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Zener problem in motion detection circuit

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Avradip

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Hi,
I am Avradip .I am studying a sample motion detection circuit which uses PIR sensor D203S and motion detetor IC BISS0001.The output of sensor directly connected to BISS0001 and after several stage of amplification it gives output(Vo) which switches SCR. SCR switches TRIAC which is used for general purpose ac switching.But i didnt understood the role of some of components . In circuit they used 7.5V zener diode across triac gate terminal.But what is the purpose of using this zener diode ? when SCR switches triac the voltage across the zener shows around 3.3 voltage then how is it possible to on the zener and triac respectively. Again they used another 7.5V zener which is shown in red marking. i didnt understood the functionality of these zener .I will be very grateful if anyone help me. motion_switching _ckt_value.JPG
 
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The zener at the right circle is using to switch on the triac on a specific voltage level, according to the voltage drop on the 1k resistor which is in series with the scr. Of course this is happens when the load is high and using the triac as a protection for the current through the scr.

The second zener, I am not sure for its purpose. Maybe, when the triac switch, the voltage goes approximately to zero. But the voltage at Anode of the scr is still more safe to be used for bias, as on this voltage depends the switching of the triac. So it's just for the supply voltage of 7133.
 

I have seen that zener voltage varies with respect to load but as we know that zener always hold fixed voltage when supplying greater voltage then the specified one.then what's happening in that case.
and i didnt understood the line 'using the triac as a protection for the current through the scr.'
the answer u gave for left zener is obscure to me .
 

I have seen that zener voltage varies with respect to load but as we know that zener always hold fixed voltage when supplying greater voltage then the specified one.then what's happening in that case.
and i didnt understood the line 'using the triac as a protection for the current through the scr.'
the answer u gave for left zener is obscure to me .

according to the voltage drop on the 1k resistor the triac is going on, If the triac goes on, then the voltage after the bridge goes approximately to zero, so the current that scr pulls goes to zero. It's a protection scheme.

For the left zener, I told you i am not sure. But I think that this is the reason.
 

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