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All about SCRs

  1. Jul 16, 2018 #1
    I have this chart from 1973 seems this is all the SCR's they had what I wanted to know is when it says typical gate trigger current is this the minimum needed to turn it on or is the maximum it can handle? I built this sound to light converter. What I can't figure out is it says SCR's latch on when turned on and you need to disconnect power to turn them off does that mean gate? If this sound to light converter where used with 12 volts DC bulb instead of 120 volt AC would this unit with this SCR still work? I also found the buzzer picture on line would not pushing the switch turn on and off the buzzer?
     

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  2. jcsd
  3. Jul 16, 2018 #2

    NascentOxygen

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    typical gate trigger current would be the minimum needed to give reliable turn on

    With an SCR, the only way to turn it off is to interrupt the anode-cathode current by some external arrangement. If the SCR is connected to AC, this drops to zero every cycle, so the SCR automatically turns off by this fact. You can't turn off an ordinary SCR at its gate. There was a type that could be turned off at the gate, a second gate, unsurprisingly this device was called a gate turn off SCR; I've not encountered one.

    As for your buzzer, the push button turns it on and it continues buzzing forever, or until you disconnect the 3V supply.
     
  4. Jul 16, 2018 #3
    Yeah an SCR is basically a controlled diode, once you tickle it, it latches "on" until the PN junction is reverse biased.

    They are made with PNP connected to a NPN (collector to base on both) so the "gate" current is the minimum base current required to trigger the latching action.

    Any particular reason a SCR was chosen for your circuit? They are really only use full when used as a controlled rectifier on AC (eg light dimmer) or as a crowbar for protection.
     
  5. Jul 16, 2018 #4
    At least I know it works on AC not DC
     
  6. Jul 17, 2018 #5

    jim hardy

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    No. Once you trigger the SCR with gate current, typically tens of milliamps, and establish anode to cathode current , the SCR will remain "ON" until the anode-cathode current drops to less than the value in your "Holding Current" column. Usually we design the circuit to force anode current all the way to zero just to assure turnoff.

    So think of it as an automatic door - opens on a trigger to gate terminal allowing anode current , remains open until anode current becomes so feeble the door can snap shut again. But be careful with terminology in your analogies - an open door is same as a closed switch...

    old jim
     
  7. Jul 17, 2018 #6

    Averagesupernova

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    When in school I built a keypad combination switch using likely the most unconventional components. You guessed it, SCRs. Each SCR was hooked to a key on the pad. The SCRs were wired in a string. The combination could not have repeat keys. The first key would fire the first SCR in the string which allowed the following SCRs to latch on. Using comparators it could be determined which if any key was pressed out of sequence which would interrupt the current in the string of SCRs and reset the whole thing but without any indication to whoever was entering the code. There was also a timer that would allow only so much time to get the code entered. Until this thread, I had forgotten about it.
     
  8. Jul 18, 2018 #7

    jim hardy

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    SCR's make a simple memory element , as in @Averagesupernova 's post. (@ still doesn't autocomplete)
     
  9. Sep 3, 2018 #8
    As noted you have to take the anode current below the holding current to turn off the SCR. The typical gate current to trigger is not the maximum that you could encounter. Somewhere in the specification there should be a maximum required gate current. However, in general you should be fine if you applied double the typical current. Be careful not to exceed the maximum gate power (or current if specified). While I have used a DC current in one application it is generally the practice to use a pulse on the gate. The SCR should have a turn on time specified and setting the gate pulse longer should work well. SCRs were popular before the power mosfets became available. They do have the advantage of being able to block voltage of either polarity.
     
  10. Sep 3, 2018 #9

    Tom.G

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    When it says typical they mean it. That is the current at which most of the devices will turn on. Some will turn on with less gate current, a few will need more current to turn on. Their is often an Absolute Maximum gate current specified, which if exceeded, will damage the device.

    As for the buzzer circuit, it depends on the type of buzzer. If an electronic type the SCR will likely stay on. If it is an old electro-mechanical type with electrical contacts that open and close during operation, the SCR may or may not turn off depending on the current thru, and the inductance of, the buzzer

    Cheers,
    Tom
     
  11. Sep 15, 2018 at 8:30 PM #10
    I was always wondering if a transistor can be used in the same way as a SCR except when not apply current to base it would be off?
     
  12. Sep 15, 2018 at 8:47 PM #11
    An SCR is equivalent to an NPN - PNP pair. You can take an npn and pnp and wire them in the appropriate configuration. I have done that several times with small signal transistors like the 2N3904 and 1N3906 just to watch them work as an SCR.
     
  13. Sep 15, 2018 at 10:28 PM #12

    Tom.G

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    Yes. Which you choose to use depends on whether it is easier in you application to supply a pulse to turn on an SCR or a continuous current to a transistor. Also, historically SCRs were (still are?) available in higher current and voltage ratings than transistors.
    diag600.jpg
    This SCR is 2.9 inches in diameter and handles 1880Amps and over 2000Volts.
    (expensive though, about USD $500)

    Cheers,
    Tom
     
  14. Sep 16, 2018 at 12:49 AM #13

    jim hardy

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    If you're careful you can.

    A transistor can be used as a switch.
    An SCR IS a switch.
    An SCR allows current to flow in only one direction, that is if you reverse the applied voltage it'll block current flow.
    A transistor does NOT block reverse current flow. Reversing applied voltage is a good way to wreck one.

    That's what @Joseph M. Zias said above.(@ not working again)
    So don't try it in that illegible sound-to-;light circuit you posted .
     
  15. Sep 16, 2018 at 12:54 AM #14

    jim hardy

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    SCR belongs to a family called Thyristors.
    A close relative of the SCR is the "Triac".
    A "Triac" is like two SCR's in one package sharing a common gate.
    Triacs can pass current either direction... Most lamp dimmers use Triacs not SCR's.

    https://www.electronics-tutorials.ws/power/triac.html

    Some folks don't distinguish, just call them "Thyristor" leaving it to you to figure out from the symbol which one they meant SCR pr Triac.. That's imprecise communication.


    old jim
     
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