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Optoisolator in a stepper controller circuit

Discussion in 'Electronic Design' started by [email protected], Apr 9, 2007.

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  1. Guest

    I will be driving a unipolar stepper motor with an old 68HC11. I'm
    thinking about using this http://www.aaroncake.net/circuits/stepper.asp)
    circuit control the motor so the microcontroller can focus mostly on
    sensor input. I've been told I need to isolate the microcontroller
    from the stepper coils via optoisolators. I have three, possibly
    three questions about this:

    1. I'm assuming this means I will need two power supplies, one for
    the microcontroller/sensors and one for the steppers?

    2. The optoisolators I've looked at don't mention a built-in resistor
    in the schematics, do I need to provide one

    3. Would I put the optoisolators between the microcontroller and the
    control circuit (IE on the clock/step and reverse lines) or would I
    need to also isolate the controller circuit (IE place the opto-
    isolators between the JK Flip-flops and the power transistors?)

    4. If I need to isolate the controller circuit as well, would the
    4027 be able to sink the (10ma if I'm reading the 4Nxx spec right)
    current from the opto-isolator? I don't see info on maximum power
    dissipation on the 4027's spec sheet (http://www.futurlec.com/
    4000Series/CD4027.shtml) though I admit I'm no EE.

    Thanks in advance!
     
  2. Chris

    Chris Guest

    Hi, Richard. Starting with your questions:

    1) You'll need two power supplies anyway, unless your 68HC11 operates
    on 12VDC. Inductive loads like motor coils create voltage spikes and
    sags on supply lines, which can cause uC problems. Optoisolating the
    uC from the inductive load is probably a pretty good idea.

    2) Optoisolators usually have an LED on one side, and a
    phototransistor on the other that picks up the LED's light and turns
    on. You do need a current limiting resistor for the LED, otherwise
    you'll stress out the uC output port pin.

    3) You can put as many or as few optoisolators wherever you please.
    Usually it's done right at the STEP and DIRECTION signal, because that
    means you'll only need two optoisolators.

    4) Probably a bad idea to drive an optodiode with a 4000-series CMOS
    output -- it just can't source/sink enough current. Again, best to
    drive the optoLED from your 68HC11 output.

    Now for the rest of the story, as Paul Harvey would say. First off,
    the 4N26 is a miserable choice for logic optoisolation. Its current
    transfer ratio (the relationship between how much current is going
    through the LED and the current you can coax out of the
    phototransistor on the other side) is a ruinous 20% -- you have to
    pump five times as much current through the LED as you get at the
    output.

    Look -- use the KISS principle (keep it safe and simple). The H11L1
    is an optoisolator with built-in schmitt trigger logic output. It's
    been around for a long time, and it's relatively inexpensive. Drive
    the LED with only 3mA (for a 5V supply on the 68HC11, use a 1K series
    resistor from the port output pin to the LED). hook up the H11L1 pin 6
    to the isolated +12V, pin 5 to the isolated power supply common, and
    attach a 10K pullup resistor from pin 4 to pin 6. You can then read a
    guaranteed logic level for your step and dir driver control signals.

    The other issue is that you've probably got the wrong transistors
    there. It looks like you're asking the 4027 outputs to drive 10mA or
    so. That's not realistic. Not only that, but the current gain of the
    TIP31 transistors isn't going to be able to drive any but the smallest
    stepper motors, even with 10mA of base drive current. Do yourself a
    favor. Replace R1-4 with 4.7K resistors, and use an NPN darlington
    transistor like the TIP121 for Q1-4. Your transistors will be glad
    you did.

    Cheers
    Chris
     
  3. If you use one supply, there is not much reason to isolate a
    signal and then reconnect it to the same supply.
    Probably. Most isolators are an LED and a photo transistor
    of some kind. You have to add the components it takes to
    drive the LED with a reasonable (for it) current from your
    signal source and to produce an output signal from the
    transistor.
    I think you should isolate the step and forward/reverse
    signals, if anything. But with correct supply connections,
    you probably don't need the isolation, at all. Use two
    supplies, one for the logic and one for the motor, and
    connect them at one point... near the emitters of Q1-4.
    The CMOS logic shown could be powered from the 5 volt
    processor supply, if you can get enough base drive through
    R1-4. You might need to add a current boost stage if the
    logic chips cannot supply enough base current to saturate
    transistors.
    Doubtful. That would make the chips pretty warm, even if
    they could. Add emitter follower outputs and they can drive
    lots more current.
     
  4. Guest

    First, thanks for the response Chris.

    I'm curious about the dual power supply situation since this will be
    powered via batteries. Can I just plop a 7805 in front of the
    microcontroller and optoisolate as discussed, or do I really need a
    second set of batteries?

    Also, do you know of a multi-package for the H11L1 so I can keep the
    chip count to a minimum? I was eying the ILQ74, but it of course
    doesn't have the built in Schmitt triggers.

    Thanks again!
     
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