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Need Help Making High Voltage Automatic Capacitor Discharge Circuit

Discussion in 'High Voltage' started by ZGrizzlyWord, Nov 19, 2011.

  1. ZGrizzlyWord

    ZGrizzlyWord New Member

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    Everything powered up when I plugged it in, but the SCR never fired. Should the diode allow or disallow current through the SCR? (the diode above the SCR)

    Thanks.
     
  2. gary350

    gary350 Well-Known Member

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    You can use an industrial mercury contactor it is a switch used as a relay I have several of these. You can buy these at your local industrial electrical supply company.

    Here is one rated 35 amps with a 24 VAC trigger only $30. Notice this is for 3 phase you can connect all 3 contactors in parallel to get 105 amps 600 VAC. You can not get any simpler than this and they are almost industructable.

    http://www.ebay.com/itm/WATLOW-MERC...768?pt=LH_DefaultDomain_0&hash=item5884458d80

    This one is rated 120 amps 600 volts. It is triggered by 120 VAC. Very simple.

    http://www.ebay.com/itm/MDI-235NO-1...&otn=3&po=LVI&ps=63&clkid=4736561968340127274

    Here is a cheap one $34 with a 24 volt trigger.

    http://www.ebay.com/itm/Durakool-Me...746?pt=LH_DefaultDomain_0&hash=item2a0ae6f59a

    You can also use a NL-7171 Mercury Ignitron Tube rated 15,000. volts 35,000. amps, 525 Meg Watts. I have 3 of these. I don't recall the trigger voltage but it is on the data sheet.

    http://www.rell.com/Pages/Product-Details.aspx?productId=187292
     
    Last edited: Dec 7, 2011
  3. alec_t

    alec_t Well-Known Member Most Helpful Member

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    You hopefully have the diac in series between the top of C2 and the SCR gate? Try increasing the value of C2 to, say, 47u (it may not hold enough charge to fire the SCR). Do you have the triggering spec from the datasheet?
    D2 (1N4006) should be polarised as shown in my previous post, to block current from R1 to the SCR. Its purpose is to 'short out' the damaging back-EMF generated when the coil current is switched off.
     
    Last edited: Dec 8, 2011
  4. dave

    Dave New Member

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  5. ZGrizzlyWord

    ZGrizzlyWord New Member

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    Ok, I decided to use a breadboard to try and debug this sucker. Here is what I can tell you:
    1) C1 is charging very quickly and reaches 185V rapidly (using 115 AC for now, will step up to 230V once I got this thing going). The green light means 185V is reached.
    2) C2 charges slowly, but always increases until it also reaches 185V.
    3) The red light was meant to simulate a load in-place of the inducer. The red light is brightest when the system is turned on, and turns off as C2 approaches 185V.
    4) I have reduced C1 from 680uF to 100uF to increase space on my breadboard.
    5) I have increased C2 from 10uF to 100uF to try and increase C2 like Alec mentioned
    6) I have increased R2 from 100kohm to 10mohm in an attempt to increase the R2*C2 time constant.

    Please see picture of breadboard. You will see the removed 100kohm R1 resistor is replaced with 820ohm resistor (trying to reduce the C1*R1 time constant), the 10uF capacitor and a potentiometer in the background. I also thought I might have a faulty SCR, so you see the new one with soildered leads and the old one laying on the wood.

    [​IMG]
     
  6. ZGrizzlyWord

    ZGrizzlyWord New Member

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    Here is the spec sheet:
    http://www.electro-tech-online.com/custompdfs/2011/12/Littelfuse_Thyristor_Sxx35x.pdf

    Here is the product page:
    http://www.mouser.com/Search/ProductDetail.aspx?R=S6035KTPvirtualkey57610000virtualkey576-S6035KTP
     
  7. alec_t

    alec_t Well-Known Member Most Helpful Member

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    According to the spec the gate needs at least 1.5V and somewhere between 5mA and 40mA to fire the SCR. It should be able to get that from C2 if things are wired ok. If the diac conducts to fire the SCR the voltage on C2 should never rise significantly above the diac breakdown voltage (~20V). Double-check the pin-out of the SCR and ensure its's wired 'right way round'. Doubl-check your breadboard wiring. Can you post the schematic of your present circuit, including the LEDs etc?
     
  8. ZGrizzlyWord

    ZGrizzlyWord New Member

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    Please see the schematic below. I accidentally had the SCR wired cathode as positive and anode as negative, but for the breadboard I got a new SCR and got the polarity right. Speaking of which, do you think the other SCR was damaged by the wrong polarity? Also, do you think maybe I should switch over to a solid state relay instead of the SCR? Thanks for the help.
    [​IMG]
     
  9. alec_t

    alec_t Well-Known Member Most Helpful Member

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    If you had the other scr reverse-polarised in the same circuit as shown above then less than 1mA of reverse current could have flowed through it and I think it's unlikely to have been harmed. That 1mA is probably insufficient to sustain conduction, so the new scr will only conduct for the very brief duration of the trigger pulse from the diac and you are unlikely to notice the red LED light up. The 2.2Meg resistor seems unnecessary.
     

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