a simple question about SCR

Thread Starter

xljin2014

Joined Nov 11, 2014
73
1636100561767.jpg

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
4,169
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.
 

Ian0

Joined Aug 7, 2020
4,169
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.CAAEFD71-349E-4431-B014-1F45CA516950.jpeg
 

BobTPH

Joined Jun 5, 2013
4,320
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
 

Thread Starter

xljin2014

Joined Nov 11, 2014
73
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?
 

BobTPH

Joined Jun 5, 2013
4,320
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
 

Thread Starter

xljin2014

Joined Nov 11, 2014
73
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.
 

bertus

Joined Apr 5, 2008
21,766
Hello,

Did you see this picture in the PDF I posted?
AN1001.12 SCR characteristics.png
There will be no current through the load as long as the gate current is below IL .

Bertus
 

ericgibbs

Joined Jan 29, 2010
14,728
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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Thread Starter

xljin2014

Joined Nov 11, 2014
73
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?
 
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