# working of an SCR

Discussion in 'The Projects Forum' started by KMK, Nov 21, 2010.

1. ### KMK Thread Starter Active Member

Feb 11, 2010
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0
Hi,
I work with electronics as hobby... i was planning to use an SCR ( PCR-406 ) in one of my projcts (using an LDR to fire the SCR)
however i dont know how SCR works.. & i looked at the data sheet, but could not understand how to connect it .
here is the link for the data sheet
also it would be better if any one can explain how to design a simple circuit (may be using single LED ) as to how i can use the SCR

Apr 5, 2008
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3. ### KMK Thread Starter Active Member

Feb 11, 2010
68
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hi burtus.. thanks for that.. i actually forgot abt it...was veru helpful...
i checked 2 out of 3 SCRs which i found in string light electronic box... 2 seem to be working fine... i used a blue LED on a 3V circuit.. they latch up with 3V, but not at 1.5V.... however the third one directly goes to latching stat.. without any gate trigger.. so does it mean this SCR is bad one ?

4. ### Wendy Moderator

Mar 24, 2008
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2,540
Odds are you need a resistor between the gate and cathode. Something in the range of 10KΩ, thought this is not guaranteed.

5. ### KMK Thread Starter Active Member

Feb 11, 2010
68
0
Thanks for that information Bill.. I did manage to get a workd using an SCR. In fact, i have built a dark detector using the SCR & ldr.. works well..
The only concern is to turn off the SCR, i havce installed a micro puch button ( which shorts the supply) to turn off the scr current..
I have read abt commutation being used to turn off the SCR.. & use capacitors of it..
http://www.circuitstoday.com/scr-turning-off-methods
this is back circuit diagram
3 things to be noted
1] v = 3 volts LED is blue
2]there is a variable resistance of 1M between +ve & the LDR (the junction before the LED ) to control the current to tthe gate & adjust the scr firing light sensitivity.
3] P is the push s/w to turn off the SCR

so can any one tell me if it is possible to redesign this so that the scr can be made to turn off automatically.. ?(again according to the light level )