Vgt Voltage gate trigger for SCR gate

Discussion in 'General Electronics Chat' started by RiG615, Jun 12, 2009.

  1. RiG615

    Thread Starter Active Member

    Nov 13, 2008
    50
    0
    So I was looking on digikey for a SCR gate when I see a column for 'Voltage Gate Trigger Max' and it goes from 200mv to 5v and the 5v ones are like a million dollars-

    So what is Voltage gate trigger (max)?

    I'm assuming its the max voltage allowed to the gate.?

    Anyway I want to use one in a system powered by a 4v battery. So do I have to get the voltage way down into the millivolts? Can I just add a resistor in front of the gate?....and if so how do I calculate how big of a resistor?

    thanks
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    Take a look at the attached PDF.
    That will make the voltages and currents used in datasheets more clear.

    Greetings,
    Bertus
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Are you aware the SCR is an AC device? Once gated on DC, it will only turn off when the voltage across it is removed.

    A FET is a better switch for DC.
     
  4. RiG615

    Thread Starter Active Member

    Nov 13, 2008
    50
    0
    beenthere, so for the SCR using DC

    If I trigger it with a short pulse(DC), current will flow from the anode to cathode until I kill the power? on DC

    Is that true?
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    Yes, you are right.
    When using a SCR on DC it will stay on until the power is removed OR the cathode and anode are shorted for a short time.

    Greetings,
    Bertus
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Yes - it's about as easy to use a switch instead.
     
  7. hobbyist

    Distinguished Member

    Aug 10, 2008
    764
    56
    If you still need to use an SCR for switching,
    then one way to get the voltage proper at the gate would be to build a voltage divider so that when the 4 volt supply signal comes in it would divide at the gate.

    Example: If you needed 0.2v. at the gate you could use a 1k ohm res. from gate to ground, then connect a switch between the 4v. supply and a 18k resistor connected to the gate, so when the switch makes contact then ((4v. x 1K) / 19k) = .210 v.

    so 210 mV. would be applied to the gate at each switch input.

    Something I learned from working with scr's is that to have the gate floating someti,es gives eroneous results, but by grounding the gate ensures that when the SCR is deactivated it stays in that state. Even if a spurous signal should reach the gate. Depending how sensitive the gate is.
     
    Last edited: Jun 12, 2009
  8. RiG615

    Thread Starter Active Member

    Nov 13, 2008
    50
    0

    After a short DC pulse (to the gate) I want current to flow and stay flowing even after the pulse ends.
    what other switches do this?
     
  9. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,675
    899
    I think VGT is being misinterpreted by some. Here are some definitions from the AN1008 (cited above), AN1001 (Littlefuse), and ON Semiconductor (emphasis added):

    The key to triggering is to have sufficient voltage to give a gate current that is 4 to 5 times the minimum IGT. Semikron and some of the Digikey links show a diagram of the safe operating area -- it is usually the very last figure in the datasheet. Look at that diagram to get a feel for the voltage/current needs of your device.

    According to the above definitions, VGT is a requirement of the device. In other words, it is the minimum voltage required to ensure the SCR fires. I suspect VGT(max) is the maximum manufacturing tolerance for the minimum value of VGT for a particular device.

    Bottom line is, first pick a device. Then from its IGT figure out what you may need for triggering. I suspect the resistances given above will not allow adequate gate current, at least not for a large SCR.

    John
     
  10. RiG615

    Thread Starter Active Member

    Nov 13, 2008
    50
    0
    thanks bertus
    your pdf helped clear me up!


    "
    VGT: DC Gate Trigger Voltage
    SCR
    VGT is the DC gate-cathode voltage that is present just prior to
    triggering when the gate current equals the DC trigger current. As
    shown in the characteristic curve in Figure AN1008.8, the gate
    trigger voltage is higher at lower temperatures. The gate-cathode
    voltage drop can be higher than the DC trigger level if the gate is
    driven by a current higher than the trigger current.
    Triac
    The difference in VGT for the SCR and the triac is that the triac
    can be fired in four possible modes. The threshold trigger voltage
    can be slightly different, depending on which of the four operating
    modes is actually used.
    "

    so I'm not going to blow out a scr with a vgt max of 800mv with a trigger voltage of 4v.
     
  11. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,675
    899
    What is your IGT? You might consider a small gate resistor to limit that, but I suspect it would not be necessary. John
     
Loading...