1. shortbus

    Thread Starter AAC Fanatic!

    Sep 30, 2009
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    Did they stop making, silicon controlled switches (SCS) or have they been given a new name? Similar to a SCR, but with a fourth lead, used to shut off the switch.

    If they don't still make them, is there a way to make a SCR turn off, once triggered? For a DC circuit.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You mean without removing the DC?
    There is a method called forced commutation, but it is not trivial to enforce.
    Max.
     
  3. #12

    Expert

    Nov 30, 2010
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    Unless you get more specific, I'd say you need a MOSFET.
    Give up some details.
     
  4. shortbus

    Thread Starter AAC Fanatic!

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    #12, a mosfet is what I was trying to replace.:)

    I've been seeing some posts lately on SCR's and started looking for something similar that would work for DC that doesn't go to zero volts. A SCR works but only for AC or DC that returns to zero volts. Came across references to SCS but couldn't find them in Digikey's site. Guess they went away when mosfets got better.

    Max, this is what I thought was the case.

    This is for my ongoing electrical discharge machine. It needs to be able to turn off when the voltage gets down to ~10Vdc from ~100Vdc at the start of the pulse.

    Thanks guy's for the answers.
     
  5. inwo

    Well-Known Member

    Nov 7, 2013
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    Like max said, there are ways to commutate scrs.

    It can be as simple as a momentary switch across anode and cathode.
    Anything to drop current momentarily to zero.
    Often a capacitor is used in the commutation circuit.

    If you post circuit or look up "scr commutation" maybe.

    I've played with scr circuits quite a bit and have an old GE scr manual that has lots of examples.

    It's amazing what they could do before high power bjt or fet.
     
  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    There are lots of easy ways to do this. One would be an opAmp (or comparitor) with voltage divider from your load (since 100 v is too much for most op Amps) and other input would be a reference voltage or zener - connect that mess to a MOSFET.

    What amp load and switching speed to you need for this project?
     
  7. crutschow

    Expert

    Mar 14, 2008
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    One method of commutation is to add a second SCR coupled with a capacitor between the two SCR anodes. The capacitor is charged to some voltage by a resistor connected to the second SCR. When the second SCR is triggered, the discharge of the cap will momentarily reverse bias the first SCR and turn it off. One limitation is that the resistor must be large enough so that the current is small enough to allow the second SCR to turn off after generating the commutation pulse.
     
  8. shortbus

    Thread Starter AAC Fanatic!

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    Right now the design I'm doing has a comparator to switch the mosfet off. It needs to be shut off at that level to keep the electrode from shorting out the spark gap.

    Amps are variable, between 5 to 20. The time/frequency is also variable, between 5kHz and 50kHz. with a variable duty cycle, between 10% to 50% on time.
     
  9. shortbus

    Thread Starter AAC Fanatic!

    Sep 30, 2009
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    Thanks for every ones answers. Thought maybe the SCS was called some thing else now, but guess not.

    I keep putting off just building this thing, trying to come up with a better way of doing it. Time, I guess to just do it the way I first thought of. I've got some big mosfets that are for a inverter welder, so I guess that's the way to go.
     
  10. GopherT

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    Nov 23, 2012
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  11. alfacliff

    Well-Known Member

    Dec 13, 2013
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    there is a componant knows as a GTO or gate turn off. a type of scr used in high power vfd's like the 1300 series allen bradley. it has another gate terminal, if you put 10%of the power flow through the scr, it turns off. I think they were made by toshiba, I do know there was a real long lead time getting replacements due to a poor output of parts from the mfgr.
     
  12. shortbus

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    I do know about the GTO, but prices of them are pretty high and not many places stock them. The GTO looks to be what took the place of the SCS.
     
  13. alfacliff

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    Dec 13, 2013
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    about 16,000 usd for each one in the 300 hp vfd I was working on. also found out that someone had removed the gto's from one vfd and put them in the other to troubleshoot. those hockey puck package parts dont like that, when you torque them down, they imprint on the soft copper the shape of the connections inside. when you take them out and put them back in, they dont always line up with the previous imprint. $16,000 x 12 and a couple of years wait for new ones for both.
     
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  14. ian field

    Distinguished Member

    Oct 27, 2012
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    The basic 4-layer device which can be SCR, SCS or PUT, depending which layers are leaded to the outside world, can be reproduced by a cross connected complementary pair of signal transistors. The collector of each is connected to the base of the other, any current flow will have regenerative (positive feedback) effect. You can include resistors in the circuit to facilitate GTO capability.
     
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