what is the use of composite signal generator, zero crossing detector
and comparator
a zero crossing detect circuit is used to indicat when and AC voltage
is crossing the 0V line or within a window of a few volts either way.
You know that AC voltages swing to a positive voltage and then a
negative and this repeats continuously right?
If you are switch something on, at the point the current starts
flowing, it can make "noise" both electrically and within the radio
spectrum (you may have seen this as lines of interference on your TV
when someone has gone past on a motorbike or similar - the Radio
interference from the ignition circuit is temporarily swamping the TV
signal and being displayed as rubbish on the screen). So, if you turn
on an AC device, you generally don't have any idea where in the swing
cycle the voltage is and if it is significantly in, current will
"rush" in to the device - with the same effect as the motorbike.
Dimmer switches on lighting that don't use resistors use a triac to
switch to the voltage on and off rapidly - because the bulb take a
while to reach full brightness, if you switch off the power before it
gets there it will appear to be dim constantly, but if you do this
without a zero crossing trigger of some sorts it will play merry hell
with your electrics and TV. A zero crossing detect will tell when it's
safe to switch the power on... because you are at zero voltage, there
is no current flowing (actually there will be a bit because 0v lasts
for a fraction of a second) so you allow the curretn to build up
nicely with the voltage. You can turn the power off anywhere in the
cycle with minimum interference.
So to recap, ZCD lets you know (in this application) when to switch
the power on to eliminate electrical and RF noise (also stops
overheating)
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A comparator lets you compare two voltages. One is usually a reference
and the output of the comparator will be at 0v so long as the input
voltage remains less than the reference. as soon as it is more, the
output will go "high" (to the positive supply voltage). A window
comparator will have two references and the output will go high only
when the input voltage is in between those - you could make a ZCD out
of a window comparator - if you set the lower reference to same -0.5v
and the upper to +0.5v, the output would go high breifly for the time
the imput voltage was between these two levels (right around 0v)
