optocoupler

im trying to design a voltmeter circuit to measure unknown resistance in a voltage divider.. because of the difference in voltage.. i use a optocoupler to isolate the circuit.. but i dont know whether just by connecting the circuit together with the optocoupler.. will the voltmeter still work..

nitE said:

im trying to design a voltmeter circuit to measure unknown resistance in a voltage divider.. because of the difference in voltage.. i use a optocoupler to isolate the circuit.. but i dont know whether just by connecting the circuit together with the optocoupler.. will the voltmeter still work..

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hi,

If the opto coupler is in the measuring meter then that method will not work.

Use a DVM [digital voltmeter], the input resistance of a good DVM is so high it will not load [affect] the resistor potential divider.

hi Eric.. thank for the feedback.. im using very high resistance in my voltage divider (about 50M ohm) and my voltmeter circuit uses PIC microcontroller.. im told that the resistance may cause interference to my PIC uC and also the difference in voltage may also damage my PIC uC.. so is it wrong..? and i dont need to have the optocoupler attach..?

there're 2 problem of using an optical coupler
the first is that the linearity of a optical coupler isn't that good. this may cause error.
the second is that the emitter of optical will suck several mA. If the voltage source doesn't has low internal resistance, the error may be significant. This problem can be solved by using a voltage follower.

a voltage-frequency modulator should be used in a high accurate isolated voltage meter circuit.

nitE said:

hi Eric.. thank for the feedback.. im using very high resistance in my voltage divider (about 50M ohm) and my voltmeter circuit uses PIC microcontroller.. im told that the resistance may cause interference to my PIC uC and also the difference in voltage may also damage my PIC uC.. so is it wrong..? and i dont need to have the optocoupler attach..?

Click to expand...

hi,
50MΩ for a divider is a high value, what voltage, etc are you dividing down from that need 50MΩ.?

actually im using this as a insulation tester.. im trying to use the 50Mohm to compare against the insulation resistance so that if the insulation resistance is higher than 50Mohm then the voltage reading will be higher than half of the voltage source.. wonder if there is a megohmmeter design which i can use..? or this method can work..? the voltage could be as high as 500VDC..

nitE