zero crossing detector

[sauravndkumar]

can anyone explain how is this working?
how can it be helpful in light dimmer circuit?

 

[panic mode]

which part is confusing?

there are 4 diodes forming rectifier bridge.
this produces DC pulses resembling halfperiods of AC sinewave.
normally this would go directly to C1 for filtering but that would ruing the signal waveform and erase any info on zero corssing. therefore, D5 is used to isolate point 'a' where we have the pulsed wave, and the C1 which smooths ripples out.
R1 and D6 form simple voltage regulator so you get 3VDC as measured by XMM2.

pulsed signal with significantly higher amplitude (point 'a') is used to turn on transistor Q1.
basically transistor is on most of the time and turns off only when zero crossing (when voltage at 'a' is less than some 1.5V).
transistor is powered from 3V derived from same AC circuit (no AC, no signal).
since transistor is off during short periods where amplitude of waveform at 'a' is small, output taken from collector is high or "logic one".

 

[sauravndkumar]

thanks!! but how does it help in the light dimming ckt? we are controlling triac for regulating the load. the output of zcd(zero crossing detector) goes to the microcotroller which is then sent through optocoupler for controlling the triac.we are using phase angle modulation

 

[ChrisP58]

In order for the microcontroller to know when to turn the triac on for a particular phase angel, it needs to know where the zero crossing point is.

If you want 100% of the voltage, you fire the triac right after the zero crossing. If you want 50%, you trigger it at 90%, about 4mS after the ZC (at 60Hz).

 

[panic mode]

remember that once you trigger triac or scr, they remain 'on' as long as power is present.
in ac circuit that happens when crossing zero. if you when zero occurs, you can use your circuit to create delay before issuing trigger.
suppose your AC is 50Hz. that means that length of period is 1/50=0.020sec (20ms).
one halfperiod is then 10ms. suppose you can manage delay that can be adjusted 0...10ms.
this circuit will be triggered after every zero crossing, which in this case is every 10ms...

if delay is 0ms, trigger is issued right after zero crossing so triac will turn on and stay on for entire 10ms (until next zero). if the load is light, it would be at full brightness.

if delay is 5ms, trigger is delayed so at first triac is off and then it is turned in (after 5ms delay) so it only conducts half of the time. if the load is light, it would be at half brightness.

if delay is max (ie almost 10ms), then triac will barely have a chance to turn on before zero turns it off. very little power is transferred and if load is light, it would be (or appear to be) off....

as you can see changing one parameter such as delay allows control of load in wide range. note that far we only mention non-reactive loads (incandescent light, heaters etc.). actually there is better way to control heaters to eliminate transients. inductive loads are also common (motors) but require special consideration.