Thevenized or Nortonized circuit is a reduced form of a original circuit

[PG1995]

Hi

A Thevenized or Nortonized circuit is a reduced form of a original circuit; in other words a simple form of a complex one. For instance, a circuit with 20 resistors could be reduced to an equivalent circuit which uses no more than one resistor. Well, why don't we make a simple circuit instead of complex one? What's the need of wasting money on 20 resistors when only one can do? Would you please tell me? Thanks.

Regards
PG

 

[Grossel]

Funny you

Could have been posted in members lounge thought

 

[crutschow]

A Thevenin or Norton equivalent is how it appears to the outside world from one output port. But internally those extra resistors may be doing something useful that is not apparent from one external connection. It may also have other external connections which likely will have a different equivalent Thevenin or Norton value. For example a passive filter will have an input and an output with the input equivalent generally different then the output equivalent. So the reason for using all those extra parts is determined by what the designer wanted them to do.

 

[PG1995]

Thanks a lot, crutschow. I'm sorry for the belated reply. I did read your reply when you replied. And a little thanks to Grossel for finding it funny!

Would it be possible for any of you to show me a simple circuit which makes it clear that Thevenin equivalent circuit cannot replace the original one? It would be very kind of you.

Regards
PG

 

[carbonzit]

Would it be possible for any of you to show me a simple circuit which makes it clear that Thevenin equivalent circuit cannot replace the original one? It would be very kind of you.

Sure; that's a trivial exercise.

**broken link removed**

The circuit inside the box requires exactly half the supply voltage (hence the two equal resistors). When we Thévenize or Nortonize the circuit, the two resistors will be combined into the single equivalent resistance.

Keep in mind that Thévenizing or Nortonizing circuits is an analytical tool, not a way of reducing the parts count. We're only conceptually reducing the circuit for the sake of analysis.

 

[PG1995]

Thanks a lot, carbonzit.

Is the "V" on the left side a voltmeter?

When we Thevenize the circuit, won't the circuit within the box also be considered? e.g. If it were a current source we will turn off it by opening it.

Please help me. Thank you.

 

[PG1995]

Help, please! I hope it's not considered premature bumping.

 

[carbonzit]

s the "V" on the left side a voltmeter?

No, it's the voltage source.

When we Thevenize the circuit, won't the circuit within the box also be considered? e.g. If it were a current source we will turn off it by opening it.

Yes, of course; whatever's in that box becomes part of the Thévenin equivalent.

 

[PG1995]

Thank you, carbonzit.

Okay. Then, we Thevenize the circuit the voltage source will be shorted. Suppose the circuit inside the box contains elements such as diodes etc. Such a circuit cannot be Thevenize (perhaps, it could be but it won't be 'exactly' equivalent, at least that's what I know). Then, what will we do with the circuit inside the box? Will we leave as it is and only find the equivalent resistance of the two resistors which divides the voltage?

 

[carbonzit]

Okay. Then, we Thevenize the circuit the voltage source will be shorted. Suppose the circuit inside the box contains elements such as diodes etc. Such a circuit cannot be Thevenize (perhaps, it could be but it won't be 'exactly' equivalent, at least that's what I know). Then, what will we do with the circuit inside the box? Will we leave as it is and only find the equivalent resistance of the two resistors which divides the voltage?

No, all circuits can be Thévenized. (It's an analytical tool, remember.) Now, I'm in an area that's a little "above my pay grade" here, but if a circuit contains diodes or other non-linear devices, their equivalent resistance (under the conditions found in the circuit) must be computed as part of the equivalent circuit. Perhaps someone else here can explain it better; I'm not exactly sure how that is done. (Same for transistors, etc.)

The point is that all of the circuit, including whatever is in my box up there, gets simplified when the circuit is Thévenized.

 

[PG1995]

Thank you, carbonzit.

Okay. As you say the Thevenized circuit cannot replace the original circuit. That means the circuit in post #5 cannot be replaced by its Thevenine equivalent. Let's say it's due to the reason that the circuit within the box is doing some special job. What simple function can you think of which enclosed circuit is doing? Perhaps, it has some output port which connect to some external device such as headphones! I hope I'm nearing the end of my confusion. Thank you.

Regards
PG

 

[carbonzit]

Okay. As you say the Thevenized circuit cannot replace the original circuit. That means the circuit in post #5 cannot be replaced by its Thevenine equivalent. Let's say it's due to the reason that the circuit within the box is doing some special job. What simple function can you think of which enclosed circuit is doing? Perhaps, it has some output port which connect to some external device such as headphones! I hope I'm nearing the end of my confusion. Thank you.

Well, unfortunately, you really need to be able to view this in general terms; in other words, it doesn't matter what any part of the circuit does. Reducing circuits using Thévenin's or Norton's theorems applies to any and all circuits, regardless of what they do. So it doesn't matter if that box in my circuit drives a pair of headphones, controls a relay, blinks a LED or any of a thousand other things it could do. These are generalized analytical tools, that's all. Does that help?

Once you master Thévenin and Norton, you should be able to look at any circuit and reduce it to its simplest possible form, for the sake of determining the circuit's voltage or current, and its equivalent resistance.

 

[Ratchit]

PG1995,

A circuit must be linear before Thevenin's or Norton's theorem can be applied to it. Ratch