Resistance measurement circuit

[AriS85]

Hello,

I'm not sure if there is a practical solution to this problem, but maybe someone can can help me with this.

I'm using a 200 ohm trimmer to test a certain device. It's not relevant what the device is, but basically I need a variable resistance and I need to be able to measure or check the resistance at a certain point. I bought a 0-200 ohm integrated panel ohmmeter from ebay and I use a 9V battery to power it.

I could use a switch to check the value of the resistance. When the switch is pressed, it disconnects the external device and connects the ohmmeter. Like in the following picture:

But I would really like to make the circuit so, that the ohmmeter shows the resistance constantly. So that I don't need to press any buttons.
Is there a way to do this? I've also measured the resistance from points 2 and 3, but then I have to subtract the result from the total (200ohm) resistance to get the resistance the device sees. I wouldn't like to do that either. Is it possible to measure the resistance from points 2 and 3 and make the ohmmeter show the opposite resistance?

Thanks for anyone who is willing to help

 

[Reloadron]

You are placing an external resistance across a "device" and likely adjusting that external 200 ohm pot until the device does something. At that point you measure the resistance that the device did something at. My guess is that whatever the external device is that device is passing a current through the external resistance. Should that be true rather than an ohmmeter you could use a voltmeter and measure the voltage drop across the external resistance as you adjust the pot. Your circuit example does not reflect a battery buy you mention using a battery? Looking at your drawing the external resistance is either in series or parallel with something in the device. If this is some sort of a go or no go test then what you are doing is the only method without knowing much more information.

Ron

 

[alec_t]

It's not relevant what the device is

If the device affects any current through the trimmer then it is relevant, since an attached Ohmmeter will give a misleading reading for the trimmer resistance.

 

[Reloadron]

If the device affects any current through the trimmer then it is relevant, since an attached Ohmmeter will give a misleading reading for the trimmer resistance.

Alec, I agree. It looks like, from his drawing, he adjust the pot till the device does something (whatever it does) and then the device is disconnected from the pot and he measures the pot's resistance that was applied to the device. Regardless, I really dislike the magical mystery tour.

Ron

 

[genIQ]

Hi can you tell me more about this 200 ohm panel meter..it's impedance or what it looks like to your circuit( load )

 

[MikeMl]

Why not just measure the voltage across the trimmer in real time with no switches?

 

[KeepItSimpleStupid]

This, sort of, reminds me of ways to check moving coil meter resistances or input Z of an amplifier.

But, if the process warrents a programmable electronic load can output a programmable resistance. DVM's with programmable interfaces can can surely do what you need done with a controller of sorts.

You could also do a binary weighted ladder and some sort of computer. e.g. 256+128+64+...+2+1

So, you can end up with a 1 ohm resolution. You just binary search for the value and it takes very few trials to zero onto the value.

 

[Tony Stewart]

External device will be some source of current or voltage. Once you measure voltage across 1& 3 and 1&2 it with the Pot then the Pot resistance , you no longer need to measure the Pot. R

Either R=V/I where either I is constant or V is constant

If the above is not true, then it is not simple and another equation is needed.

So as you indicated you simply measure the unused R (2&3) and mentally subtract from 200

more precise display of position or R requires full disclosure of the object properties.

 

[ChrisP58]

Do you really need to know the actual resistance, or do you just need to know the relative position of the rheostat?

If the latter, then there's probably an easier way to get that info. Most circuits that use rheostats as controls or position sensors apply a current to the variable resistance, and measure the variation in voltage across the device. In many cases the current source is just a fixed resistor to a voltage rail.

I recommend a test. With the system in operation, put a voltmeter across the variable resistance and move it end to end, noting the voltage along the way. Can you use the voltage reading to give you the info you need? If so, you're done.


Note: This assumes that it's a safe, low voltage, and that the measurements can be made without exposing the technician (you) to bodily harm when making them.

Expanding upon MikeMI's comment.

 

[AriS85]

The mysterious "external device" is a device that measures skin impedance. As I found out few days ago, it does this using high frequency ~80KHz signal with less than 10mA of current. I do not know how constant the voltage or current is when the load changes.

This test box would be used to test that the device alerts when resistance gets very low (almost short) ~5 ohm and when resistance gets too high, 135 ohms. This is how it's supposed to be tested, with a potentiometer. I am just wondering if a potentiometer is fully resistive at 80KHz? But as I said, this is how it's supposed to be tested

If the device works at higher frequencies and the ohmmeter at DC, could I filter these two signals with a large enough cap series with the measuring device and an inductor series with the ohmmeter and make them work at the same time?

I bought a 0-200ohm panel ohmmeter from ebay. It hasn't arrived yet and the spec sheet doesn't cover much. It does have a pot for zero adjustment.

 

[alec_t]

I've never known skin resistance to be that low. Presumably the distance between probes used for the measurement is extremely small? If the 80kHz signal drives 10mA through ~100Ω, that implies a drive voltage ~1VAC. I suspect that is going to upset your Ohmmeter if you intend connecting that to the working skin tester. Also, the 9V supply for the Ohmmeter may well upset the skin tester. I think you will have to settle for using the Ohmmeter to check the pot setting only when the pot is disconnected from the skin tester. Disconnection would be necessary when the skin tester is in normal use anyway.

 

[AriS85]

I've never known skin resistance to be that low. Presumably the distance between probes used for the measurement is extremely small?

Yes the electrodes are close to each other.

I think you will have to settle for using the Ohmmeter to check the pot setting only when the pot is disconnected from the skin tester.

It seems so. Thanks for help anyway

 

[Tony Stewart]

If you define the target geometry and material, and function, an accurate assessment can be made.

Skin effects in coax, is measured in something called Transfer Impedance, which has a bandwidth from hundreds of MHz to many GHz that depends on geometric error of conductor diameter ratios , effective braid coverage and loss tangent of insulator.

It would be inductive if geometry is axial and controlled impedance if coaxial.

Testing is trivial once this product is defined. Metcal for instance use skin effect for surface controlled Peltier heat loss to regulate soldering temp.

 

[JimB]

Reading through this thread I am reminded of this:

I cannot help but think that AriS85 was referring to biological stuff, you know the wrapping of the body which stops you bleeding all over everything you touch, rather the skin effect which affects the apparent conductivity of things like copper wire at radio frequencies.

Please feel free to correct me if I am wrong.

JimB

 

[Tony Stewart]

By jove Jim I think you've got it.

Skin being a less salty dielectric than body fluids if like a weak capacitor with lossy electrodes. The other effect is galvanic skin response from metallic electrodes which interferes with DC impedance measurements and EEG/ECG so higher frequency is used. To measure the ESR of the capacitance, one can ignore the DC resistance and use a constant current source from any oscillator to measure voltage proportional to resistance.

Can you find a CC sine or square wave source and precision OA peak detector circuit?

I knew how to test my my finger tips with an LCR meter I have so it is calibrated in kOhms and pF !! based on pressure and area. Which is very useful for debugging circuits with your fingers. From dry to wet Squeezed, I can get 10M to 10k and 1pF to 1000pF. Sweat is a contributing factor.

For ESD sandals or shoes, all one needs is leather soles or a feedthru conductor to footpad with 1M in series.