Source Transformation - Page 1
Forum section: electronics / beginners

author: JOJOElectro29
author_rank: Contributor
timestamp: May 08, 2025, 06:25:06 am
content: Hi I'm not sure how the source transformation in this question works. The resistors 125 ohmn and 10 ohmn are suddenly omitted in the first step. I tried calculating the resistors where 125 ohmn is in parallel with 35 ohmn unsuccessfully. I did not get 25 ohmn

author: iMo
author_rank: Super Contributor
timestamp: May 08, 2025, 06:43:56 am
content: A Voltage source has zero internal resistance/impedance (in theory a short).A Current source has infinite internal resistance/impedance (in theory an open circuit).

author: MathWizard
author_rank: Super Contributor
timestamp: May 08, 2025, 08:00:15 am
content: I think they are just ignoring the 125R and 10R, for finding Vout, since the 10R won't change the 8A, and the 125R won't change the 250V. So the current of the 250V, should be higher overall, then they use.

author: JOJOElectro29
author_rank: Contributor
timestamp: May 08, 2025, 11:45:02 am
content: IMO, you mean the 125 ohm is a short circuit. In that case wouldn't 250V be 0V? for the 10 ohm resistor if that section of the circuit is open, is the overall current still 8A?

author: MathWizard
author_rank: Super Contributor
timestamp: May 08, 2025, 08:32:52 pm
content: No it would be open, just since it's in parallel with 250V, and they they ignore the current it would have had. But then Vs and the series 25R would make the Inorton of 10A, and with the 25R now as a parallel / shunt resistor. And then with the 20, 25, and 100 all between the same 2 nodes, they take their parallel combo to get 10ohms.But yeah they must be shorting the 10R that was in the branch with the 8A, /pretending it's not there. Even tho it should have 80V across it, I can't remember for sure, why that's allowed. But an ideal current source will always adjust it's voltage to whatever is required, for the given current.If I did leave the extra 10R there, then I would just use nodal analysis or superposition to get the answer, and then try and make the answer w/ 1 source, and 1 resistor.

author: ledtester
author_rank: Super Contributor
timestamp: May 08, 2025, 10:41:00 pm
content: Convert this part of the circuit to its Thevenin equivalent:The open circuit voltage is 250V.The short-circuit current is 250V / 25R = 10A.Note how the 125R resistor plays no part in the calculation - none of the short-circuit current flows through the 125R resistor.

author: MrAl
author_rank: Super Contributor
timestamp: May 09, 2025, 05:25:29 am
content: Quote from: JOJOElectro29 on May 08, 2025, 06:25:06 amHello,For these kinds of problems the sources are considered to be ideal which means their values do not change with any sort of load.The whole idea for that first part is that a resistor in parallel to an ideal voltage source does not alter the voltage of that source. So you are left with a 250v source in series with that 25 Ohm resistor. That then converts to a 10 amp source in parallel with the 25 Ohm resistor.A similar idea goes for the 8 amp current source. A resistor in series with an ideal current source does not alter the current, so 8 amp flows through the 10 Ohm resistor.Once you do the transformations you end up with a simpler problem to solve. That's the beauty of source transformations.If you want to be sure this all works, try simulating the 250v source in parallel with that 25 Ohm resistor ALONE without anything else. Change the value of the resistor from 25 Ohms to 50 Ohms, to 100 Ohms, to 10 Ohms, then note that the voltage of 250v never changes. Since the 250v source stays at 250v, you can proceed AS IF the circuit did not even have that 125 Ohm resistor in it. You only need to consider that resistor if you want to figure out the total current drawn from the 250v source and the power it has to deliver to the entire circuit.

author: BennoG
author_rank: Frequent Contributor
timestamp: May 09, 2025, 05:08:45 pm
content: The whole setup is flawed. If you only look at the 8A current over the 10 ohm resistor then Vo can only be 80V and nothing else.If the voltage is higher then there is more current through the 10 ohm resistor any lower voltage will require less current through the 10 ohm resistor.Benno

author: ledtester
author_rank: Super Contributor
timestamp: May 09, 2025, 06:31:38 pm
content: Quote from: BennoG on May 09, 2025, 05:08:45 pmThe 80V occurs at the junction of the 8A source and the 10R resistor. See the attachment for all of the voltages and currents.Being ideal, the 8A current source can always change the voltage at its "tail" (Vo in this case) to the voltage required at its "head" (80V).

author: JOJOElectro29
author_rank: Contributor
timestamp: May 10, 2025, 03:43:28 am
content: found this solution from Electric Circuits-James W. Nilsson, Susan Riedel-10th-Prentice Hall (2014) Example 4.9