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Why do i have current on the voltage sensing wires of a Four-Terminal measurement ?

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SandiegoSD

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Getting higher R using Four-terminal sensing than with a Keithley mOhm-meter

Folks,
I've been experimenting with Four-terminal (Kevin) measurement to measure some low
resistance. I was trying to recreate what this guy did on a breadborad:

http://www.kerrywong.com/2011/08/14/accurate-milliohm-measurement/

Unfortunately in my tests the Device Under Test has a resistance value of under 1 milli ohm (while the 18 gauge copper wires are typically 10milli ohm per two inches roughly, which I use to connect the DUT to the breadboard)
I guess this is causing troubles for me.
On the two inner voltage sensing wires, I measure a voltage drop across the wire (which connects the DUT to the breadboard--I'm sticking the probes of my voltage meter onto the breadboard too )

Does anyone know whether this is because my DUT resistance is too low compared to the wires, that the Four-terminal sensing method just become not applicable for me?? or I'm missing something?
 
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I don't understand your problem. :confused: The current on the voltage sensing wires (to your voltmeter) carry only the voltmeter current which is tiny. You put the voltage probes directly across the DUT. The voltage drop across the wires to the DUT is unimportant since it is not measured. That is the purpose of the 4-wire measurement -- to avoid the error from any voltage drop due to the resistance of the wire going to the DUT.
 
I don't understand your problem. :confused: The current on the voltage sensing wires (to your voltmeter) carry only the voltmeter current which is tiny. You put the voltage probes directly across the DUT. The voltage drop across the wires to the DUT is unimportant since it is not measured. That is the purpose of the 4-wire measurement -- to avoid the error from any voltage drop due to the resistance of the wire going to the DUT.

GOod point.Ok.. i was measuring this because the measured value for the DUT is higher than what I got with a standalone Keithley instrument. So i was trying to see whether I'm picking up/accidentally measuring the wires or some other contact resistance other than that on the DUT.
 
For a DUT with 0.36 milli Ohm resistance(I know ,, it's really low),
when I run 500 mA across the current wires, the voltage I measure with the volt meter, at the ends of the voltage sensing wires, is .3 milli Volts, which indicates 0.6 milli Ohm resistance.


So I guess I should've change the title of the post to : why I'm getting a higher value with Four-terminal sensing than what I measured with a Keithley micro-ohmmeter
 
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Your link isn't good. You need to do a "Copy Link Location" from your browser, they ... work in the forum, but not copy and paste.

Take a look at the datasheet for a 4-terminal resistor: https://www.electro-tech-online.com/custompdfs/2012/11/vcs301.pdf . Only between E1 & E2 is the resistance known.

Resistance measurement at either the low extreme (milli-ohms) or the high extreme (GΩ) is quite difficult. I've spent a lot of time measuring in the GigOhm range and a lot of time doing 4-terminal measurements with voltages < 1 V and currents <1 mA

In any event, you want to measure the current through the resistor and the voltage across the resistor. There are always measurement difficulties. Voltmeters have a High Z and therefore even a 10 M ohm resistance in parallel with 1 ohm is negligible. Now we have accuracy problems.

The 4-terminal method is used a lot in current shunts which is a low value resistor. You might have a 50 mV burden at 100 Amps. It's used to remove the effects of contact resistance.

You should attempt the measurement with say a resistor around 0.5 ohms or get a known 4-terminal resistor.

I have no idea of your measuring setup and what your trying to measure.
 
Your link isn't good. You need to do a "Copy Link Location" from your browser, they ... work in the forum, but not copy and paste.

Take a look at the datasheet for a 4-terminal resistor:...

I have no idea of your measuring setup and what your trying to measure.

link fixed.
Unfortunately I can't change the resistance value. It's a connector pin that I'm trying to measure the contact resistance of., and I have no jurisdiction over its value.

So the accuracy problem... I measure the current from my current source, it's dead-on 500 mA. but the voltage is a bit off.
There gonna be a finite reason for that inaccuracy right? i mean it's either some other contact resistance I'm counting in, or maybe it's what happens to four-terminal setup when load resistance is smaller (to some degree) than the wires
 
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I don't understand your problem. :confused: The current on the voltage sensing wires (to your voltmeter) carry only the voltmeter current which is tiny. You put the voltage probes directly across the DUT. The voltage drop across the wires to the DUT is unimportant since it is not measured. That is the purpose of the 4-wire measurement -- to avoid the error from any voltage drop due to the resistance of the wire going to the DUT.

