How It's Measured

I have a circuit (can be assumed as Black Box) I want to measure the input resistance (R2) at DC. I do the common method of using a signal (V1) with a known source resistance (R1) in series with the black box circuit. I look at the voltage across the R2 while changing R1 until it's half of V1. At that point I know R1=R2 and have my resistance under test.


I want to verify R2 by changing R1 to another value and calculating (with an online calculator) the voltage across R2. The problem is, it's far off from what the theoretical value should be. Any ideas what might be happening or further steps to take?

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    \$\begingroup\$ How can we tell anything about that if it is "black box"? One reason could be that an evil demon is sitting there with a potentiometer and turning it. \$\endgroup\$ – Eugene Sh. Jul 30 '18 at 17:16
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    \$\begingroup\$ That black box is probably the cookie box my grandma lost in 1972. These are phenomenal, but will leave you with a sugar rush, so eat with care. No, seriously, you only consider specific boxes, probably some with a network of linear inside, and only with a specific number of "pins". Without knowing any of your assumptions, however, we can't help you :( \$\endgroup\$ – Marcus Müller Jul 30 '18 at 17:20
  • \$\begingroup\$ I've seen many attempts to measure input impedance of black boxes, is it common for those folks to also get inconsistence values? \$\endgroup\$ – D. Kravchuk Jul 30 '18 at 17:21
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    \$\begingroup\$ @D.Kravchuk you don't understand: We know nothing about the black boxes you or "these folks" are measuring. How should we know what's common?? I'm afraid you'll have to make a lot more statements about your box (as said above), or this will be closed as "too unclear". \$\endgroup\$ – Marcus Müller Jul 30 '18 at 17:22
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    \$\begingroup\$ @Tyler Then we can conclude it is alive. At least some relief. \$\endgroup\$ – Eugene Sh. Jul 30 '18 at 17:29

Your "black box" might contain some element with a non-linear resistor, which is dependent on voltage applied. Like a diode. When you use your "method" of varying an external resistor until it matches V1 = V2, you will have different results for different R1s.

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  • \$\begingroup\$ I do have a few diodes in there, I will short them and try again. I have a DAC, Processor, Motor Driver, transceiver, and will study their affect on the measurements. I guess it's never as easy as it may seem. Thanks for the feedback. \$\endgroup\$ – D. Kravchuk Jul 30 '18 at 17:41

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