thyristor

Discussion in 'General Electronics Chat' started by cheeseycheese, Dec 19, 2006.

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  1. cheeseycheese

    Thread Starter New Member

    Dec 19, 2006
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    hi
    ive bought 6 stud scrs but when I came round to testing them they did not work, I cannot find the datasheet anywhere but I guessed the pinouts: small connector-gate case-cathode big connector-anode. I am begining to wonder if dropping them damaged it?... I hope not.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    Got some type numbers? Are they anode-to-case or cathode-to-case?
     
  3. cheeseycheese

    Thread Starter New Member

    Dec 19, 2006
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    The number on the case is: cr12 u02 jy 1 im not sure if the spaces are ment to be there..
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    Don't think you can get much from those numbers...

    The usual way to test SCR's is to use a meter and confirm that there is no conduction anode-to-cathode, or cathode-to-anode. Using the diode check, look for open going gate-to-anode. The gate-to-cathode may look like a short - there may be very little difference in conduction when you reverse the leads. And the conduction may have very little resistance - not at all like a pn junction in a transistor.

    If they look ok on a meter, then you will need to check your trigger circuit. If you can post up the circuit, maybe we can see something not quite right.
     
  5. cheeseycheese

    Thread Starter New Member

    Dec 19, 2006
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    Ok I have tested between terminal 1 and 2 and there is no connecton +/-. There is a resistance of 32milliohms between gate and terminal 1 only +/- polarity
    Refer to the picture below.
    [​IMG]
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    The 32 mohms does sound awful. Try one other check. Put your meter across the SCR reading ohms, and connect the positive lead to the gate - just touch it briefly. If you have the polarity correct, the resistance anode-to-cathode should go to zero - the device is on and stays on because the voltage across it is DC.
     
  7. cheeseycheese

    Thread Starter New Member

    Dec 19, 2006
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    I did what you said, if I join gate to terminal 2 it reads 32milliohms regardless of the polarity. But if I join gate to terminal 1 it reads nothing with the polarity around either way... doesn't sound so good.
    Would dropping it affect it, are they fragile?
     
  8. Johann

    Senior Member

    Nov 27, 2006
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    Hi!
    The Stud on this type of thyristor usually is the Anode, the small terminal is the Gate and the one next to it is the Cathode. Use a bench power supply or a car battery and connect the positive thereof to the stud. Now connect one terminal of a 12V automotive lamp to the Cathode and return the other terminal to the Negative terminal of the battery or power supply. Take a 10 ohm or so resistor and connect one side also to the positive and momentarilly touch the gate with the free end of the resistor. If all is well, the lamp should light up and stay on until the circuit is broken again.
    Usually when it comes to this size thyristor, your multimeter does not have enough current to trigger the device into conduction, so a simple circuit as described above is quite a useful way of testing SCRs and TRIACS.:)
     
  9. cheeseycheese

    Thread Starter New Member

    Dec 19, 2006
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    Well I did that and it turned on for about 2 seconds then cut out, for a expirment I removed the cathode and found out it was doing the same thing... all of the currant was passing through the gate!!
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    Gates should not carry conduction - it looks like they are toast.
     
  11. cheeseycheese

    Thread Starter New Member

    Dec 19, 2006
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    Ahhhhh!! I broke one of my scrs, how I dont know, but I tried another one it worked! the weird thing is when I touch the gate to v+ it turns on then if I touch it to v+ again it will turn off, is this normal??
     
  12. Johann

    Senior Member

    Nov 27, 2006
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    Hi All!
    The thyristor should turn on and latch when the gate is made positive with respect to the cathode and although it turned off when you touched the gate with a negative polarity, it is not the correct way to do this. Thyristor turn-off should be done by interrupting the current from anode-to-cathode. One method is to put a switch in line with the main current path and another way is to temporarilly short out between anode and cathode. This is also accomplished by means of commutating capacitors (which effectively do what is explained above).
    However, there are devices known as GTOs (Gate-Turn-Offs). They are designed to turn off with a negative pulse on the gate (or a special second gate provided for this purpose), but they are not freely available through the normal component suppliers and were used in limited numbers in the past.

    Another important factor to remember is that a thyristor has a minimum holding current in order for it to stay on after gate triggering, also it must have a certain minimum and maximum gate current. Unfortunately, this can only be found on the data sheet for a particular thyristor. :rolleyes:
     
  13. cheeseycheese

    Thread Starter New Member

    Dec 19, 2006
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    Hmmmm maby I have the GTO but on positive, it latches on and off whenever i touch it to positive. Somehow I am doubting it is ment to do that but as long as it does not damage it I dont mind. I hope I have not got the rare type, I feel really bad damaging moast of them. Thanks for all of your help, Now I can get one step closer to making my coilgun :)
     
    Last edited by a moderator: Nov 11, 2011
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