+ Post New Thread
Results 1 to 6 of 6

9th October 2019, 01:09 #1
 Join Date
 May 2019
 Posts
 10
 Helped
 0 / 0
 Points
 135
 Level
 1
Impedance Calculation
In the attached image I am showing a voltage divider with the values of 50M (fixed) , 4.7k (fixed) and an unknown impedance Z.
Is it possible to calculate the value Z only using Vin, 50M and the 4.7k without using Vout directly?
I am looking for a simplified equation for Z that doesn't explicitly need Vout.
For this same application I saw a formula that didn't include Vout here so I was curious on the details.
Thank you for your thoughts.

Advertisement

9th October 2019, 01:55 #2
 Join Date
 Oct 2006
 Location
 Real Homeless
 Posts
 2,684
 Helped
 725 / 725
 Points
 17,428
 Level
 31
Re: Impedance Calculation
Drive by current source of known value.

Advertisement

9th October 2019, 06:44 #3
 Join Date
 Apr 2011
 Location
 Minneapolis, Minnesota, USA
 Posts
 13,023
 Helped
 2589 / 2589
 Points
 53,276
 Level
 56
Re: Impedance Calculation
Vout is a percentage of Vin. The formula is:
(4700 + Z)
divided by
(50M + 4700 + Z)
To find Z, make the above formula equal to your desired percentage, then solve for Z algebraically.

Advertisement

9th October 2019, 07:29 #4
Awards:
 Join Date
 Apr 2014
 Posts
 16,082
 Helped
 3643 / 3643
 Points
 79,246
 Level
 68
Re: Impedance Calculation
Hi,
Using Brad's formula works for pure ohmic values.
But for true impedances (and this is how I understand your question) you need graphic (vector) solution.
I assume even with the value of Vout it is not possible to calculate Z as an unknown impedance.
(Combination of R L C, or R C, or R L, or L C)
Klaus
Added:
Maybe with:
* Vin, sinewave, known amplitude, known frequency
* Vout, amplitude, phase shift
* and the knwledge it is no R LC impedance
...you may be able to calculate ZPlease don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.

9th October 2019, 17:34 #5
 Join Date
 May 2019
 Posts
 10
 Helped
 0 / 0
 Points
 135
 Level
 1
Re: Impedance Calculation
Thanks for the posts everyone. A followup to what BradtheRad posted.
My followup question is that if Vout is a percentage of Vin how do I know what this percentage is? Do you mean the transfer function, i.e. Vout / Vin ?
For example, if I capture some data for Vin and Vout should the ratio of Vout/Vin always been the same?
This is a simple case but I was not make sure I am following you.
In this case the ratio would be Vout/Vin that is ~0.091
If one of the resistors was changed I should be able to solve for this because I know the ratio and the other resistor value. Right?
Thanks.

Advertisement

9th October 2019, 22:48 #6
Awards:
 Join Date
 Apr 2014
 Posts
 16,082
 Helped
 3643 / 3643
 Points
 79,246
 Level
 68
Re: Impedance Calculation
Hi,
Now it's a completely different situation:
* only two resistors
* no complex impedance
You now have a simple resistive voltage divider.
The formula can be found million times in the internet.
KlausPlease don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.
+ Post New Thread
Please login