# Problem In Measuring Equivalent Resistance

• March 2nd, 2011, 12:11 PM
mr.science
Problem In Measuring Equivalent Resistance
Open the link for circuit diagram
http://i864.photobucket.com/albums/a...me/circuit.jpg

Keeping in view the circuit provided in the link if we find the equivalent resistance of the circuit by using the formula
Equivalent resistance = open circuited voltage/short circuited current
Equivalent resistance = 3V/0.4mA = 7.5kohm

But when we generally calculate the equivalent resistance then the 30kohm resistance and 10kohm resistance are in series because they share only a single node and in this way equivalent resistance comes out to be
Equivalent resistance = 30k + 10k = 40kohm

Why is this difference?
• March 2nd, 2011, 12:44 PM
Harold14370
In the one case you are calculating the equivalent impedance looking back at the circuit from the output terminals, and that is the parallel combination of the 10K and 30K resistor. In the second example you are calculating the impedance which is seen by the power supply. That is the series combination of the same two resistors.

Here is another way of looking at it. Thevenin's theorom says we can treat any linear circuit as a "black box" which is equivalent to a voltage source in series with an impedance.
http://en.wikipedia.org/wiki/Th%C3%A9venin%27s_theorem
If you take everything to the left of your output terminals and enclose it in a "black box" the equivalent voltage and resistance are as determined by your stated method.
• March 5th, 2011, 08:06 AM
mr.science
Thank you very much for the response.

I have got the point clear that where i am actually making the mistake.

Now I would like to ask that what is the difference between looking circuit from source side or from the output terminal side?

I mean that suppose i have a circuit in real and i want to remove all the resistances and replace all the resistance with one resistance i.e, equivalent resistance.Now when i look at my real circuit from source side it has got different equivalent resistance and when i look at the circuit from output terminal side it gives different equivalent resistance.Which equivalent resistance shall i substitute in the circuit and why?the one that is calculated from seeing the circuit from source side or the one that is calculated by seeing the circuit from output terminal side?
• March 5th, 2011, 09:39 AM
Harold14370
It all depends on what you are trying to calculate. If you need to know the current and power consumption of the battery, you will have to use the resistance of the network that is connected to the battery terminals. If you need to know the current in a load, which is to be connected to a pair of output terminals somewhere in the network, then you could use Thevenin's theorem to determine an equivalent voltage and resistance.

The Thevenin equivalent voltage and resistance would only be used to calculate the current in the load. It would not tell you anything about what is happening inside the "black box".
• March 6th, 2011, 02:42 AM
mr.science
Thanks.