help required on ac motor speed control

[sherazi]

got the zero crossing solution its good...

u can have the details from http://www.hoelscher-hi.de/hendrik/english/dimmer.htm#zc detection

 

[(*steve*)]

since you already have a transformer producing lower voltage AC, I'd use that instead of connecting directly to the mains. It's safer -- less wiring at 240V.

I would actually recommend you use an optocoupler with bidirectional LEDs (like a TLP120) because that way the detection will be symmetrical. With this circuit, the error differs for positive and negative half cycles.

With the device I recommend, you'll get a narrow pulse at each zero crossing. If you're clever you can measure the duration of the pulse and estimate the actual point where the mains crosses zero.

 

[sherazi]

yes iam definately going to use a 12v input... this pic was taken from the web thats why it shows such a circuit

 

[sherazi]

i think it should work when i would connect the iputs to the bridge rectifer out put before filtering ripples... it would then be ok to use the normal op/coupler

is'nt it?

 

[(*steve*)]

you will have to use a separate bridge rectifier otherwise the filter capacitors will stop it working.

The problem in using a bridge rectifier as opposed to operating it from AC is that the voltage across the diodes will add to the error. It would be best to use the highest (safe) AC voltage available (24V?) and a resistor. If you have 2 of these optocouplers you could connect the outputs in parallel and the inputs around opposite ways. It will do the same thing.

edit: This is simple and potentially more accurate than the optocoupler solution.