# a simple question about SCR

#### xljin2014

Joined Nov 11, 2014
120

hi, friends,

the circuit above is a resistor connects with a scr(silicon - controlled rectifier). my question is:if point A is 0V in the beginning, and Vin is of high voltage, will voltage of A go up and SCR can start conducting? I mean if there's parasitic capacitor between SCR anode and cathode.

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#### Ian0

Joined Aug 7, 2020
10,247
A can never be 0V (Unless Vcc=0V) Even if the SCR is on, “A” will never get lower than 0.8V.
Vin can never get higher than about 1V, because there is a diode between gate and cathode.
If A is at a low voltage, then the SCR is on, and it will remain like that until Vcc is switched off (more specifically, until the current through R falls below the holding current of the SCR) regardless of what happens at Vin.
If A is at Vcc, then the SCR is off, and it can be switched on by passing a current greater than the trigger current between Vin (gate) and cathode. The voltage at A will then fall to about 0.8V.

#### xljin2014

Joined Nov 11, 2014
120

#### Ian0

Joined Aug 7, 2020
10,247
why? what if we let it be at the beginning?
If Vcc is present, then A will equal Vcc if the thyristor is OFF and A will equal 0.8V if the thyristor is ON.
(look up “on-state voltage” in the datasheet, you will generally find a figure of about 1.2V at full current, down to 0.8V at low current.

#### xljin2014

Joined Nov 11, 2014
120
A can never be 0V (Unless Vcc=0V)
at the beginning, when VCC is adding , and A is 0v, how does it come to 0.8v?

#### Ian0

Joined Aug 7, 2020
10,247
What is your VCC voltage? What is the value of R?

#### BobTPH

Joined Jun 5, 2013
9,256
If nothing is connected to Vcc, the voltage at A is unknown, not zero. As soon as Vcc is connected, it is at Vcc.

Bob

#### xljin2014

Joined Nov 11, 2014
120
What is your VCC voltage? What is the value of R?
we can randomly give VCC as 5v,and R as 5k.

#### xljin2014

Joined Nov 11, 2014
120
If nothing is connected to Vcc, the voltage at A is unknown, not zero. As soon as Vcc is connected, it is at Vcc.

Bob
so when Vin about 1v is added, and Va is high, then current flows through r and scr, and A comes down to 0.8v . is it right?

second, if we connect A to zero at first, how will it goes to 0.8v after VCC is added?

#### bertus

Joined Apr 5, 2008
22,303
Hello,

You might want to have a look at the attached PDF.

Bertus

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#### BobTPH

Joined Jun 5, 2013
9,256
If you connect A to 0V, then the SCR is shorted out, not even in the circuit.

What are you getting at? I suspect you have in mind some exploitation of what you think is happening. Tell us what it is, and we can tell you whether or not it will work.

Bob

#### xljin2014

Joined Nov 11, 2014
120
If you connect A to 0V, then the SCR is shorted out, not even in the circuit.

What are you getting at? I suspect you have in mind some exploitation of what you think is happening. Tell us what it is, and we can tell you whether or not it will work.

Bob
I just want to know how point A goes to 0.8V if it is 0V originally , I suspect there are parasitic capacitors between anode and maybe gate or cathode and VA can go up as capacitors charge.

#### xljin2014

Joined Nov 11, 2014
120
Hello,

You might want to have a look at the attached PDF.

Bertus

#### BobTPH

Joined Jun 5, 2013
9,256
just want to know how point A goes to 0.8V if it is 0V originally
It is not at 0V initially. When the SCR is off it is at Vcc. So your question is assuming a false premise.

Bob

#### BobTPH

Joined Jun 5, 2013
9,256
just want to know how point A goes to 0.8V if it is 0V originally
It is not at 0V initially. When the SCR is off it is at Vcc. So your question is assuming a false premise.

Bob

#### xljin2014

Joined Nov 11, 2014
120
It is not at 0V initially. When the SCR is off it is at Vcc. So your question is assuming a false premise.

Bob
we can make it to be 0V originally, why not?

#### bertus

Joined Apr 5, 2008
22,303
Hello,

Did you see this picture in the PDF I posted?

There will be no current through the load as long as the gate current is below IL .

Bertus

#### xljin2014

Joined Nov 11, 2014
120
Hello,

Did you see this picture in the PDF I posted?
View attachment 251966
There will be no current through the load as long as the gate current is below IL .

Bertus
i don't know much about scr, but I think your pic is for static state, and I'm talking about dynamic state, about parasitic capacitors......

#### ericgibbs

Joined Jan 29, 2010
19,081
hi 2014,
This is what LTspice shows, remember this is in simulation.
1. The Gate is ON, with Vss Low
2, Gate still ON, Vss pulled High.
3. Vss switched off after a delay.

Expanded the Xaxis for the Vss going High

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#### xljin2014

Joined Nov 11, 2014
120
hi 2014,
This is what LTspice shows, remember this is in simulation.
1. The Gate is ON, with Vss Low
2, Gate still ON, Vss pulled High.
3. Vss switched off after a delay.

Expanded the Xaxis for the Vss going High
may I ask a question pls? in pic EG985, does it mean when vs added, it causes vg goes up, and va goes up from 0v?