Current constant across h-bridge load

Discussion in 'General Electronics Chat' started by kevin0228ca, Jun 16, 2015.

  1. kevin0228ca

    Thread Starter Member

    Jun 5, 2015
    I am testing how to use h-bridge to allow current go both directions across load.
    My schematic

    I can get current going both directions as I see current square wave in simulation.
    But my Rload will be varying, which lead to current changing.
    I simulated when Rload is 100 ohm, Iload is +-10mA.
    but when Rload is 500 ohm, Iload is +- 3mA.

    How to design a h-bridge that will produce constant current across Rload?
    Or is there any simple IC?
    My I source will be constant less than 10 mA.

    Thank you.
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    With the circuit shown there is shootthrough if Rload is >100 Ohms, i.e Q3 and Q5 conduct at the same time and rob the load of current.
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
    The bigger problem is that the current goes straight through the BE diode of Q6 into BE diode of Q4 and straight to ground. You need to swap top and bottom transistors.
  4. Sensacell

    Well-Known Member

    Jun 19, 2012
    Base current in the transistors steals some of the current.

    Change it to a complimentary MOSFET pair and things will get better.
    You will still have some charge injection when the gates switch, but at DC it will work perfect.

    OR... put your current regulator inside a rectifier bridge- at the output of a normal H-bridge.
    This would only work with a 2- terminal current regulator.
  5. kevin0228ca

    Thread Starter Member

    Jun 5, 2015
    Thank you for your answers.

    Please clarify my confusion.
    My V1 and V2 has a phase so that when V1=5v V2=0v.

    From what I read, Q5Q6 are activated by 5V and Q3Q4 by 0V.
    So when V1=5 V2=0, Q4Q5 activate, so current goes source->Q5->load->Q4->ground
    when V1=0 V2=5, Q3Q6 activate, so current goes source->Q6->load->Q3->ground

    Is my circuit correct?
    How should I modify my circuit?
    And why is Rload varying current?

    Sorry I am new electric engineering which is why I use a simple example.
  6. DickCappels


    Aug 21, 2008
    It might require a transformer to couple power to the circuit, but you could put a full wave bridge rectifier in series with the H-Bridge output (connect one AC input to the bridge output and the other to the load). Connect a circuit that draws a constant current between the + and - bridge terminals. I know it can be done because am doing this myself at 240 VAC 50 Hz.
  7. ronv

    AAC Fanatic!

    Nov 12, 2008
    I'm going to guess a little bit because you don't show the most important part - the current source. But I bet it must use 5 volts or you could not always turn off the top transistors. If so. the most voltage at it's output is probably around 4 volts, so if it all went thru your 500 ohms that would only be 8 ma. But it does not all go there 4 ma goes to base drive current for the top transistor leaving only about 4 ma for your load.
  8. crutschow


    Mar 14, 2008
    That circuit has the PNP bases directly connected to the NPN bases so there is nothing to limit the base-emitter current and both transistors will always be on (remember the base-emitter junction looks like a forward biased diode).
    Place a separate resistor from each base the square-wave drive source to prevent that.