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To SCR or not SCR?

Discussion in 'General Electronics Discussion' started by bonedoc, Jan 8, 2012.

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  1. bonedoc

    bonedoc

    122
    0
    Dec 21, 2011
    Hey everyone, I have been using a SCR for discharging some high voltages. (up to 300V, few hundred uf). I recently found out that my inductive load would lock the SCR into the active state. This was annoying, but manageable by cutting off power to the area. However, I decided to add an optocoupler for more chip protection. The optocoupler has also made it difficult to keep the SCR from locking up, because it is on its own area of power. Furthermore, I am not using AC voltage, only DC, so I question the need for the SCR.

    So....I have 2 questions:

    -Can anyone recommend a NPN that can discharge higher voltages, at high energy (around 20J)? I have some IRF730....but I dont think they can handle much energy..just voltage.

    -In the case of the SCR, Is there a simple way of keeping it from locking....like resistors or capacitors? In other words, keeping the thing explicitly off until and obvious signal happens?

    Thanks.
     
  2. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    This is the type of circuit that is used in electric fence energisers, they work well.
    In order for the SCR to turn off, the current in the SCR must drop to zero and there must be no trigger voltage. If you are driving an inductance, it can be arranged that the inductance can ring with a small capacitance to try to reverse the current, this will turn off the SCR. If the capacitor is constantly charged, it may not be possible to get the SCR current down to zero.

    I had to look up the details of the IRF730, perhaps you could have done this.
    The IRF730 can stand 400V and pass 5.5A. The amount of energy will depend on the time that it is passing current and the current will depend on the inductance which is being driven.

    There are many high voltage transistors, used in switch mode power supplies, and I am not up-to-date with current practise. I have some ttransitors removed from faulty TVs and power supplies.
    2SD820 600v 5A
    BU508 700V 5A
    BUT11A 450V 5A
    BUX87 450V 1A

    FETs
    IRF830 500V 4.5A
    IRFR310 400V 1.7A (ex CFL Nmos)
    IRFR9310 400V 1.7A (ex CFL Pmos)
    You can use a parameter search provided by one of the suppliers for others.
     
  3. bonedoc

    bonedoc

    122
    0
    Dec 21, 2011
    Ok, I did a rough calculation on the inductance of my inductor. It will be around 1.5H. In the case of a discharge of a capacitor at voltage X, shouldnt the peak current flow be at the instant the cap is discharged? Also, there should be no frequency involved, so shouldnt there be an easy way to calculate the max current seen?

    I looked at electrical fence schematics, and it looks like they either have a 0.1 uF decoupling cap from the drain to ground, or an IN4005 diode from the source to drain. Is either just as acceptable?
     
  4. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    Assuming no losses, the energy in the capacitor will go into the inductor.
    0.5 * C * V*V = 0.5 * L * I*I so I = V * sqrt(C/L)
    Taking 300V, 200uF, E=9J, with L=1.5H, Ipeak=3.5A

    The current will continue until the capacitor is charged in the reverse direction which is disastrous if it is an electrolytic. Diodes may be added to stop this happening.

    There will be an oscillating frequency involved due to the inductance and capacitance but presumably you are sending energy somewhere so it may be highly damped.

    P.S. I could do with a tame bone doc!
     
  5. bonedoc

    bonedoc

    122
    0
    Dec 21, 2011
    Thanks! Now I can do some calculating.

    Let me guess, you have a lumbar disc herniation? :)
     
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