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Battery charger power supply/circuit question...

Discussion in 'Electronic Basics' started by [email protected], Jun 6, 2006.

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  1. Guest

    I am using a battery charger for a power supply for a circuit i am
    building to control the speed of a DC motor. here is my circuit (view
    in notepad)

    | +5 V
    | |
    | .-.
    | 470 | |
    | 1/2 W | |
    | '-'
    | |
    | o---------> to MOSFET
    | | Gate
    |PRN ___ |/
    | o-|___|-o-|2N3904
    | 2.2K | |>
    | .-. |
    | 10K| | |
    | | | ===
    | '-' GND
    | |
    | ===
    | GND

    V+ from battery charger
    (18.8 V, 10A)
    | |
    .-. |
    Motor ( X ) -
    '-' ^
    | |
    | D1
    ||<- PM
    from 2N3904 -->-----||-+

    Power MOSFET IRF510

    Anyway, there are serious problems with this circuit. It keeps blowing
    up, haha. Especially the 2N3904 transistor. Any ideas? Is a battery
    charger suitable for a power supply? Are there modifications i need to
    make? Any help would be great. Thanks, Lucas
  2. You need a resistor from the 3904 to the irf510 . You are most likely
    getting the current from the gate feeding back into the 3904 and burning it
    up. Try adding about a 10K resistor there.
  3. ehsjr

    ehsjr Guest

    You need an entirely different circuit. When controlling
    the speed of a small DC motor, you want PWM - pulse width
    modulation. A sample circuit is attached:

    +12 ------------------+--------------+
    a| |
    [D3] |
    | |
    +---+----------+ +-----+
    | | | | |
    [1K] | ---------- [D2] [MOTOR]
    | | | 8 | a| |
    | +-----|4 | | |
    / | | +-----+
    25K \<---+----|7 555 | |
    / | | | /
    | a| | 3|---[150R]---| TIP31
    [1K] [D1] | | \e
    | | | | |
    +----+----|6 | |
    | | | | |
    | +----|2 5|---+ | Diodes: 1N4001
    | | | | |
    [C1] ---------- [C2] |
    | .1uF | .01uF |
    Gnd ---+------------------------+----------+

    Also, the battery charger may put out DC with a good
    deal of ripple. Add a hefty electrolytic, say 470uF,
    from pin 8 to ground. D3 consists of 5 1N400x diodes
    in series, to drop the 18 volts from the charger
    down closer to 14 volts. If you had a regulated 12 volt
    supply, D3 would be eliminated.

  4. Guest

    Sorry, i forgot to inform everyone i was controlling the PWM from my PC
    with the printer port (thus the prn in my origonal circuit). Thanks
  5. Guest

    Another thing, I used this same schematic with a homade power supply
    (transformer, full wave rectifier, some capacitors, etc..) and it
    worked fine. This is why I think the problem lies within using the car
    battery charger (again 18v 10 amps). Anyway, ehsjr brought up a good
    point with the ripple. Maybe this is causing the problem. And James
    Thompson ill try to throw a resistor in there for safe measures. Here
    is a more general question however, if I am using a power supply with
    the same voltage but allows more current (10 Amps v. 2.8 amps from my
    home-made power supply), but am powering the same motor with the same
    circuit, the same amount of current should be running through the
    circuit right? That is, just because the battery charger is rated at
    10 amps, doesn't mean it is indeed supplying more current since im
    powering the same motor, right? Anyway, I think the problem lies
    withing the power supply, so any idea on what exactly the problem is
    would help A LOT!!! Thanks so much everyone.
  6. ehsjr

    ehsjr Guest

    Ok, your circuit diagram shows +5 to the collector of
    the 3904 through 470 ohms. Is that the same when
    you use the car battery charger as it is when you
    use your homemade supply? (Implying a 5 volt
    vr in there) If not, add a 7805. Filter out the
    ripple. Make your battery charger supply equal to
    your homemade supply.

    Make sure you have common ground (prn and power
    supply). Put a 100 ohm resistor between the
    3904 and the gate.

    Who knows if the current is the same? The circuit
    between the load and the source may behave differently
    when powered by a high ripple source. If that circuit
    behaves the same, then it does not matter if your
    source is capable of 10 amps or 2.8 amps, provided
    the motor can draw no more than 2.8 amps. A supply will
    provide only the amount of current drawn from it,
    not its full current rating in excess of what is drawn.

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