# 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

Apr 5, 2008
15,799
2,385
Hello,

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

Greetings,
Bertus

File size:
127.7 KB
Views:
627
3. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
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?

Apr 5, 2008
15,799
2,385
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

Apr 20, 2004
15,815
283

7. ### hobbyist Distinguished Member

Aug 10, 2008
773
62
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
5,699
909
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
5,699
909
What is your IGT? You might consider a small gate resistor to limit that, but I suspect it would not be necessary. John