Understand source transformation

1. Feb 28, 2009

Air

I'm trying to understand source transformation and with my research, I have had some confusion. I have two question regarding them and would appreciate any help.

1. I understand that the thevenin resistance is found by short circuiting the voltage source. Is that also the case when you short circuit the current source? What if it has both sources, what resistance do we find when we short circuit them?
2. By using source transformation, can we only change to voltage source if the current source and the resistors are in parallel?

Thanks in advance.

2. Feb 28, 2009

Kruum

Re: Sources

1. You simply make a current source an open circuit. If you think of it a current source keeps the current in one branch constant, if you take it out, there would be no current.

2. Yep.

3. Feb 28, 2009

Air

Re: Sources

1. So if you take out the current source, it's not the same as taking out voltage source. By taking voltage source out, you can find thevenin resistance. By taking current source out, you find nothing? Is that what you are saying?

2. Great. So if the current source and the resistor are in series then we cannot use source transformation to change to voltage source?

4. Feb 28, 2009

Kruum

Re: Sources

No, you can take the current source out. You just can't short circuit it otherwise your resistance will be wrong.

Unfortunately, no. But there are usually some other tricks we can use to make the circuit easier.

5. Feb 28, 2009

Air

Re: Sources

So, if I'm to find the thevenin resistance and have been given both voltage source and current source, is the best method to convert the current source to the voltage source then short circuit the current source?

Hmmm... Can you briefly explain what to do if current source and the resistor are in series and we wish to change to voltage source?

6. Feb 28, 2009

Kruum

Re: Sources

You could do that. But then again you can take the current source out and replace it with an open connection.

If you have a current source in series with a resistor, there is nothing you can do to change the current source into voltage source. I meant that you can make the circuit easier by using some other tricks. Sorry for confusing you, I'm not natively English.

7. Feb 28, 2009

Air

Re: Sources

Great, Thanks.

Cool. I have a final question regarding this. If the voltage source and the resistor is in series, then it can be changed to current source? If the voltage source and the resistor is in parallel then it cannot be changed to current source?

It's ok. You have helped me alot and your language has been very clear.

8. Feb 28, 2009

Kruum

Re: Sources

That's right.

9. Feb 28, 2009

CEL

Re: Sources

If you have a current source in series with a resistor, you can eliminate the resistor from your circuit.
The same is true for a resistor in parallel with a voltage source.

10. Mar 1, 2009

Kruum

Re: Sources

Providing you have the appropriate circuit to do so.

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