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12 Volt power Supply

Discussion in 'Electronic Basics' started by [email protected], Jun 22, 2006.

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  1. Guest

    please forgive the supidity of this question but I am a mechanical
    engineer and have very linited knowledge of electronics.

    I have tried to biuld a 12 volt power supply for testing
    motor/gearbox/pump units and the problem is that it keep blowing up

    I have only a very simple circuit of a Transformer to drop the voltatge
    down from 240 then feed this through a bridge rectifier ( International
    Rectifier Part No. GBPC3502A ) and the connect the output via an
    ammeter to the motor. once it is running it is fine, but somtimes blows
    the refitier when it is turned on ( switching the AC input to the
    transformer ) do I need more components in the circuit, if so what ?

    The motors will draw up to 20 amps, when running the rectifier does get
    hot but has a heat sink and fan from an old PC connected. but it
    allways blows when you turn it on, not when it is running

    All assistance greatfully received

  2. Guest


    It must be the 'surge' current thats blowing the rectifier.
    If the motor draws 20Amps with a moderate load, then I bet it draws
    80Amps at switch on (Very high load during the first few hundred
    Your fan and heatsink wont help with the initial surge.

    You'll need a more suitable bridge rectifier.

    Steve Balstone
  3. Guest

    Cheers for that , Sounds like that is the problem, is there any way to
    limit the intial current draw, a coil or somthing like that ?

  4. Guest


    To keep it really simple, you could build a crude 'soft start' circuit.
    Get a 0.5 ohm very high wattage (50W).
    Connect this in series with the transformer output and the rectifier ac
    Now put a high current (20A) switch ACROSS the resistor.
    To start.....
    Turn switch off, turn on mains, motor will start slowly then throw on
    the new switch after a few seconds
    Putting these on the AC side NOT the DC side and that will save the
    switch contacts from weilding themselves together!!

    Steve Balstone
  5. Guest

    Cheers sounds like a nice easy way of getting it going, will put it on
    a rely and push button to short out the resistor, so cannot be started
    with the resistor shorted, and put a thermal switch on the relay so if
    button not pressed then unit shuts down before getting too hot

    Cheers again

  6. If you move that relay to the primary side, you will not need such
    large contacts. The resistor will need to be about 200 ohms, in that
    case. If left on, it could produce almost 300 watts of heat. But
    this reduces the contact current to about 1 amp.
  7. The bridge rectifier shorts out any motor inductive current, so that
    is no problem. I don't that the transformer will generate anything
    damaging with the motor connected across the rectifier. The main
    power switch will have no inrush spike (caused by transformer core
    saturation from the last power off half cycle being in the same
    direction as the first one on power up) if there will be a 200 ohm
    resistor in series for some time period. The contact that shorts that
    out always has 200 ohms across it to act as a spark suppressor. I see
    no problems if the primary side contacts are rated for 5 amps or so.
  8. I would rather see an X capacitor of a microfarad or so across the
  9. Jamie

    Jamie Guest

    switches on the primary side of the transformer can be
    dirty and thus generate some high voltage spikes on the
    secondary side when switched.
    i guess the question here is, does this happen if you
    simply turn on the supply with no motor load on it?
    also, motors can be very inductive and having no capacitor
    in the circuit can generate some nice HV energy if you are
    getting arcs in the switching process.
    i would try 2 things., a small non polarized cap on the
    switch contacts and one on the secondary side ! something like
    a .1 ceramic with the proper handling voltage. and for the
    output of the bridge a larger cap to help suppress the pulse.

    we had the same problem at work with a simple DC motor on a
    Varistat with a bridge rectifier. , some times just turning on
    the unit if the motor was on a light or heavy start, it would
    short the bridge.
    you may also want to look at getting some TVS diodes
    (transient voltage suppression), don't use MOV's because
    they only work x number of times before they short.
    diodes/rectifiers have 0.6 volt on the average cutoff
    of no current flow, this lack of load can allow for the xformer
    to generate some nice HV with noisy switches and out of phase switching.
    because the pulse width is generally short and wiring in
    your application after the bridge produces some nice inductive
    reactance compind with the motor, HV could simply be able to jump the
    diodes voltage limits
    and short for that moment.
    the only other thing i can think of is that you have a heavy starting
    load on your motor? if the motor starts quickly then the rectifier should
    handle that.
    i would at least try to use a 20 amp fast blow fuse on the output. if
    you blow that on start up, then you have way too much starting torque
    in which case you should have a bigger unit or current limited supply.
    but try the caps first. i think that will cure your problem.
  10. Jamie

    Jamie Guest

    say what you want, that was a cure we did to solve a long nasty
    problem that was given to us after to many times of that part of the
    equipment being down. used a set of HV probes on the scope to monitor
    this problem and it was exactly that. out of phase/noisy contacts on the
    primary side randomly generating HV pulses combined with motor the
    bridge didn't like. the other guys try to keep a note on that equipment
    to instruct the operators to make sure the variac was down to zero
    before starting ! , but like any one else that works in a production
    type job they don't give a crap about machinery.
    when testing with just the motor on line with no caps, i was getting
    aprox 2000 V or more pulse generated when testing across the bridged.
    that is the AC-DC leg of the bridge., remove the motor load and i only
    saw a aprox 1/4 of that which the bridge seem to handle ok.
    put the .1 mica caps on the switch, secondary side and one on the +&-
    output along with a small DC cap, switching noise gone, no HV over that
    which was suppose to be there. and that unit has been running with the
    same bridge for at least 3 or more years now.

