Help with Thevenin equivalent circuit

[jj5]

Hi there. I am working my way through Thevenin-Norton class notes from G. Tuttle and I built this circuit as shown on page 6.

The results I get are these:

RL	vRL	iRL
50 Ω	9.56 V	3.07 mA
500 Ω	10.32 V	2.91 mA
5 kΩ	15.37 V	1.91 mA
50 kΩ	22.8 V	0.42 mA
500 kΩ	24.71 V	49.1 µA

The results from the author are these:

RL	vRL	iRL
50 Ω	0.2475 V	4.95 mA
500 Ω	2.273 V	4.55 mA
5 kΩ	12.5 V	2.5 mA
50 kΩ	22.73 V	0.455 mA
500 kΩ	24.75 V	49.15 µA

For 500 kΩ and 50 kΩ my results are very similar to those in the instructions, but for resistance values lower than that my results differ considerably.

My setup is a Riden RD6006 bench power supply providing 10 V in and a FNIRSI SG-002 current source providing 4 mA in. R₁, R₂, and R_L are provided by three decade boxes. The Fluke meter is measuring voltage across the load and the EEVblog meter is measuring current through the load.

I would like to understand why I don't get the same results as the author? Thanks for your help!

[PGPG]

Look critically at your results.
If you have 9.56V at 50 Ω then 9.56/50 = 191mA and not 3.07 mA.

The difference is too big to be due only to measurement inaccuracies.

[jj5]

Look critically at your results.
If you have 9.56V at 50 Ω then 9.56/50 = 191mA and not 3.07 mA.

The difference is too big to be due only to measurement inaccuracies.

Some of my measurements must be wrong. But which ones and why?

[PGPG]

Some of my measurements must be wrong. But which ones and why?

My crystal ball is out of order.

[RFDx]

Where exactly did you insert the current meter? Remove the current meter, measure just the voltage across the load and compare with theory and your previous values.

[golden_labels]

Measurements seem correct. The circuit — not so much.

A hint: does your current source behave like a current source when the current drawn would exceed 4 mA?

[Sensorcat]

Some of my measurements must be wrong. But which ones and why?

There are two ways to solve this: Waiting for a sudden inspiration or systematically work through every component to check for errors. It often needs a surprisingly low number of components for the need of the boring second option.

Check your meters against the sources, both meters, both sources, several values set on the sources each. Check your resistor decades, they include lots of switches, which sometimes fail. Check your wires. If finished and nothing found, check how your sources behave in the presence of another source. It is important to understand that the instructions that you have used are circuit theory, which uses ideal sources. You don't have ideal sources, the ones you have try to come close to ideal. For instance, no real current souce is able to maintain its current with any load resistance. Reason: It must increase the voltage across its terminals for high load resistors, but there's a limit, which the ideal source does not have.

[trilerian]

I looked at the circuit in the book.  Vth = 25V, Rth = 5000kohm, In = 5mA.  The 4mA current source has to raise the voltage to node A to 25V, make sure your current source can supply enough voltage to do this.  In real life voltage sources and current sources are not ideal... 

[jj5]

Thanks to everyone who helped me with this problem. In the process of applying some of your suggestions I took a new set of readings, and this time they were pretty much exactly the same as what was shown in the instructions. So it must have been something mundane like a bumped switch on the decade box or a loose wire or something... anyway all is well that ends well, thanks again!