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[SOLVED] How to use SCR as a Latch?

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abuhafss

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Hi

I want to use SCR as a latch.

The triggering voltage is 5V and the supply voltage is 9V. But I am getting only 4.5V at the output which gets off when the trigger is off.
How could I get constant 9V at the output?
 

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  • SCR Latch.gif
    SCR Latch.gif
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V1 should NOT be connected between ground and gate, it should be between cathode and gate.
 

schmitt trigger said:
V1 should NOT be connected between ground and gate, it should be between cathode and gate.
Click to expand...

I am sorry, I am confused. Do you mean to say that the ground should be disconnected from the negative of the Triggering Source & the cathode of SCR?

Actually, I wanted to replace SCR for flip-flop SR latches in a circuit as shown in the diagram. Of course, the negative of the triggering source is common ground. How shall we use an SCR to get 9V at the output?

Thanks for the help.
 

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  • Latch.png
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abuhafss said:
I am sorry, I am confused. Do you mean to say that the ground should be disconnected from the negative of the Triggering Source & the cathode of SCR?

Actually, I wanted to replace SCR for flip-flop SR latches in a circuit as shown in the diagram. Of course, the negative of the triggering source is common ground. How shall we use an SCR to get 9V at the output?

Thanks for the help.
Click to expand...

In that case, the load should be connected on the cathode side, as shown in this image from the General Electric SCR Manual, 5th edition


scan0005.jpg
 
crutschow said:
Does the load have to be grounded?
Click to expand...

Actually, there is no load. The supply voltage (9V) is to be latched for future requirement when the trigger voltage turns off.

- - - Updated - - -

Here is the equivalent transistorized circuit.

The zener D3 is used to increase the thresold voltage from 0.6V to 4.5V.
 

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  • Transistor Latch.gif
    Transistor Latch.gif
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Your circuit only latches ON. Do you need to latch it off? The only way to turn that circuit off is to remove the 9V power. Is that okay?
 

Besides other problems, the circuit doesn't latch at all because the BT151 latching and holding current are never achieved. Refer to datasheet.
 

Going back to your original circuit with the BT151 SCR, you need a diode in series with the gate to prevent reverse voltage breakdown of the gate-cathode input and a resistor in series with the gate to limit the maximum trigger current.

Also, as FvM noted, the maximum latching current is 40mA so R1 can be no larger than 200 ohms with a 9V supply and a 1V drop across the SCR.
 

I am sorry, I could not get it worked even with EC103 small TO-90 type SCR.

Shall appreciate if you please make me an .ASC file for LTSpice.



Lastly, Wish You All a Happy and Prosperous 2014 !!!
 

crutschow said:
Post the .asc files for your simulations (along with the scr model and icon files).
Click to expand...

Here are the files. Change the extensions of the first two files to .lib and the last file to .asc
 

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  • BT151.txt
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  • SCR.txt
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  • SCR_Latch.txt
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It is. Forgot it was there. ;-)

- - - Updated - - -

Here's a simulation that works as a latch. As I noted previously the holding current of that SCR requires a load resistor no larger than 200 ohms. (You should be able to go to 400 ohms with the EC103).

I also added a 1k ohm resistor from the SCR gate to cathode to take care of any gate leakage current.

SCR Latch.gif
 
Last edited:

Thanks for your help.

Just a last question, is the output 8.3V because of the usual 0.7V drop caused by a diode (in this case within the SCR)?
 

abuhafss said:
Thanks for your help.

Just a last question, is the output 8.3V because of the usual 0.7V drop caused by a diode (in this case within the SCR)?
Click to expand...
Yes, it is actually the base-emitter drop of the NPN equivalent transistor in the SCR (look at the SCR equivalent circuit).
 
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