ac frequency measurement with isolation

[zeroskj1988]

i want use measure the frequency of ac(normaly 50hz).since i have problem with common ground,i want to isolate the neural line. so i use a optocoupler to isolate the ac
my circuitis-------half wave rectifier---100k-1k voltage divider---then optocounpler to ccp2 ofpic16f877a. i use timer 1 to find the period. can any body spot, any problem with this logic. thanks in advance actually it is not working

 

[Reloadron]

If you want to measure the AC mains frequency (you don't mention voltage) then just use a small step down transformer. 220 vac to something like 6.3 vac. You could hack a simple wall wart to get the transformer or just find an ac output wall wart. Can you post a drawing of what you have? Also, what type of timer? What exactly are you using to measure the frequency/period?

Ron

 

[mdorian]

Loose the 1k resistor , use only rectifier diode ,100k resistor , optocoupler because the optocoupler is current driven. If still doesn't work ,in the processor side must be a pull-up resistor , check if your Current Transfer Ratio of the optocoupler is sufficient. Some have CTR = 1 or less and for 120v main voltage R pull-up must be higher than 20Kohm. The transformer is to big for such a simple task , post a drawing , the value of the main voltage (I guess 120v if you're from Ohio) and the optocoupler you use to help you make-it work.

 

[alec_t]

Assuming the mains voltage is 230V rms the peak current through the opto-coupler diode is only ~3.2mA. Is that enough to give a meaningful output (you don't specify which opto-coupler you're using)? For a 110V mains supply the diode current (~1.5mA) would almost certainly be inadequate for most opto-couplers.

Alec