Help please with this simple transistor circuit

Thread Starter

bryankerr

Joined Apr 1, 2015
15
Hello all and thanks for taking a look at my thread.

I was hoping you could help me understand this circuit. But how is this transistor doing that exactly? Can you help me understand the circuit both intuitively and mathematically? I have never seen a transistor circuit like this. Thank you!
 
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Thread Starter

bryankerr

Joined Apr 1, 2015
15
I copied it correctly. The only thing I left out is that the incoming voltage is actually from an AC rectifier bridge.
 

ErnieM

Joined Apr 24, 2011
8,126
An NPN transistor needs current to flow from base to emitter to get current to flow from collector to emitter.

Both devices as drawn have a short across base to emitter. Thus no current, no function.
 

blocco a spirale

Joined Jun 18, 2008
1,546
I'm wondering whether with an AC input it might actually flash the LED. On the positive cycles the capacitor will charge and the transistor will be off but when the polarity reverses on the negative cycle the transistor's base would be positive wrt its emitter and would turn on and flash the LED.

I assume there should also be a resistor to limit base current.
 
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Thread Starter

bryankerr

Joined Apr 1, 2015
15
But there is no negative side of the voltage since its been rectified? This circuit indeed works. I checked the output pulses and they are perfect square waves 2.5 volts @ 120 hertz.
 

Dodgydave

Joined Jun 22, 2012
9,920
think your drawing is a Zero Crossing Detector, using an opto coupler and transistor driving it, the led lights when the voltage is near to zero ,(prob 3v), but it doesn't look right...
 
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Thread Starter

bryankerr

Joined Apr 1, 2015
15
If I take a multimeter and put the positive on the base, and negative on the emitter I get -.3 dc and switching on/off .42 ac
 

Thread Starter

bryankerr

Joined Apr 1, 2015
15
There is 3.5 volt ac applied to a bridge rectifier, and if I am careful not to leave anything out, there is also a resistor in parrallel with that whole circuit above. but I assume it has no function in the opto coupler function
 

blocco a spirale

Joined Jun 18, 2008
1,546
I know what's happening here; The capacitor charges as the input voltage rises (the diode is forward biased, transistor and LED are off), when the voltage starts to fall, the lower end of the capacitor tracks the input voltage and drops below 0V (the diode is reverse biased) taking the transistor emitter with it. The transistor base current comes from the 0V rail as it is now at a higher potential wrt the emitter, the transistor turns on and flashes the LED.

The LED will flash on the falling edges of the incoming waveform.
 
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Thread Starter

bryankerr

Joined Apr 1, 2015
15
blocco, I think you're on to something, because the base has somewhat square waves on the oscope at 120Hz, and I believe the peaks are at the zero crossing of the input. But can you explain why the capacitor voltage drops below 0V?
 

blocco a spirale

Joined Jun 18, 2008
1,546
As the input voltage rises, the capacitor will charge to the peak value of the incoming supply (minus the diode drop), let's say 5V. As the voltage falls, the capacitor will maintain this potential across its terminals (it can't discharge because the diode is now reverse-biased). So, when, for example, the input falls to 4V, the top end of the capacitor will also be at 4V wrt 0V but there will still be 5V across the capacitor which means that the lower end will be at -1V wrt 0V. The most you will see on this circuit is around -0.65V equal to, and limited by, the transistor's Vbe .
 
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