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What R is used for RC constant?

767pilot

New Member
In this circuit to calculate the RC do I use the 2 resistors in parallel ie 690ohms or is it just R1?

Thanks
 

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AllVol

New Member
Is this an AC circuit?
The resistors are in series, not parallel.
Why is R1 tied back in?
 

Hero999

Banned
What does that circuit do?

It's a high pass filter followed by a rectifier but other than that there isn't enough to go on.

I take it this is part of a larger project, please post the entire schematic if you expect to get any help.
 

767pilot

New Member
sorry i uploaded the incorrect drawing

thats all the circuit is, a high-pass rectifier. I am learning about thevenins therom and think that R1 and R2 are seen in parallel but would like clarification on that please
 

Hero999

Banned
When the wavefrom is above about 600mV R1 is in parallel with R2 which is in series with D1, when the waveform is below 600mV R2 can be ignored.
 

767pilot

New Member
thanks for the answer

how would the current in the circuit be calculated?

the way I see it, which could be wrong in parts, is

if F3db = 491 hz

a) Rt = R1//R2 = 690 ohms
b) XC = 1/(2PI*F*C) = 690 ohms

therefore

c) Z=SQRT(XCsqr + Rtsqr) = 976 ohms
d) I=V/Z = 5/976 = 5.1 mA

this isnt the same result I get in the simulator. How does the .6V drop across the diode affect the current? ie when V>.6v the diode conducts but surely the above calculations assume the diode is conducting all the time? or at least not in the circuit?

thanks
 
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