e.g.
    could be motor).
    be pump/motor).
    is marked -, so it could be polarised capacitor (looks like an
    electrolytic capacitor).
    connections marked S10 K275 0013
    small components looking like surface mount capacitors)
    positive, other end marked - . Looks like electrolytic capacitor 16V
    100uF).
    is found next to the connection to the relay).
    Please let me know if you need more information. The presence of a
    polarised capacitor C2 of 400V connected to the mains supply via diode
    D1 would seem to indicate half wave rectification? If so, I can't
    quite understand why it is rectified prior to the transformer.

    Thanks for the feedback.
    Yes, at present I have disconnected all connectors (motor, pump etc.)
    from the circuit board.
    Thanks for the feedback. How do I actually test the pump? Should I
    fill the washing machine with water before testing the pump and see if
    water exits the drain? Or is it possible to test the pump without
    fluid? Anyway, at present the pump is not connected to the circuit
    board.

    If the triac fail shorted it could explain the excessive current
    drain?

    The triac is connected to the circuit board via a large heatsink, so
    it seems difficult to remove (hence test for shorts). Any suggestions?

    When I trace the 3pin connector going to the pump back to the circuit
    board, it seems that a relay (10A 125V AC, 12VDC) controls the pump.

    Visual inspection of the circuit board indicates damage to resistor
    R1. Black soot is seen in the vicinity of diodes D1, D2, D3. The
    diodes does not seem damaged from visual inspection, although the
    wires of the diodes are not shiny, indicating current drain.
    Sorry, it is actually TOP209P. At the top is written K021 and at
    bottom 1067 1B,

    Thanks for all your kind help.
    Best regards,
    Theo
     
  6. Dave

    Dave Guest

    A picture is worth a thousand words... just remove the word "delete" from my
    email address.
    Nor can I. The transformer would only produce the other half of the wave on
    the secondaries. This can't be right.
    Test the pump wiring harness. Look for a short.

    Assuming the pump to operate at mains voltage, just figure out which pins on
    the pump connector are for power, connect up a reasonable-guage power cord
    and plug it in. We're not trying to determine if this thing pumps water...
    at this point we don't really care. We just want to make sure that the
    windings aren't shorted and that the pump will start and run. It won't hurt
    it to run it dry.
    That was my original thought, but I had some reservations, hence my
    questions about the existence of a relay which you have confirmed. The pump
    only operates at one speed, so can be controlled using a relay as opposed to
    a triac. The triac likely controls the motor so you can have slow-speed
    agitation and high-speed spin with no mechanical transmission. IF the triac
    controlled the pump and IF the triac were shorted, the pump would simply run
    all the time. This is not the case.
    Confirm that the wash/spin motor works and that it's controlled by the large
    triac and move on.
    Are diodes D1/D2/D3 and resistor R1 on the controlling 12VDC side of the
    relay or the load side?

    If they're on the load side, and somehow I doubt they are, it's gotta be the
    pump or wiring. If they're on the control side, look for a smoked resistor
    (and perhaps diode) which limits current through the relay coil. If this
    failed it may take down any number of other components in the relay control
    circuit.
    Well well well, that's a 3-terminal off-line PWM switch, as predicted. How
    many windings/leads on the transformer. The typical circuit diagram in the
    TOP209 datasheet shows it being used to derive a stable DC voltage via a
    transformer with a bias winding. It's possible that this guy produces the
    <12VDC to operate the relay. A schematic would be really helpful here.
     
  7. Guest

    Unfortunately Google only shows <>, not the
    complete email address (presumably to prevent spam). Can you send a
    message to my email address to which I can reply?

    Thanks for all your feedback so far.

    Best regards,
    Theo
     
  8. Marra

    Marra Guest

    How about getting a qualified engineer in to fix it before you kill
    yourself !

    Advice to an amateur? not a chance mate, you might end up frying
    yourself.
     
  9. Dave

    Dave Guest

    Yes, lots of people kill themselves every year troubleshooting unpowered
    appliance circuits.

    these types of equipment are far too complicated for the average individual
    to ever understand without a degree in engineering... turning a pump on and
    off on a timed cycle, whew! that's WAAAAYYYYY beyond me!

    Best leave it to the "experts".

    The OP obviously has some deductive skills and many times, that and some
    perseverance are all it takes to fix something. Thanks for your advice, but
    why don't you take your attitude and shove it where the sun don't shine?

    Have a great day.

    Dave
     
  10.  
  11. Guest

    According to the connection diagram I sent previously, the mains is
    connected directly to diode D1 via a (blown, unknown value) resistor
    R1:
    Furthermore, diode D1 is also connected to the transformer:
    If C2 is indeed a polarised electrolytic capacitor (hopefully you'll
    be able to visually confirm from the JPEG I sent), it would seem to
    imply DC (direct current) - possibly half-wave rectification. Why a
    transformer is then connected to the diode D1 is, at present, unclear
    to me - I just don't understand it yet.
    Thanks for the feedback. The electrician that helped us with the
    washing machine is unfortunately overseas at present, but the washing
    machine seemed to be working correctly (he started a program without
    any problems, then stopped the washing machine and pumped out all the
    water). However, when we started using the washing machine it would
    stop with a flashing light after 20 minutes. When I set the switch to
    leave the water in the washing machine, it carried on longer (up to
    about 1 hour 20 minutes) - but still did not complete the washing
    cycle. When I tried to pump out the water manually the problem with
    the shorted resistor occurred. The reason for the flasing light seemed
    to be related to the pumping out of water - I set the program to pump
    out water, but just heard a 'humming noise', which seems to indicate a
    problem with the pump. This would possibly explain why the machine
    stopped with a flashing light after 20 minutes - presumaby it tried
    (without success) to pump the water out for the next cycle to start.

    At present I am not too worried about the pump, as I can always buy a
    cheap 'universal' pump to replace if needed. My big worry is about the
    electronics. At present I do not know the value of the damaged
    resistor (to replace) and whether additional components are damaged.

    By the way, the electrician already made a modification with a switch
    that we now manually press to start the heating cycle (he also
    replaced the thermostat recently).
    I have measured about 370R between the terminals of relay RL1
    connected to the transformer windings (infinfity between other
    terminals):
    So it would seem as though diodes D1/D2/D3 and resistor R1 is
    connecting to RL1 (via a transformer) on the controlling side.
    The other terminals of RL1 are connected to the connector on the
    circuit boad (presumably pump/motor) and the mains, so I assume it
    would be the switch controlled by the windings of RL1.

    Is it possible to infer the likely value of resistor R1 from the text
    description of the connections? I would suspect that activating the
    relays could take a few hundred milli-amperes. A 2.5k resistor
    (damaged R1 resistor) would produce 240/2500 = 100mA, but in actual
    test got very hot in a few seconds - hence indicating a short
    somewhere?
    I don't quite know how to draw a schematic diagram with text, but most
    connections to TOP209 have already been described:
    I can add the following:
    marked negative. Value is 22uF, 50V.
    terminal of electrolytic capacitor C3 (16V 100uF).

    The (small) transformer have 4 terminals.

    Thanks for all your kind help.
    Best regards,
    Theo
     
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