SCR firing troubles

Discussion in 'General Electronics Chat' started by fizzit, Jul 1, 2010.

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  1. fizzit

    Thread Starter New Member

    Jul 1, 2010
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    Hi, I'm new here, and this looks like a really cool forum. I was wondering if someone could help me with my SCRs.

    I've been having some problems firing them. I had two Teccor/Littelfuse S8040R SCRs (in parallel) hooked up to a capacitor charged to about 400v. The load (a coil) is connected to the + side of the capacitor, and the other end of the coil is connected to the anode of the SCR. The SCR cathode is connected to the capacitor minus. The gate is connected to a pushbutton switch in series with a 2.4v (two NimH AA's in series) supply, and the - of this is connected to the cathode. It would not fire when I pressed the button. I then disconnected the capacitor and measured the voltage between the gate and the cathode when the button was pressed, and it was only 1.44 volts! The SCR gates draw a lot of current which drags the battery voltage down a lot. When I hooked it up to my variable supply, the SCRs drew 3v, 3a! This makes sense to me because the SCR gate is supposed to be a diode, but then it is rated for 1.5v 30ma gate.

    I then measured the resistance between the tab and anode pin of the SCR, and they're supposed to be connected, but overloaded my multimeter...

    And the resistance between the SCR gates and cathodes is 82 ohms measured either way, and the diode voltage drop is around .1 volts, either way...

    I'm very confused. Why won't the SCR fire? Why does the gate draw so much current? And why aren't the tab and anode pins connected? Thanks in advance, for any assistance.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    This sounds distressingly like another coil gun. Fooling with high voltages and uncontrolled currents can result in injury or death. We do not support the discussion of such devices on these forums.
     
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