h-bridge

Discussion in 'General Electronics Chat' started by z3ngew, Jun 21, 2013.

  1. z3ngew

    Thread Starter New Member

    Apr 18, 2013
    13
    0
    Hi everyone,
    i have a problem, i have designed an h bridge circuit, and its working fine, but there is a problem that is the output current of the circuit is very low to make the motor turn, i need help!
    take a look at the design in the attachments

    Thanks in advance,
    z3ngew
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    I think the current out of the opto-isolators will be very low. These devices are typically spec'ed by collector current forward gain, but as you have connected them in a CC configuration, you can't depend on the forward gain spec. You should do a detailed analysis on that part of the design.
     
  3. z3ngew

    Thread Starter New Member

    Apr 18, 2013
    13
    0
    thanks for the reply, but if the opto-coupler output current is small but enough to switch on the bridge.
    I can't see how this affect the bridge output current.
    Please, provide me with more explanation.

    Thanks in advance,
    z3ngew
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,018
    3,235
    Since the transistor is completely isolated in an opto-isolator, it makes no difference whether it is connected in a cc or ce emitter configuration, the gain is the same.

    z3ngewz, what is the motor voltage and current?
    What is the voltage from the MCU source.
     
  5. z3ngew

    Thread Starter New Member

    Apr 18, 2013
    13
    0
    I am using a 40 pin microcontroller, each pin output volt will range from 3 to 5 volts.
    I am using a 6v volt with load current 150 mA.

    Thanks in advance,
    z3ngew
     
  6. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    OK you got me there. However, the 10K resistor is still gonna kill his forward current. For example, if he is using 10V rails, the best he can get from the opto is ~900uA. The 2222 might bump that up a 100 or so, still not much current. I still say he should re-evaluate that part of the design.
     
  7. z3ngew

    Thread Starter New Member

    Apr 18, 2013
    13
    0
    what you suggest i should do, i have tried to change the 10k ohm resistor at the base of the 2n2222 transistor with 330 ohm and 1k ohm, it is giving the same result, how can i troubleshoot this circuit?

    thanks in advance,
    z3ngew
     
  8. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    Your bridge uses transistors. They need suffiient drive in the base to produce the current your motor requires. For example, if your motor requires 500mA and your transistors have a forward gain of 50, you'll need at least 500mA/50 = 10mA at the base. Just "turning on" the transistors isn't enough.

    I made up these numbers, you need to know what your circuit requires and the parameters of your components, and then to the analysis for your circuit.
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,018
    3,235
    Where is the motor connected?
    What is the voltage across the motor and the current through the motor when the bridge is ON?
     
  10. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    The 330ohm resistor sould have given you much more current. Time to break out the meter and start taking measurements, I think.
     
  11. z3ngew

    Thread Starter New Member

    Apr 18, 2013
    13
    0
    I'm sry, but if i am using TIP 141 instead of TIP142 with the pnp TIP147 could this make a problem???

    z3ngew
     
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