How to solve a circuit mystery box?

• Engineering
• Sage4
In summary, the resistors in this circuit are not in the correct order, and so it cannot be the circuit as illustrated.
Sage4
Homework Statement
deduct the value of each of the 5 resistors of the same value in the 2 black boxes and their organization in a circuit explaining what is being measured.
Relevant Equations
Serie: Req= R+R+R...
Parallele: 1/R_eq =1/R+1/R+1/R….
How can I solve a ciruit mystery box only knowing there are 5 identical resistors inside. I have two boxes to solve, one they're all in serie and the other is a combination serie and parallele. Each mystery box has 6 nodes and i can connect 2 nodes at a time to an ohmmeter.

If i take the one all in serie (it's the only one I've started) this is what I've concluded:

All resistor are 10k ohm and they're all in serie. With 6 nodes i have the 15 following combinations. What other conclusions could be made? The colors are because I'm more of a visual learner.

This is what @Baluncore told me:

Measure and record the resistance between;
Terminal; t1 and t2=30k, t3=20k, t4=30k, t5=20k, t6=10k
Terminal; t2 and t3=10k, t4=20k, t5=30k, t6=20k
Terminal; t3 and t4=10k, t5=20k, t6=10k
Terminal; t4 and t5=30k, t6=20k
Terminal; t5 and t6=10k

Notice the 5x 10k ?
The 5x 10k resistors are between; t1-t6, t2-t3, t3-t4, t3-t6, t5-t6.

Notice there are three arms from t3 and from t6.
But t3 and t6 are connected by 10k.

t2 and t4 are each connected by 10k to t3.
t1 and t5 are each connected by 10k to t6.
t1, t2, t4 and t5 are the terminal ends of the circuit.
Now check all the 20k and 30k values are series connected paths on the same diagram.
This is what I’ve been tryng to do with those info:

The one in the bottom I'm tryng to see if 2 nodes can be connected at the same place/switch but I am still blocked...

Last edited by a moderator:
.Scott
You noticed that there were 5 10K measurements. What does that suggest about any internal nodes (nodes not connected to the outside of the box)?

There are 5 10K readings. That should be a big hint.

These resistors are not lined up like this: -R--R--R--R--R-

Also, none of the measurements indicate an open circuit, so everything is connected - directly or indirectly.

.Scott said:
You noticed that there were 5 10K measurements. What does that suggest about any internal nodes (nodes not connected to the outside of the box)?

There are 5 10K readings. That should be a big hint.

These resistors are not lined up like this: -R--R--R--R--R-

Also, none of the measurements indicate an open circuit, so everything is connected - directly or indirectly.

That's what i was suspecting...this is what i have if the resistors are not lined up like this: -R--R--R--R--R-
But doesn't that resistor between 3 and 6 make the circuit parallele? I opened the circuit at 1, 2, 4, and five for that but I'm not sure.

.Scott
.Scott said:
These resistors are not lined up like this: R--R--R--R--R
@Sage4: Ceci est comme le son de mon camion quand j'essaye de le démarrer par une journée froide ##-## That's like the sound of my truck when I try to start it on a cold day.

You would prebably benefit from studying some fundamentals of resistor black box analysis.

In addition to Ohm's law, which is of course de rigeur here, Thévenin's theorem and Norton's theorem can help you to simplify things.

You can also apply some principles of algebraic graph theory ##-## with the terminals and junctures as nodes and the resistors and connectors as edges.

Hint: you can use the readings to produce a system of simultaneous equations.

https://diyodemag.com/education/fundamentals_black_box_circuit_analysis
https://www.cs.cmu.edu/~tdear/ee/thevenin_norton.pdf

Sage4 said:
sysprog So I'm wrong ...?

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Sage4 said:
So I'm wrong ...?

So you should calculate all resistances in your proposed solution and see if they match.

It doesn't sound like you've been very methodical. The way to attack this is 1) figure out where the resistors are, and then 2) figure out how they are connected.

sysprog
Sage4 said:
But doesn't that resistor between 3 and 6 make the circuit parallele? I opened the circuit at 1, 2, 4, and five for that but I'm not sure.

