Delay Sharp response

Discussion in 'General Electronics Chat' started by Top-Dog, May 9, 2013.

  1. Top-Dog

    Thread Starter Member

    Mar 1, 2013
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    Hi, I need a fast response/low rise time RC timer. I have tried a few different BJT configurations, but I can't get the fast response/slew rate that I need. The only way that I know that gives decent results is an op-amp comparator against a fixed bias, but I need multiple timers for my project and I don't want to use an op-amp for each one. Any suggestions?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    A quad comparator, like the LM339, has 4 comparators in each package. That likely will take fewer parts then a discrete solution to your problem.
     
  3. chuckey

    Well-Known Member

    Jun 4, 2007
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    have you tried putting a diode across the 220K resistor. The cap then charges through the forward conduction of the diode, but decays via the 20K and the base current of the transistor.
    Frank
     
  4. bertus

    Administrator

    Apr 5, 2008
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  5. crutschow

    Expert

    Mar 14, 2008
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    What exactly do you mean by "fast response/low rise time RC timer"? :confused: What is the output you want with respect to the input?
     
  6. Top-Dog

    Thread Starter Member

    Mar 1, 2013
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    Basically, I want the output signal to go high after the RC timer has reached its time constant. At the moment you can see from the current through the LED that the circuit takes over 10 seconds to completely switch to fully conducting. I want to reduce this transition time to < milliseconds if possible.

    As an aside, is it possible to generate a pulse like this i.e. two different RC timers and a discrete component logic gate, and when both RC timers are fully conducing use an XOR gate to pull the output low (and use a push button to short the caps to reset all the timers)?
     
  7. Top-Dog

    Thread Starter Member

    Mar 1, 2013
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    Thanks chuckey. Putting a forward bias diode really helps, as you can see the rise time of the response signal is about 1.1 seconds now. I've had a look at the wikipedia BJT schmitt trigger, but is there a way to simplify it i.e. make it switch when the input voltage > V_threshold (I don't need it to switch back when the signal falls, because the signal never falls in this implementation)?

    Is the simplified circuit the one shown here: http://en.wikipedia.org/wiki/File:Schmitt_parallel.svg
    I assume the input is connected to the base of Q1 via R.
     
    Last edited: May 10, 2013
  8. crutschow

    Expert

    Mar 14, 2008
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    You could generate your desired delay and subsequent pulse with two 555 timers (or one dual 556 timer) configured as one-shots. The first one-shot provides the delay which then triggers the second one-shot at the end of the delay. Basically the one-shot period of these timers is the RC time-constant of their timing components.

    The 555 has a reset input to reset the timers.
     
  9. Top-Dog

    Thread Starter Member

    Mar 1, 2013
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    Thanks for the idea, but I can't use 555 or 556 timers in this project (it's one of the limitations of the design criteria).
     
  10. crutschow

    Expert

    Mar 14, 2008
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    Well, it would be nice to know the limits of the design rules up front so we don't spin our wheels. :rolleyes: Any other limits we should know?
     
  11. Top-Dog

    Thread Starter Member

    Mar 1, 2013
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    No digital components (inc. gates (IC versions), uC and registers, and monolithic timers), but I can implement them with discrete components and op-amps - it's an all analog project. I also only have a limited supply of parts, so I can't order online (hence no exotic IC's). I am also trying to have a low component count as possible, and reduce the complexity of the circuit to make simulation/analysis/debugging/budgeting easier.
     
    Last edited: May 10, 2013
  12. crutschow

    Expert

    Mar 14, 2008
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    In post #6 you suggested using an XOR gate but now you say no digital components. Your limits seem to be shifting. ;)

    If you have op amps then just use them as comparators from the RC output to give a fast turn-on. Use them with a little positive feedback to provide hysteresis (of say 100mV) and prevent oscillations at the trip point.
     
  13. Top-Dog

    Thread Starter Member

    Mar 1, 2013
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    yea, I see what mean. I can't use IC's (other than op amps) to implement logic functions, but something like this is fine.

    Is there another way to get the same (or better) response with passive components? I'm not against using op-amps for the RC timers, but it's a last resort.
     
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