    if you read a lot of the instruction manuals for drives or sensitive
    electronics, they will tell you to not! switch the primary side of a
    transformer feed because of this problem. switching the secondary side
    is much better to avoid this damaging noise.
    higher end electronics have suppression components on the line to
    remove that noise but a lot of the low end simple mini drives do not
    go to far when protecting it self from this kind of damage in which
    case you should be using a set of contacts to feed the drive at its
    shortest point.
    many supplies also use series inductors/chokes with a cap to remove
    this problem.
  11. ehsjr

    ehsjr Guest

    Yes there is, as discussed. However, it may well
    be that the limited current will prevent your motor
    from starting. Then it's cook the motor, cook the
    circuit time.

  12. Alan B

    Alan B Guest

    This brings to mind an interesting story. I used to write and maintain
    software for automatic testing of electronic circuit cards. One particular
    card had a "soft start" circuit similar to this, where a pair of resistors
    were in the path at initialization, then shorted by relay action some
    milliseconds later. The original test software author neglected to program
    the shorting relays after turn-on. Cards were being sent to software as
    defective, with comments such as "smoked resistors," "resistors glowed red"
    and so on.

    The resistors were hardy wire-wound types, so showed no signs of damage
    when the circuit was cold. The cards routinely were returned to the system
    with no defect noted. This started a circular process that took a bit of
    time to figure out!
  13. Alan B

    Alan B Guest

    I agree with what's been posted so far, that you could either beef up your
    bridge diodes, or create a "soft start" circuit at the primary. But I also
    notice that you haven't indicated that you have any filter capacitors as
    part of your design. I wonder if some smoothing at the DC output might be
    a good idea?
  14. Alan B

    Alan B Guest

    Yeah, this could depend on how much torque the motor is required to have at
    startup. If there was a way to limit the torque load until the motor is
    turning, i.e. a clutch on the load, this might both allow a soft-start
    circuit to work, and at the same time eliminate the need for it.
  15. Alan B wrote:

    He has an inductive filter. The motor inductance averages the effect
    of the voltage wave.
  16. Alan B

    Alan B Guest

    OK, something to study up on. Motors have always been spooky to me.
  17. ehsjr

    ehsjr Guest

    When you first start a motor, it draws a *huge*
    amount of current with respect to the current it
    draws once it is up to speed. At start up, you
    want to be able to deliver that relatively hugh
    current - and soft start does the opposite. The
    motor needs that current to get started properly.
    Restricting it with a soft start may cook the
    both the motor and the powersupply.

    Build or buy a supply that can deliver the needed
    start current. Maybe an auto battery charger -
    some of them have a very high current spring return
    "start" position on the charge selector switch. You
    could wire in a relay that transfers for a second or
    two to provide that "blast" of current. I say maybe,
    because I don't know how those chargers provide that
    starting current, so further investigation would
    be needed.

    Even better would be to use an auto battery which can
    supply hundreds of amps for the brief startup period.
    There's no maybe in that solution.

  18. jasen

    jasen Guest

    it seems to me that doing that would soak up some of those voltage spikes
    that the experts have been blaming for the rectifier failure. it probably
    doesn't need capacitors of a size to fill the valleys of the rectified AC,
    just something to soak up the spikes.

    the experts were saying to put the capacitirs on the primary... I don't see
    why they couldn't go after the rectifier...

  19. Guest


    The motor torque is roughly proportional to the current, so limiting
    the current to no more than full speed current just slows the initial
    acceleration, unless there is some break away static friction or other
    starting torque peak added to the inertial one.

    The O.P. said nothing about being in a hurry to get the motors moving.
  20. ehsjr

    ehsjr Guest

    You need some minimum torque to get the motor going,
    at all. If there is insufficient torque, it just
    sits there cooking. Full speed current is a small
    percentage of starting current, and isn't even
    a valid figure. An unloaded motor vs a loaded
    motor comes to mind. The valid figure is FLA -
    full load amps. Compare FLA with LRA. LRA (locked
    rotor amps) is the startup current - the current the
    motor draws when it is stationary - and can be huge
    with respect to the FLA (full load amps) - as much
    as 6 times FLA. And who knows how much greater it
    will be vs full speed amps, which is meaningless.

    If the motor sits there drawing full speed current
    and not moving, there's no airflow, so not only is
    it drawing LRA, it is not being cooled by air movement
    its spinning would cause via the attached fan, or
    even just the armature if there is no fan incorporated
    with the motor.

    The posts recommending soft start - and not pointing
    out the above - are playing fast and loose with the
    OP's motor. Soft start, *properly* applied, is a
    good thing. It may well be that something posted
    might work. But the warning that it could cause the
    motor to cook stands. The issue is not about getting
    the motor up to speed quickly, it is about avoiding
    the situation where the motor doesn't start at all.

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