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Measuring Resistance

B

Bob H.

Jan 1, 1970
0
I have a basic series-parallel circuit, it is power by my DC power supply.
From the Neg to R1, then to the parallel group of R2 & R3, then to the Pos.
Question : Why is it when i measure the series resistance of R1 in the
circuit with the power connected ( not turned on) I get a different reading
than if I take it right out of the ciruit? Do I have to disconnect the power
to read resistance because the power supply is actually in parallel to the
series resistor of R1. that would mean the same is taking place with the
parallel group but I haven't noticed since I was splitting that up anyhow to
measure the resistance there.
Is this something I should keep in mind for the future, disconnect the power
line to the circuit before measuring resistance - even if only a series
resistance.

Thank you in advance one and all... ..
Bob
 
E

Eeyore

Jan 1, 1970
0
Bob H. said:
I have a basic series-parallel circuit, it is power by my DC power supply.
From the Neg to R1, then to the parallel group of R2 & R3, then to the Pos.
Question : Why is it when i measure the series resistance of R1 in the
circuit with the power connected ( not turned on) I get a different reading
than if I take it right out of the ciruit? Do I have to disconnect the power
to read resistance because the power supply is actually in parallel to the
series resistor of R1. that would mean the same is taking place with the
parallel group but I haven't noticed since I was splitting that up anyhow to
measure the resistance there.
Is this something I should keep in mind for the future, disconnect the power
line to the circuit before measuring resistance - even if only a series
resistance.

You can't measure resistance with the circuit live.

Graham
 
J

John G.

Jan 1, 1970
0
Eeyore said:
You can't measure resistance with the circuit live.

Graham
And yes the power supply forms part of the cct when turned off.
 
G

gearhead

Jan 1, 1970
0
I have a basic series-parallel circuit, it is power by my DC power supply..
From the Neg to R1, then to the parallel group of R2 & R3, then to the Pos.
Question : Why is it when i measure the series resistance of R1 in the
circuit with the power connected ( not turned on) I get a different reading
than if I take it right out of the ciruit? Do I have to disconnect the power
to read resistance because the power supply is actually in parallel to the
series resistor of R1. that would mean the same is taking place with the
parallel group but I haven't noticed since I was splitting that up anyhowto
measure the resistance there.
Is this something I should keep in mind for the future, disconnect the power
line to the circuit before measuring resistance - even if only a series
resistance.

Thank you in advance one and all... ..
Bob

I think your meter probably measures resistance by applying a
regulated current through the resistor under test, and then taking a
reading of the voltage across it. Or maybe it applies a voltage and
measures current; either way, some power has to come from the meter
and go through the resistor.
If that's true, then having the circuit turned on and applying more
voltage and current through the resistor would really screw up the
reading.
And you don't have to connect a battery to the circuit to mess up the
reading. Consider connecting a resistor in place of the battery. You
then have a different configuration altogether, with an added resistor
in parallel with your network. So in general, you may not be able to
trust measurements "in-circuit" of static values such as resistance.
Dynamic measurements such as with an oscilloscope present the same
concern; that's why o-scopes have such high input impedance.
 

neon

Oct 21, 2006
1,325
Joined
Oct 21, 2006
Messages
1,325
Measuring any active conponent even thought the power is off will have an inpedance.One thing is for sure you have a lot of caps across the output which are a short untill the meter charges them up if ever.You will never get a true reading as parts are connected and the bigger the resistance the bigger the error.
 
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