### Author Topic: EEVblog 1472 - Resistor Cube Problem SOLVED  (Read 5028 times)

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#### EEVblog

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##### EEVblog 1472 - Resistor Cube Problem SOLVED
« on: May 04, 2022, 09:38:19 pm »
How to solve the resistor cube problem using equipotential nodes, short and open techniques, and circuit simplification.

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#### ziq8tsi

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##### Re: EEVblog 1472 - Resistor Cube Problem SOLVED
« Reply #1 on: May 04, 2022, 11:44:12 pm »
0:36 "they happen to be 10K here, point 0, 5%"

are they not 33R, 1%?

#### hulk69

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##### Re: EEVblog 1472 - Resistor Cube Problem SOLVED
« Reply #2 on: May 05, 2022, 10:20:58 am »
This exercise got me looking for the Weathstone bridge, but the more I look at it the more I cannot find any use for it nowdays.

Any op-amp circuit like a non inverter would do a better job...

As I see it the Weathstone bridge is just a good exercise for beginners to learn the Kirchhoff law

#### Kleinstein

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##### Re: EEVblog 1472 - Resistor Cube Problem SOLVED
« Reply #3 on: May 05, 2022, 10:55:45 am »
The Weathstone bridge circuit is still relevant for resistive sensors like strain gages and sometimes still used with PT100 and the like. The highest resolution resistance measurements still used a bridge.

#### hulk69

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##### Re: EEVblog 1472 - Resistor Cube Problem SOLVED
« Reply #4 on: May 05, 2022, 11:16:20 am »
I lookup "strain gages" and it seems that the gage itself is a bridge

#### tggzzz

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##### Re: EEVblog 1472 - Resistor Cube Problem SOLVED
« Reply #5 on: May 05, 2022, 11:32:15 am »
This exercise got me looking for the Weathstone bridge, but the more I look at it the more I cannot find any use for it nowdays.

Any op-amp circuit like a non inverter would do a better job...

As I see it the Weathstone bridge is just a good exercise for beginners to learn the Kirchhoff law

Too simplistic, as others have pointed out.

Bridges allow important types of "errors" in sensors to be avoided. Of particular interest are imprecise values, drift over time, and drift with temperature. Simple opamp circuits can only amplify "errors"; complex opamp circuits might enable compensation for the "errors".
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#### StillTrying

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##### Re: EEVblog 1472 - Resistor Cube Problem SOLVED
« Reply #6 on: May 05, 2022, 01:46:05 pm »
This is how I'd solve it.
.  That took much longer than I thought it would.

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#### free_electron

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##### Re: EEVblog 1472 - Resistor Cube Problem SOLVED
« Reply #7 on: May 05, 2022, 01:58:52 pm »
This is how I'd solve it.
it is exactly this kind of braincrackers that spice was invented for !

now do it for a set of resistors that is 1, 1.1 1.2 1.3 1.4 and so on in different permutations. and try also opposite corners fo the cube...

These kind if school puzzles are nothing but timepass. you might as well teach how to solve sudokus . Nobody will ask you in  the industry to solve the "resistor cube."
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#### HighVoltage

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##### Re: EEVblog 1472 - Resistor Cube Problem SOLVED
« Reply #8 on: May 05, 2022, 02:21:03 pm »
The Weathstone bridge circuit is still relevant for resistive sensors like strain gages and sometimes still used with PT100 and the like. The highest resolution resistance measurements still used a bridge.
Agree!
Almost all automotive mass air flow sensors for combustion engines use a Wheatstone-Bridge circuit with a PT20 as a measurement resistor.
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#### erosenberg

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##### Re: EEVblog 1472 - Resistor Cube Problem SOLVED
« Reply #9 on: May 05, 2022, 10:54:39 pm »
Nice method of simplification!
What is the drawing software being used here? Looks like it is helpful for teaching and presentations.

#### EEVblog

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##### Re: EEVblog 1472 - Resistor Cube Problem SOLVED
« Reply #10 on: May 06, 2022, 07:31:29 am »
Nice method of simplification!
What is the drawing software being used here? Looks like it is helpful for teaching and presentations.

Drawboard PDF, aussie company, originally designed for marking up architecture drawing. i.e. make changes and save as a PDF.

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#### armandine2

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##### Re: EEVblog 1472 - Resistor Cube Problem SOLVED
« Reply #11 on: May 09, 2022, 12:22:35 pm »
This exercise got me looking for the Weathstone bridge, but the more I look at it the more I cannot find any use for it nowdays.

Any op-amp circuit like a non inverter would do a better job...

As I see it the Weathstone bridge is just a good exercise for beginners to learn the Kirchhoff law

still in use? definitely still taught
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#### twospoons

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##### Re: EEVblog 1472 - Resistor Cube Problem SOLVED
« Reply #12 on: May 10, 2022, 03:56:42 am »

now do it for a set of resistors that is 1, 1.1 1.2 1.3 1.4 and so on in different permutations. and try also opposite corners fo the cube...

The resistance between opposite corners of the cube (A and G in the video) is even easier to solve. By inspection you can see that points B,D,E must be the same voltage, and points C,F and H must be the same voltage ( the joys of symmetry). So the three resistors from point A may be considered to be in parallel, and the three resistors from point G may be considered to be in parallel. The remaining 6 resistors must also have identical voltage drops since they all connect from the B,D,E point group to the C,F,H point group so they may be considered in parallel too.

Therefore RAG = R/3+R/6+R/3 = 5/6 R

#### Vtile

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##### Re: EEVblog 1472 - Resistor Cube Problem SOLVED
« Reply #13 on: June 23, 2022, 04:22:27 pm »
This exercise got me looking for the Weathstone bridge, but the more I look at it the more I cannot find any use for it nowdays.

Any op-amp circuit like a non inverter would do a better job...

As I see it the Weathstone bridge is just a good exercise for beginners to learn the Kirchhoff law
Bridges are important still today for high precision instrumentation.

The beayty of bridge circuit is that is provides integrated high stability voltage reference (assuming one unknown) and passive (pseudo) amplifier.

It also still can be made to be totally passive so no interference from electronic amplifier and it is also considered instrically safe for EX-spaces ie. with Pt100.

It definedly is not a mainsteam, but it have never been mainstream.

It is one of those rare circuits which can be used to push the limit further than the used components by iteration.

Ps. The common "equation" floating around is oversimplification when effect of sensing device is assumed to be zero ie. infitely high resistance over the bridge halfs and equilibrium state.
« Last Edit: June 23, 2022, 04:25:01 pm by Vtile »

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