Cap Pulser Issue

Discussion in 'General Electronics Chat' started by Joster, Oct 6, 2013.

  1. Joster

    Thread Starter Member

    Jun 12, 2013
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    Hi All,


    I'm looking for some help getting this Cap Pulser circuit going. It's on the right track but I need some help getting the comparator section to work correctly. Right now Vin (Va) will not go above Vref (please see lab notes). When it is operating correctly, Vin will rise above Vref causing Vout to hit Vcc, which in this case is equal to Vz (15V). This turns Q1 all the way on and discharges C1 down to the battery voltge. It the should charge up to a voltage determined by the R1 pot.


    -Joe
     
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  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    So I guess there is something charging up the big cap that isn't shown?
    If that is the case since the cap is the power source its voltage can rise until it hits the threshold then turns on. But as soon as it turns on the voltage will drop and it will turn off. The result is something that hovers around the threshold voltage.
     
  3. Joster

    Thread Starter Member

    Jun 12, 2013
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    Yes that is the case. And yes, that is what is happening. But, there has to be a way to achieve the operation I outline in my notes...
     
  4. ronv

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    Nov 12, 2008
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    Is it always the same voltage that you want it to turn on and off at? If not what voltages? What is the voltage on the cap?

    In other words lets say the cap gets charged to 14.6 volts and you want it to stay on until it drops to 13.8? Something like that.
     
  5. Joster

    Thread Starter Member

    Jun 12, 2013
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    Yes, exactly! I need the cap to charge to 24V and then have Q1 on long enough for the cap to discharge down to somewhere near the battery voltage (around 13V or so)
     
  6. Joster

    Thread Starter Member

    Jun 12, 2013
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    The on time of Q1 would be short because these are photo flash caps. It's ment to pulse current into a battery to charge it.
     
  7. Joster

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    Jun 12, 2013
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    Also, I am getting the same problem when I use an external battery to run the LM741
     
  8. ronv

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    Nov 12, 2008
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    There are a couple of things.
    1- The turn on and turn off thresholds are the same. The turn on needs to be say 20 volts and the turn off 13 or whatever you choose.
    2- There is nothing to limit the current (except internal resistance) so the discharge time is very fast so the fet may not survive. Take a look at this one.
    http://www.mouser.com/ds/2/200/irfb4110pbf-78040.pdf
    I added some resistance (0.1 ohms) that limits the current to about 75 amps.
    I also changed the IC to a comparator - 741's are not very fast.
    Still need to know what is charging the cap as the duty cycle may be to high.
     
    Last edited: Oct 7, 2013
  9. Joster

    Thread Starter Member

    Jun 12, 2013
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    Wow! this is incredible. That circuit looks amazing! Turns out by adding a 47uF 100V Electrolytic Cap between pin3 of the 741 and ground I fixed the issue. I was very surprised that it worked because I was just trying it for the heck of it.

    I will definitely be taking a close look at the circuit you have here as time permits. Thank you again. It appears your are an extremely talented EE.

    What is charging the cap is high voltage spikes from a pulsed coil.

    Regarding the MOSFET handling the current spikes, could I not just put a bunch in parallel? When I do this I will need separate resistors for each driving circuit correct?

    Cheers!
     
  10. ronv

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    Nov 12, 2008
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    That might work. ;) That would let the voltage change slowly at the op amps so it would stay on or off longer.

    If you look above the big spike in the simulation you will see it is I V2 which stands for the current in the battery, ` 77 amps for 4 ms.

    Yes you can just parallel some FETs-no extra resistor required.
     
  11. Joster

    Thread Starter Member

    Jun 12, 2013
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    Ok Thanks! This has been more than helpful!
     
  12. Joster

    Thread Starter Member

    Jun 12, 2013
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    Can you please specify the Zener values in your circuit?

    Also, what is meant by "higher = higher trip on lower" and "lower = higher trip on high"

    Thanks!
     
  13. ronv

    AAC Fanatic!

    Nov 12, 2008
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    The diodes are just schottky diodes.

    "lower = higher trip on high - lower value resistor will make the turn on point higher on the cap voltage.

    "higher = higher trip on lower - higher value resistor will raise where the fet turns off.
    These are the resistors in series with the diodes
     
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