Verify current limiting built into H-bridge

Discussion in 'The Projects Forum' started by masked, Jul 27, 2010.

  1. masked

    Thread Starter Member

    Jul 1, 2010
    48
    1
    Hi all,
    Thanks to SgtWookie I was able to work out building my h-bridge for a variable current circuit.
    I just did the equations for made-up limit of 10Ω, on the grounds that the conductance in my electrolysis chamber will never approach that of the 10Ω element in my house water heater.
    ...also, it makes the math easy: 27V/10Ω = 2.7A.

    I came up with the attached schematic which seems to handle any resistance down to about 10Ω at which point it draws ~2.7A
    Since I want to run at a constant 2.64mA, and I'm still waiting for my current limiting diode to arrive, I popped in a 10.2k resistor to simulate the correct current draw.
    It works - but: I noticed the bridge was pulling 64.33mA total to drive my 2.64mA load, and from what I've learned on the forums, I knew I could do better. The drive transistors don't need nearly as much current to run this small load, so I swapped out my resistors in the schematic as follows:
    [​IMG]
    Viola! It now draws only 8mA - a savings of 56ma. hooray. (I got as low as 2.65 when I refactored without the LEDs).
    From my tests, it looks like this circuit WILL restrict the current to the desired 2.64mA. So my question is this: Why wait for a CLD to arrive in the mail, when it looks like I can already create a constant current with this modification?

    The only drawback I see is that I've limited current by depriving the drive transistors of base current, so when the chamber resistance drops below 10.2k and the collector wants more than the 2.64mA (probably about 6mA) then the drive transistors will be in forward-active mode, and presumably heat up a bit.

    Is this bad? or is it an acceptable way to limit current?

    Thanks in advance,
    -masked
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    That's a lot of parts, when you could do the whole thing with 3 channels of a quad opamp and a few support components.

    Your 555 will receive very poor voltage regulation. The Zener needs a constant current through it in order to properly regulate voltage. As it is presently configured, your Zener will have a widely varying current flow through it. You will need to consult the particular Zener's datasheet to determine it's voltage @ current requirement, just for starters.

    If you want to keep the 555 timer's supply at around 9v-12v, you'll need to use a different regulation scheme.
    [eta]
    You do not show bypass caps across the 555's supply. The minimum requirement is an 0.1uF/100nF metal poly or ceramic in parallel with a 1uF cap, which can be electrolytic.
     
    Last edited: Jul 27, 2010
  3. masked

    Thread Starter Member

    Jul 1, 2010
    48
    1
    I don't even know what an opamp is yet - I'll go read.
    Out of everything you see, the only components I've used before are batteries, wire, and LEDs. I'm trying to learn about the parts I have on hand to see how this can be done.

    About the bypass cap: None of the examples I've seen use one or mention one. (http://www.kpsec.freeuk.com/555timer.htm for example). Can you help me understand what function it would perform?

    About the voltage: I started a thread specifically on that about 2 weeks ago, and as the Zener was suggested by Bill Marsden, I went with that method. Some people told me to simply connect the timer to 1 of the 3 9V batteries and some people told me that was bad.... but nobody explained why.

    Thus far I've physically built and tested about 10 versions of this but they all used the same timer circuit, and it has shown very stable function in all cases. My setup only switches the timer state once per minute, and the amps go smoothly from 0 to 2.64mA over the course of 30 minutes - so maybe that's why I haven't seen a problem.

    Can you explain what factors you were seeing that would potentially make the voltage regulation poor? Or if I use another method, can you explain why to (or not to) hook my timer supply up to just 1 of the 3 batteries?

    ...and the main question this time was whether allowing my transistors to run in forward-active mode would be bad for my device which otherwise runs perfectly.

    Thanks again,
    -masked
     
    Last edited: Jul 28, 2010
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