View attachment 272553
There are no parallel circuits between any 2 terminals in the illustration.
Measure and record the resistance between;
Terminal; t1 and t2=30k, t3=20k, t4=30k, t5=20k, t6=10k
Terminal; t2 and t3=10k, t4=20k, t5=30k, t6=20k
Terminal; t3 and t4=10k, t5=20k, t6=10k
Terminal; t4 and t5=30k, t6=20k
Terminal; t5 and t6=10k

Given the measurements listed, the circuit as cannot be as illustrated here. For explanatory purposes, the illustration is here re-posted with the resistors (arbitrarily) labeled R1 ##-## R5:

(edited to emend this part, as it had errors, and the errant vesion is quoted and discussed below).

Last edited:
sysprog said:
There are no parallel circuits between any 2 terminals in the illustration.Given the measurements listed, the circuit as cannot be as illustrated here. For explanatory purposes, the illustration is here re-posted with the resistors (arbitrarily) labeled R1 ##-## R5:

View attachment 272558

For example (using ##\leftrightarrow## to mean 'is connected via ammeter to'), if ##\mathtt{T1\leftrightarrow T3 = 20k}##, then ##\\##
##\mathtt{R1=20k}##, and if ##\\##
##\mathtt{T1\leftrightarrow T4 =30k}##, then ##\\##
##\mathtt {R3+R4=20k}##, but if ##\\##
##\mathtt{R1=20k}## and ##\mathtt{R3+R4=20k}##, then ##\\##
##\mathtt{T1 \leftrightarrow T2 = R1+(R3+R4) =20k+20k=40k}##, ##\\##
but the measurement says that ##\\##
##\mathtt{T1 \leftrightarrow T2 = 30k}##, ##\\##
Hum I am afraid to be wrong but it works everytime i try. Taking your example, how is R1=20k R1 is between t2 and t3 not t1 and t3.

Plus there is no way only one resistor is 20k, they are identical and 10k

I was still editing when you quoted me (I missed some things while using Preview), and I recognize that the problem statement says the resistors are all of equal value, but the connection from T2 to T3 inccludes only R1, so if they're all 10k, how do you get a measurement of 20k from T2 to T3 ?

sysprog said:
I was still editing when you quoted me (I missed some things while using Preview), and I recognize that the problem statement says the resistors are all of equal value, but the connection from T2 to T3 inccludes only R1, so if they're all 10k, how do you get a measurement of 20k from T2 to T3 ?
Haha Haha that's what I am saying t2 to t3 is 10k lol

Sage4 said:
Haha Haha that's what I am saying t2 to t3 is 10k lol
oops ##-## my mistake ##-## sorry . . .

So is it right? I am confuse now haha

Sage4 said:
So is it right? I am confuse now haha
I'll have to review more carefully before I can in good conscience reply to that question.

(edit)
I looked again, and it now looks right to me. I can't not have said what I said, but I can admit that I was hasty, and that my statement that "given the measurements listed, the circuit cannot be as illustrated here", was as far as I can now tell, incorrect. Sorry about the accidental incorrectness.

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Ok thanks. I am now going to try the mixed one using what you sent me earlier. Thank you again.

sysprog said:
@Sage4: Ceci est comme le son de mon camion quand j'essaye de le démarrer par une journée froide That's like the sound of my truck when I try to start it on a cold day.
I'm embarrassed to say how long it took me to get this. LOL

BvU

1. How do I determine the circuit mystery box's components?

To determine the components of a circuit mystery box, you will need to use a multimeter to measure the resistance, voltage, and current of each component. You can then compare these values to known values of different components to identify them.

2. How do I troubleshoot a circuit mystery box?

To troubleshoot a circuit mystery box, you will need to start by checking for any loose connections or damaged components. Then, use a multimeter to test the voltage and resistance at different points in the circuit. This will help you identify any faulty components or areas of the circuit that need to be repaired.

3. Can I use Kirchhoff's laws to solve a circuit mystery box?

Yes, Kirchhoff's laws can be used to solve a circuit mystery box. These laws, which include Kirchhoff's current law and Kirchhoff's voltage law, can be used to analyze the flow of current and voltage in a circuit and help you identify any errors or issues in the circuit.

4. How do I use Ohm's law to solve a circuit mystery box?

To use Ohm's law to solve a circuit mystery box, you will need to know the voltage, current, and resistance of the circuit. You can then use the formula V=IR to calculate the values of any unknown component in the circuit.

5. What should I do if I can't solve the circuit mystery box?

If you are unable to solve the circuit mystery box, you may need to seek help from a more experienced individual or consult online resources for troubleshooting techniques. It is also important to double-check your measurements and make sure you are using the correct values for components in the circuit.

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