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H-Bridge Contoller

Discussion in 'Electronic Basics' started by James, Jan 9, 2006.

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  1. James

    James Guest

    Hello,

    I am trying to cycle an h-bridge from forward to reverse at a frequency
    of anywhere between 1 and 500Hz. I am using a function generator to
    trigger the bridge, and I have separated a square wave output of the
    bridge into a negative and positive component using diodes. The
    H-bridge schematic that I am using is from Eugene Blanchard's website,
    it uses MOSFET switching transistors. Anyway, I need to trigger the
    bridge using the function generator, and I can't devise a way to
    trigger one leg of the bridge using the negative f.g. output. Can
    anybody tell me how to modify the bridge to accept this input? Or
    invert the f.g signal to a positive value? Thanks in advance!

    -- JJ
     
  2. BobG

    BobG Guest

    Are you using it as a pwm fwd/reverse speed control, or just as an
    onfwd-off-onreverse switch/relay?
     
  3. James

    James Guest

    The H-Bridge is used as a relay to cycle a low resistance load from 8
    to -8V, in a square wave type signal. During testing I am using a 0.5
    and a 2 Ohm power resistors to simulate different amounts of load. I
    can create two pulsed signals using the f.g. and diodes to trigger the
    bridge, but one logical input signal has a negative sign, which is not
    activiating one side of the bridge. The "off" time for the bridge
    should be negligible, I am trying to switch directly from fwd to
    reverse. I can supply a schematic of the bridge if necessary. thanks!
     
  4. James

    James Guest

    just as an switch/relay as you said, basically I am driving a low
    resistance load that needs to be cycled from 8 to -8 V, with about 10
    Amps of current. The load has negigible capacitance, and I am using a 2
    Ohm and 0.5 Ohm power resistors. to simulate the load during testing. I
    have a f.g. that can provide two logical input pulses via a square wave
    and diodes, but the only problem is that one of the logical inputs is
    negative, which is causing me problems. Also, I may have to vary the
    driving voltage from 1 to 8V. do you need a schematic?Thanks for the
    interest.
     
  5. BobG

    BobG Guest

    Schematic would help. If you have N chan mosfets on the top and the
    bottom, the 'top' one needs to get its gate really pulled up to turn
    on. Or you could have a 'complementary' rig where the top gate (P chan)
    get pulled down to turn on, and the bottom is N chan and the gate gets
    pulled up to turn on. What volts from the f.g. and what batt volts on
    the bridge?
     
  6. James

    James Guest

    The schematic can be found here
    http://www.geocities.com/fet_h_bridge/np-s.pdf. As far as the voltages
    are concerned, I can put 0 to 8 DC volts on the bridge, and from 0 to
    10 volts peak to peak on the f.g., which is used to activate inputs A
    and B. C and D are connected as shown in note #4. Anyway, basically
    what I need is to either activate B using a negative voltage, or invert
    the sign of the voltage that is going to B. I hope this makes
    everything clearer.
     
  7. BobG

    BobG Guest

    Looks like you put a pos pulse on A to put out pos volts, or you put
    the pos pulse on B to put out neg volts. Need to have the jumpers in
    the 2-3 position. So you need a spdt switch from the fg to A and B or
    an inverter from A to B, so they are opposite from each other.
     
  8. James

    James Guest

    Exactly
     
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