Measuring Resistance

[Bob H.]

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

 

[Eeyore]

"Bob H." wrote:

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

 

[John G.]

"Eeyore" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:49BE0116.A2B72AAE@hotmail.com...

"Bob H." wrote:

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

 

[John Fields]

On Mon, 16 Mar 2009 02:10:32 -0400, "Bob H." <rhamm618@verizon.net>
wrote:

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?

---
Because your power supply looks like a resistance when it's turned off.
(View in Courier)



+--+-[R3[-+
|+ | |
[R] +-[R2]-+
|- |
+----[R1]-+


JF

 

[gearhead]

On Mar 15, 11:10 pm, "Bob H." <rhamm...@verizon.net> wrote:

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

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.