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Old 10-19-2016, 10:48 AM   #1
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Default Unknown Test Points

Say you have a box with an unknown circuit inside are there are 10 test points A, B, C, D, E, F, G, H, I, J. How would you go about finding out what is between them? You have a multimeter, oscilloscope, and analog meter. If asked to find the resistance and voltage between A and B that seems pretty straight forward. You just measure between it and record your findings, correct? But what if you're asked to FIND out what is between each point? What method do you use? Do you simply plug in the three tools one at a time, cycle through them, and record your readings, or is there a different method?
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Old 10-19-2016, 10:52 AM   #2
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What do you mean "between" them? ...length of wire, devices, equipment...wagos...?
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Old 10-19-2016, 11:00 AM   #3
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It's a small box with test points on it. The points make it look like it's an in series circuit. 4 on the left, 4 on the right, 2 on the bottom
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Old 10-19-2016, 11:37 AM   #4
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I would just open it up before I wasted my time testing every connection.

the two on the bottom are likely power, and the 4 on either side likely in/out.

If it is a homework problem, then it is a problem in logic, as well as electronics.

since the circuit could include any number of components which behave differently depending on the state(s) of the machine, determining the components or connections would be a logical process of finding the simplest
components first. A caveat to this is that some components could be damaged
if the circuit is active when the tests are being done, or if the test instrument
is imposing a voltage/current greater than the component can withstand (ie cascade failure on a diode or transistor in an active circuit, etc).

A scope measures waveform, but unfortunately if you don't know the circuit's properties beforehand, you would likely have to isolate simple connections with voltmeter/ohmmeter (hopefully high impedance). Then possibly waveform analysis if that applied. Then you would have to repeat the measurements with each state. Examining a black box as described is a lot more complicated than, say, doing a visual inspection of the components to get a basic understanding of the device, then inspecting the states for the terminals.

just my 02
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Old 10-19-2016, 12:58 PM   #5
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Okay, if this is some kind of homework problem...then, yeah, you have to approach a black box and figure out what it is without the bomb going off...? Is that it?

If that is the case, then I would approach it first with a rated oscope and a 10:1 optically isolated (passive) probe.

If you are into circuit analysis (electronic as well as electrical) you'll be able to pretty much discern what is plugged in, how much load, etc.

If everything is off, then call the NSA...maybe they have some test equipment you can borrow.
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Old 10-19-2016, 05:17 PM   #6
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I hate homework.

Especially someone else's.
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Old 10-19-2016, 06:01 PM   #7
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Quote:
Originally Posted by charlie111 View Post
Say you have a box with an unknown circuit inside are there are 10 test points A, B, C, D, E, F, G, H, I, J. How would you go about finding out what is between them? You have a multimeter, oscilloscope, and analog meter. If asked to find the resistance and voltage between A and B that seems pretty straight forward. You just measure between it and record your findings, correct? But what if you're asked to FIND out what is between each point? What method do you use? Do you simply plug in the three tools one at a time, cycle through them, and record your readings, or is there a different method?

what are they likely to be ?
electronic components ?
start with the ohm meter
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Old 10-19-2016, 06:14 PM   #8
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what are they likely to be ?
electronic components ?
start with the ohm meter
Always voltage before resistance readings, it saves blowing up a meter in your hand.
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Old 10-19-2016, 06:15 PM   #9
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Homework is so much easier when you have all the criteria of the problem spelled out for you and not just summarizing.
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Old 10-19-2016, 06:21 PM   #10
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Homework is so much easier when you have all the criteria of the problem spelled out for you and not just summarizing.
It gets even better if you stayed awake during class.
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Old 10-19-2016, 07:36 PM   #11
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It gets even better if you stayed awake during class.
I did not stay awake and I earned evey bit of my F
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Old 10-19-2016, 07:41 PM   #12
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I did not stay awake and I earned evey bit of my F
If I napped in class I just intimidated the teachers into extra credit points.
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Old 10-19-2016, 10:45 PM   #13
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So I should start off with measuring voltage. Then resistance? What about measuring capacitors, inductors, diodes, etc? And is there an instance I should use the analog meter instead of the multimeter? When should I use the oscilloscope?
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Old 10-19-2016, 10:49 PM   #14
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Also, someone mentioned isolating connections. I'm unable to open the box or do anything to the circuit. Is there a way to isolate a circuit with just the meter/scope?
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Old 10-20-2016, 12:07 AM   #15
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So I should start off with measuring voltage. Then resistance? What about measuring capacitors, inductors, diodes, etc? And is there an instance I should use the analog meter instead of the multimeter? When should I use the oscilloscope?
1- CAPACITORS test with an ohm meter,the reading should start low then increase slowly.
2 - DIODE check for forward voltage drop, should be around .5 to .6 v
many meters have a diode check function.
3 - INDUCTORS -use the osciloscope and it's calibration output.

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