# 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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