Post subject: Thevenin Equivalence Question Posted:
Wed Aug 24, 2005 7:50 am
Joined: Wed Aug
24, 2005 7:43 am
Hello all- I am studying for my
PE after 20 years away from school...
In a very simple circuit
of a dependent current source (value= .75* voltage across resistor)
in parallel with a resistor of 4 ohms, how would one compute the
equivalent Thevenin resistance of that circuit (measured across
the parallel combo). Yeah, I know it's probably very simple... The
answer is -2 ohms, I'm told. Not sure how to arrive at that answer...
Post subject: Posted:
Wed Aug 24, 2005 11:32 am
Aug 23, 2005 6:33 pm
You need to think out of the
box on this one. Since you are dealing with a voltage-dependent
current source, you need to to know what the voltage across the
4ohm resistor is in order to get the source's current value. Try
applying a 1V independent voltage source across the output (i.e.
parallel to the 4ohm R). Then you know that Vresistor = Voc = 1V.
That means that Iresistor = 1V/4ohm = 250mA. Also, Idependentsource
will be 0.75A*1V = 750mA. The remaining 500mA will flow through
your 1V source at the output and be considered your Isc.
So, Voc/Isc = 1V/500mA = 2ohm. This result is dependent upon the
position of the current source. If, in normal circuit convention,
the arrow points up, then the above answer is correct. However,
if the arrow points down, the current flow is reversed, and Isc
becomes negative. This may be why you are told the answer is -2ohms.
Good luck with your PE!
Post subject: Thank you so much!Posted: Wed Aug
24, 2005 1:25 pm
Joined: Wed Aug 24, 2005
Great clear explanation-