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I am stuck finding the Thévenin voltage in this circuit.

enter image description here

What I did first is remove the current source and replace it with an open circuit. After that I got the Thévenin resistance which is the equivalent resistance of the circuit. R-equivalent = 10 + 20 + 10 = 40. So now how do I get the Thévenin voltage (Thévenin equivalent)?

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  • \$\begingroup\$ You might want to check your first calculation. \$\endgroup\$ Commented Mar 27, 2020 at 17:28
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    \$\begingroup\$ It's simply 10 volts in series with 10 ohms by visual inspection and the mildest mental arithmetic. Can you see why? \$\endgroup\$
    – Andy aka
    Commented Mar 27, 2020 at 17:29
  • \$\begingroup\$ @Andyaka how did you get the 10 ohms ? sorry but i'm really bad at circuits...like really really bad lol. I thought i am supposed to get the equivalent resistance of the 3 resistors! and why is the thevenin equivalent 10 volts \$\endgroup\$
    – AhmadBenos
    Commented Mar 27, 2020 at 17:34
  • \$\begingroup\$ @SpehroPefhany did i get the resistance wrong ? \$\endgroup\$
    – AhmadBenos
    Commented Mar 27, 2020 at 17:36
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    \$\begingroup\$ Draw it out. Yes. \$\endgroup\$ Commented Mar 27, 2020 at 17:47

2 Answers 2

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Observe that the circuit can be redrawn as follows:

schematic

simulate this circuit – Schematic created using CircuitLab

Calculate the total resistance across the current source. Use Ohms Law to determine the voltage across R1. That same voltage is across the series combination of R2 & R3. Now calculate the voltage across R3.

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  • \$\begingroup\$ That's wrong what you have said. You have to measure the resistance from the side of ab. And the thevenin voltage across R3. Not R1. I will include reference from book of Alexander, Sadiku \$\endgroup\$
    – Sadat Rafi
    Commented Mar 28, 2020 at 7:16
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There are two processes to find it. The first one is using source transformation. The 2A current source and the 10 ohms resistance in parallel can be replaced by a voltage source of 20 volts and a series resistance of 10 ohms.

schematic

simulate this circuit – Schematic created using CircuitLab

So, from this circuit, you will get Thevenin resistace= 10 ohms and Thevenin voltage = 10 volts.

Another method is applying the rule;

Thevenin voltage = Thevenin resistance x Short circuit current of the load terminal

According to this,

schematic

simulate this circuit

The short circuit current is calculated using the current divider law. This will also give Thevenin voltage = 10 volts.

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  • \$\begingroup\$ Note the direction of the current source. When doing the source transformation, it should really be -20V or change the polarity of your voltage source. \$\endgroup\$
    – Big6
    Commented Mar 27, 2020 at 18:01
  • \$\begingroup\$ Oh, thanks. Didn't noticed that \$\endgroup\$
    – Sadat Rafi
    Commented Mar 27, 2020 at 18:03
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    \$\begingroup\$ Please don't give out answers to homework problems so easily. We expect the OP to do some work and tell us specifically where they are stuck. \$\endgroup\$ Commented Mar 27, 2020 at 18:07
  • \$\begingroup\$ Look closer. I've applied the current divider law. 1 amp flowing through the short-circuited line then through 10 ohms. Another 1 ampere is flowing through the 10 ohms resistance at the left side. No current will flow through the 20 ohms resistor under short circuit condition. \$\endgroup\$
    – Sadat Rafi
    Commented Mar 27, 2020 at 18:35
  • \$\begingroup\$ I've checked again. Please, provide your answer. It would be helpful for finding a mistake if it is made. \$\endgroup\$
    – Sadat Rafi
    Commented Mar 27, 2020 at 18:44

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