# Non-linear device

1. Feb 2, 2010

### ian2012

Given any 'blackbox' circuit, for which the values of the thevenin voltage and thevenin resistance are known. And given that a non-linear device is placed across the terminals of the circuit. What relation would one use to find the current across the device. Since it is non-linear, i am guessing V/R does not apply.

2. Feb 2, 2010

### uart

If the nonlinear device had a simple static V/I characteristic in the form V=f(I) then you would solve the equation,

V_thv - R_thv I - f(I) = 0

3. Feb 2, 2010

### ian2012

Thank you for that. Is there an intuitive explanation?

4. Feb 3, 2010

### uart

Yes it's just a simple application of KVL (Kirchoff Voltage Law) around the loop.

5. Feb 5, 2010

### turin

However, are you also including nonlinear source circuits in your consideration?

6. Feb 5, 2010

### uart

There's actually no problem with that turin. The "black box" itself must be linear in order to have a simple Thevenins equivalent, but there's no restriction on the nature (linear/nonlinear) of the external circuit when using a Thev equiv.

7. Feb 5, 2010

### turin

That's precisely the problem. I just didn't want the OP to think that they could make a few measurements to derive a Thev. eq. circuit for any blackbox. There are some blackboxes that are not even approximately linear.

EDIT: I reread the OP carefully, and now realize that the OP probably meant what you said. Sorry.

8. Feb 19, 2010

### jsgruszynski

The part you had the Thevenin of didn't stop being linear so it still looks the same electrically.

BTW Thevenins are only strictly valid if the circuit they are representing is either linear or nearly linear (small signal assumption). Otherwise the Thevenin itself would be a problematic representation - though often enough it's still done and then you throw in new specs like THD and IMD to account for the variance.