circuit analysisparallel
I have a simple question on resistance calculation but I can't work it out. The diagram is from the site all about circuits.
I can't figure out why the top parallel total resistance for example is 71ohms. I thought the calculation for this would be
1/Rt=1/100+1/250
1/Rt=0.1+0.04=0.104
Rt=1/0.104=9.6?????
Have I missed something obvious? Appreciating that the answer is probably simple but if I dont ask I'll never find out….
The Thevenin equivalent resistance is one that looks to the circuit you want to replace. To find this value, all independent sources must be passivated.
Passivate ideal voltage source means replacing it by a short circuit. Passivate ideal current source means replacing it by an open circuit.
If the 5V supply is passivated in this specific case, the two resistors are connected in parallel, and here the value of the Thevenin equivalent resistance.
simulate this circuit – Schematic created using CircuitLab
ERRATA: In the schematic, "V2" must be \$V_{th}\$, Thevenin's equivalent voltage source.
If by "parallel and series analysis" you mean combining resistors in series and parallel until you have a single resistor then no, you can't use that kind of analysis because there aren't any resistors in series or parallel to combine. I would be inclined to use node voltage analysis myself.
However, other people might prefer other methods of analysis, such as source transformation or delta-wye transformation. I usually just go for a nodal analysis because it is a general technique and I know it will work.
Best Answer
The way you do it is correct, but the numbers don't check out:
\$\frac{1}{100 \Omega}\$ is 0.01 S and not 0.1. \$\frac{1}{250 \Omega}\$ is 0.004 S and not 0.04. So you end up with 0.0104 S, of which the inverse is 71.429 \$\Omega\$ (which has way to many digits).