# How many measurements are needed to determine a black box with four terminals?

I am having a black box with four terminals and just resistors inside. I can model it like this: simulate this circuit – Schematic created using CircuitLab

If I have four different currents and four different potentials at the four different terminals, that seems not enough to determine the circuit inside, as the determinant of my matrix to solve that seems 0, when I create equations from Kirchhoff rules. Either I created them wrong, or I must make multiple measurements with, e.g., one or two terminals open. What is the minimum amount of measurements I need to do to determine the resistors in such a black box?

• The above isn't topologically the same between any two terminals. (Easy to see that fact.) Do you know, a priori, which terminals have which relationships to this specific topology? If not, if this is truly a black box and any terminal may be connected to any of these, then you have another variable, which is the orientation of this to the terminals at the black box. (Of course, perhaps you mean something even more complex inside? With this just being an example of many.) Jul 17 at 22:14
• How many such boxes you need to measure? Seems to me like doing it manually should take less time than even writing up the question. You should be able to measure one box in a minute going manually and capturing measurements in the memory of the bench multimeter. Use a pedal from music instrument store for trigger source. Very handy. Jul 18 at 0:39
• I'm not even sure what the question actually is. A current source between R1 and R3 yields a voltage at R1 and at either of R2 or R4, which tells you both resistances, R1 and R3 with a single setup and 2 voltage measurement values. Similarly, R2 and R4 can be had. Finally, one more measurement and you get R6. Not so? (That's assuming the topology is known with respect to the terminals, of course.) Is this more about the matrix setup that validates the approach? Jul 18 at 2:18
• What is "a measurement"? Is an oscilloscope trace one measurement or thousands? Jul 18 at 13:16
• Where is R5? The numbering jumps to R6.... Jul 18 at 17:53

You have 5 unknowns. You will need at least 5 measurements to determine them.

If I have 4 different currents and 4 different potentials at the 4 different terminals, that seems not enough to determine the circuit inside, as the determinant of my matrix to solve that seems 0, when I create equations from Kirchhoff rules.

If you were thinking these 4 currents and 4 voltages were a total of 8 measurements, that's not correct.

In simple terms, 4 of these values depend on the sources you applied externally and not on the internal structure of the unknown network. If you did the measurements by, for example, applying ideal voltage sources to each terminal, then this should be obvious.

Even if you actually used a more complex source (for example a Thevenin source with an internal resistance), it will turn out that you added 4 degrees of freedom from the sources, and measured 4 responses; rather than obtained 8 independent measurements.

Another way to think of it is, you are trying to determine 5 resistances and what you measured was 4 resistances at 4 terminals, not 8 independent characteristics of the circuit.

• So with 4 currents and 4 fixed voltages, I get 4 information from it, so I guess that there should be a solution with just 2 measurements e.g. one measurement with all measurements and one measurement with one open terminal. I will try that. Jul 17 at 19:06
• @blablubbb if you're applying voltage sources to all the terminals at the same tie, that only gives 3 independent measurements, because only the differences between the terminal voltages affect the currents, not their values relative to some arbitrary ground that the unknown network isn't connected to. Jul 18 at 5:57

You do need 5 measurements to get 5 variables out. Say:

• R1+R3 (left arm)
• R2+R4 (right arm)
• R1+R6+R2 (upper branch)
• R3+R6+R4 (lower branch)
• R1+R6+R4 or R2+R6+R3 (cross branch)

It is possible to do them all at once by using floating AC sources - a different frequency for each measurement. The sources have to act like open circuits outside of their tuned frequency. In practice that means gyrators or digitally controlled sources that synthesize this floating-like behavior.

• The AC sources idea is a nice way to get it in one measurement, but I'd like to stay DC and I think there should be a solution with less than 5 measurements with 2 terminals, when the measurements involve more than 2 terminals, just the math is more annoying. Jul 17 at 19:01
• You won’t get 5 unknowns solved with 4 measurements only. It’s not about math being “annoying”, it’s not possible. All those 5 resistances are unknown and independent. If they were not then you could make approximations with fewer measurements. But as it stands, for a general solution to a general problem, you need 5 measurements. With 4 measurements you should get one variable free and the solutions will be in terms of that variable. That just from linear algebra, the matrix will be rank 4 instead of rank 5. And it is thankfully a linear problem. Jul 18 at 0:36
• @blablubbb Note that you need at least 5 measured variables. But you don't necessarily need 5 different measurement setups, if you measure multiple variables in one setup. The term "measurement" is not precise enough to distinguish between these.
– jpa
Jul 18 at 9:11

For the standpoint of a network with n accessible nodes, measurement of steady voltages and currents on/in the nodes can be summarized as n-1 voltages (one node serving as reference) and n-1 currents (the last current is the negative of the sum of the others), thus 2n-2 quantities characterize the network state. Further, in a network made of resistors, scaling the n-1 voltages scales the n-1 currents linearly. Without proof: this means we need 2n-3 resistors to model an arbitrary resistors network.

For n = 4 in the question, a network of 5 resistors indeed is enough. However no all resistor networks with 4 external nodes can be represented by the question's H construction when we want non-negative resistances and force the order of nodes. For example this: Proof: in the above circuit the resistance measured between nodes A and B (with C and D open) is R7 ∥ (R8+R10+R9) < 1Ω, when in the question it's R1+R6+R2. It follows we need R1 < 1Ω. Similarly, we need R3 < 1Ω. But the resistance measured between A and C in the above circuit is R8 ∥ (R7+R9+R10) > 50Ω, and is R1+R3 in the question's circuit, leading to the contradiction 50Ω < 2Ω.

What is the minimum amount of measurements I need to do to determine the resistors in such a black box?

If we count as measurement a reading of a multimeter, 2n-3 = 5 measurements at least, since we have as many unknowns. And exactly so if we are cautious in how we measure.

It's convenient to put a multimeter in resistor mode with autoranging, and measure 2n-3 = 5 apparent resistances across node pairs with the other n-2 = 2 disconnected. For the question, labeling accessible nodes of the H model ABCD in reading order:

1. RAC = R1+R3
2. RBD = R2+R4
3. RAB = R1+R6+R2
4. RCD = R3+R6+R4

We then compute