SCR Triggering

  1. I have a puck-style SCR with a cathode, anode and gate. I'm planning on using it to dump two series capacitors (450v + 450v) into a coil for a "coilgun" project.

    I understand that the SCR is triggered when a circuit is completed through the gate and anode using, for example, a battery and a simple switch.

    My SCR's datasheets specify IgtVgt (in mA/V) to be 300/3. I'm assuming that this means I need 300mA at 3 volts through the triggering circuit to turn the SCR on.

    My question is, do I need a resistor in the triggering circuit to protect it from the anode that is connected to the capacitor bank charged to 900v? Or, do I just need a resistor to get the 3 volts (probably a couple of AA batteries) to produce 300mA?
     
  2. jcsd
  3. Triggering an SCR by touching a wire to a battery may risk damaging the SCR by a series of excessively short pulses due to poor contact to the batteries. I would suggest using a NE555 timer set up as a monostable using 4 AA batteries. This will give a solid pulse of preset length. See

    http://focus.ti.com/lit/ds/symlink/ne555.pdf

    Otherwise, make a "bounceless switch" using two NAND gates.In either case You probably will need to use an NPN emitter follower to get 300 mA. Don't connect the gate to the anode.

    Bob S
     
  4. Thanks for your response, Bob S.

    The simple switch I was talking about would consist of slightly more than touching a wire to a battery. It would probably involve an industrial momentary push-to-close switch.

    I'm fairly familiar with the NE555 timer (not with programming one however), but I'm not familiar with a NAND gate or NPN emitter follower.

    The main thing I'm confused about, is how the single gate terminal is used to turn the SCR on. I mentioned connecting it to the anode -- I was referring to doing so in the manner depicted in the picture below.

    [​IMG]
    (coilgun.info)

    The triggering circuit above consists of a battery, switch and resistor. I was confused about what size resistor to use, but now I'm confused wholly about how to use the SCR in my circuit.

    Thanks for your time.
     
  5. vk6kro

    vk6kro 4,059
    Science Advisor

    The battery would need to be the other way around (+ towards the gate) and there should be a resistor across from pin 1 to pin 2.
    If this was 1000 ohms, then the resistor R3 would need to be about 4700 ohms.

    Leaving the gate open would invite false triggering.

    I would also move the SCR so that its cathode was grounded and it switched the coil connected to its anode.
     
  6. dlgoff

    dlgoff 3,067
    Science Advisor
    Gold Member
    2014 Award

    Also note that Bob S was referring to switch bounce as a cause for damaging the SCR.
    http://en.wikipedia.org/wiki/Switch#Contact_bounce

    Here's how to debounce with nand gates:
    [​IMG]
     
  7. That depends on what you call 'ground'. There is no reference in the schematic, so....
     
  8. What would the resistor from pin 1 to pin 2 be needed for? And what did you use to calculate those resistor values?

    Are the NAND symbols the logical representations of transistors? Or are they some other physical device?
     
  9. vk6kro

    vk6kro 4,059
    Science Advisor

    Quite true. :)
    That would be another good modification, to ground the negative line.

    I'm also a bit worried about the SCR not turning off due to voltage across the capacitors not disappearing completely even when they are shorted. I've seen electrolytics develop 50 volts or so even after they have been shorted.
     
  10. I don't think I'd worry about them not shutting off. The SCR will continue to drain the caps until there isn't enough charge left in them to maintain conduction. I know that due to dielectric absorbtion a cap can appear to take on a charge again, but I would say that once an SCR is out of conduction, it's a done deal and won't conduct until there is activity on the gate lead. Of course, R1 needs to be sized right so that it will not allow the SCR to continue to conduct indefinitely. This may end up being so large as to require very long charge times for the capacitor.
     
  11. vk6kro

    vk6kro 4,059
    Science Advisor


    The resistor would be there to require a substantial current to turn the SCR on. If it was 1000 ohms, there would have to be 1 mA flowing in the 1000 ohm resistor to get 1 volt of gate voltage.
    If the resistor was not there, very small leakage currents of a few microamps would possibly turn on the SCR. Even induced voltages from the coil would be enough to fire the SCR when it wasn't required.
    With a rail gun, this could get dangerous.

    The resistors I suggested would give about 1.58 volts which should fire the SCR easily

    NAND gates are available in integrated circuits. They are part of a family of logic gates that are used with digital signals. In the case of NAND gates, they might have two inputs and an output which is either high (5V) or low (0V) depending on the combination of voltages applied to the inputs.
     
  12. The two resistor shown in the above circuit (~5k ohm) should be pull-up resistors attached to Vcc (The + supply to the NAND gates). The negative side of the power supply should be attached to ground, like the switch.
    Bob S
     
  13. dlgoff

    dlgoff 3,067
    Science Advisor
    Gold Member
    2014 Award

    Yes. The article from where the schematic came from says,
    Looks like they are using the open triangle symbol as the positive rail.
     
  14. Contact bounce is almost never an issue with the SCR. Once fired it stays fired until the current drops below the holding current (its on the datasheet for your SCR.)
     
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