SCR Clarification

Discussion in 'General Electronics Chat' started by will1234, Nov 10, 2007.

  1. will1234

    Thread Starter New Member

    Nov 10, 2007
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    0
    I have seen a lot of posts here concerning how to fire an SCR and after reading through most of them I'm more confused now than before. I have been told that for the SCR I have the primary load goes across the large lead (anode) and body of the SCR (cathode) and the trigger signal goes across the small lead (gate) and body. To fire it I needed to add a resistor (2K or so) on the gate lead and apply a standard TTL signal (5v) to the gate which will glose the primary conenction and pass current. In my implementation (YAG laser trigger) the primary voltage is 240v so is this stating that I am to apply the 240v to the gate lead? This will burn the SCR up wouldn't it? If I were to try using a 9v battery would connecting the negative to the SCR cathode and the positive with the resistor to the SCR gate, should this cause the current to flow on the primary?

    Thanks in advance....
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    If you have a particular SCR in mind, the data sheet for the device will explicitly state how to trigger it.
     
  4. awright

    Well-Known Member

    Jul 5, 2006
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    7
    will1234, your statement, "I have been told that for the SCR I have the primary load goes across the large lead (anode) and body of the SCR (cathode)...," is incorrect if I understand what you are saying. Your statement appears to say that the load is connected in parallel with the SCR, i.e., with one load lead connected to the anode and the other load lead connected to the cathode. If the load is "across" the SCR in this fashion, and the power is connected to the parallel combination, there is nothing in series with the SCR and the power line to limit current through the SCR except the branch circuit fuse or circuit breaker.

    If you applied line power to the paralleled SCR and load, before triggering the SCR, the load would be energized since it is in series with the line and the presence of the "off" SCR in parallel would have no affect. At the instant of triggering the SCR, destructive current would surge through the SCR and either blow out the SCR or blow out the fuse or circuit breaker powering the branch circuit you are using. One would expect the failure of the SCR to be fairly dramatic, but I suppose it could be subtle and go unnoticed if it was a low current SCR that failed quickly.

    Did you, in fact, connect the load in parallel with the SCR and have you tested the SCR to see if it is still functional?

    It sounds like you might benefit from reading a basic tutorial on SCR applications and trigger circuits. The old General Electric SCR Manuals were excellent information sources on all aspects of SCR applications and testing. Try a search of on-line used book stores.

    By the way, an SCR is an SCR is an SCR. While specific SCRs might be optimized for a specific tasks (phase control, static switching, capacitor discharge, etc.), they are all applied in the same manner, i.e., with the load in series with the SCR and the power source. So I'm not sure why anyone would tell you that, "... for the SCR I have ...," the primary load goes across....

    Additionally, depending upon the application, triggering can be somewhat critical to be sure you turn the SCR on fully before current builds up to a high value. This is especially true for a capacitor discharge application (like a flashlamp driver) because of the high current available from the capacitor very quickly. Triggering requirements are discussed in the spec sheet for the SCR you are using. I'm not sure a TTL signal applied to the SCR gate, either directly or through a resistor, would be adequate for your application, as the voltage can be as low as about 3 volts (I have not looked up the TTL standard) and the current available from the specific TTL gate you are using might be too low for a capacitor discharge circuit.

    What happens if you do not have an adequate gate drive pulse is that a small region of the SCR die can be turned on and all the current to the load will pass through the limited portion of the die that is turned on, causing too high a current density and damage to or failure of the SCR. With inadequate gate drive it can take longer than normal for the entire die to turn on, although it will turn on within microseconds.

    Have fun.

    awright
     
  5. will1234

    Thread Starter New Member

    Nov 10, 2007
    8
    0
    I believe I'm so far gone in the wrong direction that I'm going to pull back and start again.

    So, I have an RCA JAN2N690CRC but am unable to find a datasheat for it. I'm attaching a mspaint scetch of how it looks. The more I try to think of how to connect the further back I go. I believe the output (taller connector on scr) should be the high voltage lead headed to the trigger transformer and the other shorter connector would be connected to the positive side of the 'triggering signal', a 9v battery with a 2K resistor on it but I don't see any way this could be correct so far. Can anyone make sense of this and guide me a little? It can't be as hard as I'm making it but it's driving me insane...

    Thanks in advance.
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    Go to www.datasheetarchive.com and search for part number 2n690.

    The prefix JAN means that it is a military-grade device.

    hgmjr
     
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