linear capacitor charging

Discussion in 'The Projects Forum' started by waltsar, May 10, 2013.

  1. waltsar

    Thread Starter New Member

    Nov 10, 2012
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    0
    I'm trying to find a way to get a linearly rising voltage output from a motor control circuit. I've done this in the past by using a bi-polar P/S and op-amps. I'd like to be able to the same with only one P/S.

    The attached is the circuit I've used before and it works very well. How can I do the same using a single P/S and either an O/A or transistor?

    Thanks,
    Walt
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,151
    3,058
    I don't think there's anything stopping you. But you might want to choose rail-to-rail op-amps.

    What is Q2? I see only one input.
     
  3. waltsar

    Thread Starter New Member

    Nov 10, 2012
    10
    0
    Sorry about that - Q2 input : the inverting input is shown, the non-inverting input is tied to common.

    I thought that a single polarity P/S causes an offset in the output. I'm not sure I know what a "rail-to-rail" op-amp is.

    Thanks,
    Walt
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,151
    3,058
    A rail-to-rail op-amp allows the inputs to come very close to the power rails, and can produce outputs very close to the rails. A "normal" op-amp could be limited to no closer than, say, 3V to one of or both rails. And of course there are amps that include one rail or the other but not both. For instance I have a bunch of LM358 op-amps on hand because they operate down to the negative rail. Look for "common mode voltage" range on the data sheet.
     
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