SCR to Discharge Capacitor Bank

Discussion in 'General Electronics Chat' started by jct891, Sep 30, 2016.

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  1. jct891

    Thread Starter New Member

    Sep 30, 2016
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    Hello,

    I have a 7334uF capacitor bank rated at 400V with a power supply of 400V and 1A. I am looking at a way to discharge them with the least amount of noise and light production. I've read that SCR triggering is the best way to go. If so, what will the rating of my SCR need to be? Which type of SCR should I use? And would you be able to provide a sample circuit? Thank you.

    Juan
     
  2. DickCappels

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    Aug 21, 2008
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    It depends on how fast you need to discharge the circuit, how much energy needs to be removed from the cap and how much current the cap can handle.

    It is easy to kill a low ESR capacitor with an SCR.

    Will the SCR be dumping the energy into a load? If so, please describe the load.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    If you have a 1A supply connected to the capacitor then the SCR may not turn off when discharging the cap.
    You may have to turn off the supply when discharging the cap.
     
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  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    The bank has allmost 600J of energy when fully charged, so this energy will need to be dissipated somewhere, like for example a resistor.
     
  5. shortbus

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  6. jct891

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    Sep 30, 2016
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    @ DickCappels, thank you for your response. I forgot to mention that the caps are electrolytic at ~100uF each and totaling 7334uF when measured with a DMM. I would like to discharge the circuit in 100us or less and be able to dissipate about 588J of energy. The idea is to discharge the caps into a coil of ~52uH (wire AWG 14) and generate a flux density of 1 - 3 Teslas.
     
    Last edited: Oct 3, 2016
  7. jct891

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    Sep 30, 2016
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    @crutschow, thank you for your response. How would I be able to make the SCR turn off when discharging the cap without turning the supply off?
     
  8. jct891

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    Sep 30, 2016
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    @kubeek thank you for your response. Correct the bank has about 580J of energy. Would the SCR resist that much energy? The resistor discharges the bank slowly. I would like to discharge the bank in about 100us or less.
     
  9. #12

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    Nov 30, 2010
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    I'm thinking you can't buy an scr that is rated for kiloamps.
    Would somebody please check my numbers?
     
  10. jct891

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    Sep 30, 2016
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    @shortbus, thank you for your reply. I'm trying to build a transcranial magnetic stimulation device. Not as powerful as the "coin shrinker" but same concept. However, I'm trying to use an SCR to discharge the caps instead of using a spark gap. That way I can reduce the noice and light production.
     
  11. #12

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    I was wrong. Here's one for $300
     
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  12. jct891

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    Sep 30, 2016
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    @#12 I have TRIACs rated at 600V, 16A and thyristors rated at 1kV, 12A. How would I be able to calculate the current generated by the capacitor bank at the time of discharge? Would I be able to control the current produced with a small value resistor? like an 27-Ohm 20W resistor and then use a diode to control the current dissipated to the resistor as to not burn it out since we will have a about 6kW generated at time of discharge?
     
  13. #12

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    You're outside my ball park. I'm not talking until somebody runs the numbers.
    580 watt-seconds in .0001 second is in the megawatt range.
    400 volts imposed on 52 uH gives a rate of rise in the mega-amps per second range.
    I can't seem to wrap my head around that right now.
    I will be getting a root canal job in 2 hours.
    Maybe tomorrow I will be able to think.
     
  14. jct891

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    Sep 30, 2016
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    @#12, 580J in 100us will equal 5800W. 5800W/400V = 14.5A.
     
  15. #12

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    580 watt seconds in 0.0001 second is 5.8 megawatts for 100 microseconds.
    Isn't it?

    I have doubts that you can accomplish this, even if you reconsider how to minimize the resistance at every conceivable part and connection.
     
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  16. jct891

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    Sep 30, 2016
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    @#12 Thank you for that! I was doing my math wrong! Instead of it being 100us I will try 100ms and that way reduce the amps going through. I was doing my math at 100ms and not at 100us, thanks again!

    So at 100ms would I then be able to use the SCRs mentioned above?
     
  17. #12

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    Again, when you slam the SCR "on", 400 volts into 52 uH gives a rate of rise of 7.7 million amps per second.
    The resistance in the wire will become a limiting factor.
    This is where you have to consider the physical properties of the entire layout to find workable numbers.
    You need to provide a lot more information for us to find these real-world limits.

    Gotta go. Dentist in 30 minutes.
     
  18. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    In other words, to discharge 580 Ws from inital 400V in 100ms will require 14.5A if I am not mistaken.
    Nevertheless, even if the SCR withstands the current through it, where do you think this eneregy will end up? Like #12 said most of it will heat the coil to very high temperature.
    I am not sure what you are trying to achieve, but I wouldn't put my head anywhere close to such device.

    My magnetics are a little bit weak, how much current do you need in order to achieve 1T peak? I have a vague feeling that depends on the area of the coil, but really am not too sure. Do you have this number?
     
    Last edited: Oct 3, 2016
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  19. #12

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    I survived!:)

    I was in a 7 (seven) Tesla MRI, 2 years ago. It's survivable, but it gets really hot in there!
    You can feel the energy pulses heating your skin.:confused::eek:

    So far, the only information we have is 52uH, 400V and 7334 uf's.
    Working with nothing but that results in huge numbers and a similar amount of doubt.
    Repeating myself: The resistance of the wire is going to be a limiting factor.
    You can't slam mega-amps around in the basement like a battleship mounted rail gun.
    There's a lot more to this than a capacitance, some microhenries, and a switch, and those other parts will set the limits.
    Provide that information to get useful answers.
     
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  20. BertHickman

    New Member

    Oct 3, 2016
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    Plugging in some trial numbers:
    C = 7334 uF
    V = 400 VDC
    0.5VC^2 = 587 joules
    L = 52 uH
    Wire gauge = #14 AWG
    Assumed helical coil, diameter = 6"
    Assumed number of turns (close-wound) = 15 turns
    Approx coil inductance = 53 uH or so...
    Coil wire length (assuming two 4" leads off coil) = 291"
    Approx coil resistance (copper wire) = 61.2 milliohms

    Assuming a "perfect" switch and no capacitor ESR, you'd have an underdamped oscillatory circuit with a natural resonant frequency of about 250 Hz with the following results:
    Coil peak current = 4625 amps
    Time from switch close to peak current = 996 us
    Peak field at 1st current peak = 3.09 T

    The actual peak current will be less due to capacitor and switch ESR, but it looks like you'll be in the right ballpark IF the coil does not self-destruct from heating and inter-turn magnetic forces. You'll also need some way to protect your polarized capacitor bank from reverse current (such as a heavy rectifier across the capacitor bank). A high-current (>300A) SCR or IGBT should work as a switch. Not sure I'd want to put my head inside the coil though... :)
     
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