# a simple question about SCR

#### ericgibbs

Joined Jan 29, 2010
17,141
Hi,
The simulation shows that point 'A' on your diagram shows a voltage high spike when the Vcc goes from 0V to the Vcc value.

As you can see from the plot, Vgate is partially high, before Vcc goes high, then it jumps to a higher voltage level as Vcc is applied.

Is that what you are asking.?
E

#### BobTPH

Joined Jun 5, 2013
6,291
we can make it to be 0V originally, why not?
Yes, you can short out the SCR, but for what reason?

I still don't know what it is you are trying to understand.

Bob

#### xljin2014

Joined Nov 11, 2014
96
Hi,
The simulation shows that point 'A' on your diagram shows a voltage high spike when the Vcc goes from 0V to the Vcc value.

As you can see from the plot, Vgate is partially high, before Vcc goes high, then it jumps to a higher voltage level as Vcc is applied.

Is that what you are asking.?
E
Hi,
The simulation shows that point 'A' on your diagram shows a voltage high spike when the Vcc goes from 0V to the Vcc value.
why is like that?

#### xljin2014

Joined Nov 11, 2014
96
As you can see from the plot, Vgate is partially high, before Vcc goes high, then it jumps to a higher voltage level as Vcc is applied.
I think vg is added from outside, not decide by vs.

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#### xljin2014

Joined Nov 11, 2014
96
Yes, you can short out the SCR, but for what reason?

I still don't know what it is you are trying to understand.

Bob
to see how it goes up to on scr

#### BobTPH

Joined Jun 5, 2013
6,291
If you hold point A at 0V, the SCR can never be on. You are not making any sense.

Bob

#### xljin2014

Joined Nov 11, 2014
96
If you hold point A at 0V, the SCR can never be on. You are not making any sense.

Bob
maybe I'm wrong. I mean weather there are parasitic capacitors in charge to lift va and on scr.

#### ericgibbs

Joined Jan 29, 2010
17,141
I think BG is added from outside, not decide by vs.
Hi,
Look at the sim circuit you will see that the Vgate is Enabled by the 6V driving current thru the R2 resistor, then thru the Gate tthen via the SCR internal resistance to 0v.

This gives a voltage at the Gate of approx 600mV. [It is Enabled]

When the Vcc is applied, the extra current thru the SCR raises the Gate voltage to approx 800mV

E

#### BobTPH

Joined Jun 5, 2013
6,291
Perhaps this answers your question. Staring Vcc at 0 and ramping it up, here is what happens to the voltage at A. As to whether there is a parasitic capacitance, yes, there is always a parasitic capacitance in every circuit element. Usually, it can be ignored.

Bob

#### xljin2014

Joined Nov 11, 2014
96
Hi,
Look at the sim circuit you will see that the Vgate is Enabled by the 6V driving current thru the R2 resistor, then thru the Gate tthen via the SCR internal resistance to 0v.

This gives a voltage at the Gate of approx 600mV. [It is Enabled]

When the Vcc is applied, the extra current thru the SCR raises the Gate voltage to approx 800mV

E
I think this involves inner structure of scr. still don't know how 600MV and 800MV come.

#### xljin2014

Joined Nov 11, 2014
96
Perhaps this answers your question. Staring Vcc at 0 and ramping it up, here is what happens to the voltage at A. As to whether there is a parasitic capacitance, yes, there is always a parasitic capacitance in every circuit element. Usually, it can be ignored.

Bob

View attachment 251974
yes, that's what I think. and now I'm wondering about va spike(fig eg984) it goes to 0v again, and scr still on , right?

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