Yes, in theory there should be negligible currents on the voltge sensing wires. but what's odd is that when I use four longer wires to connect the same DUT to the breadboard and measure,
I get higher voltages, thus higher resistance. That's also a reason why Im thinking I'm picking up these extra resistance from the wires.
 
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**broken link removed**
I cropped out the four-terminal portion on the schematic. As shown, red paths are the current wires to the DUT.
Green paths are connected to the Positive/Negative input of an differential amplifier.

Now I might be making mistakes here. The question marks above: 1) I've disconnected the Negative input on the amp (-IN) from the GND (pin 3 and 4,etc), this seems to give me better results
2) Where should I connect the current wire that comes out of the DUT? It seems it's different depending on which GND i'm hooking it up to .the power supply gnd , or the sideline on the breadboard (which is eventually connected to power supply GND)
 
I think I see what you want to do. You want an actual 4-terminal meter whereas this one is bassically a two terminal device.

I think you want to do the following. Delete the horizontal wire going to pin #1(+IN). Take (+IN) and this will be what I would call the V+ terminal for the DUT. (I+) will be the top left terminal of the DUT as shown.

Move the ground symbol about 1/2" to the right on the schematic that is near the Vout label. Remove the resulting horizontal line. So that 2, 3 and 4 becomes -IN.

At the DUT. Relabel the left ground as I-. At the DUT V+ and I+ are connected together and I- and V- are connected together.

As an aside, 10 mA of current is sometimes not enough wetting current. It would be more advisable to measure the contact resistance at the currents encountered. For reverence, you would not use a 10 A relay to switch a 1 mA current.

See if there is a minimum current suggested for the pins of the connector you are using.

The output reference ground would probably be better taken from the new pins 2,3, and 4.

I hate drawing. It's like work.
 
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Yes, it sounds like you have a sneak ground path. You can't have the input of the voltage measurement circuit connected to ground of the DUT circuit. If you do, some of the DUT current can go through the sense leads and cause a false reading as you are seeing.
 
I think I see what you want to do. You want an actual 4-terminal meter whereas this one is bassically a two terminal device.

I think you want to do the following. Delete the horizontal wire going to pin #1(+IN). Take (+IN) and this will be what I would call the V+ terminal for the DUT. (I+) will be the top left terminal of the DUT as shown.

Move the ground symbol about 1/2" to the right on the schematic that is near the Vout label. Remove the resulting horizontal line. So that 2, 3 and 4 becomes -IN.

At the DUT. Relabel the left ground as I-. At the DUT V+ and I+ are connected together and I- and V- are connected together.

As an aside, 10 mA of current is sometimes not enough wetting current. It would be more advisable to measure the contact resistance at the currents encountered. For reverence, you would not use a 10 A relay to switch a 1 mA current.

See if there is a minimum current suggested for the pins of the connector you are using.

The output reference ground would probably be better taken from the new pins 2,3, and 4.

I hate drawing. It's like work.
just want to clarify this: that Vout on Pin 6 of the the LMP8358 is not part of the 4-terminal sensing. Vout is the amplified value of +IN (the LMP8358 provides a programmable gain of say 100 to the signal at +IN)
The current i'm generating is 500mA so i think it's fine.

Yes. I separate the GND of the voltage meter from the current GND of the DUT. The value is better than before , but still slightly larger. For the 0.4mOhm, with 500mA current across the DUT, the voltage drop is 0.3mV, which converts to 0.6mOhm
 
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the OP said:
just want to clarify this: that Vout on Pin 6 of the the LMP8358 is not part of the 4-terminal sensing. Vout is the amplified value of +IN (the LMP8358 provides a programmable gain of say 100 to the signal at +IN)

Almost. The LMP8358 amplifies the difference between -IN and +IN. e.g. ( {+IN} - {-IN} ) * 100. the things in the {} are labels and not not to be mathematically operated on. Hence the term differential amplifier.

Pin6 and ground just needs to be connected to a voltmeter.

With a lower programmable gain of the LMP8358, you can increase the ohms measured. A voltmeter across the DUT can also help, but you just have to do the math manually. R = V/Iset

This thing isn't true 4-terminal as built. The current source and the voltage measuring has to be broken up a little. This "basically" means that -IN and +IN need to go to the DUT.

I should probably look at the datasheet. I'll look at it later: https://www.electro-tech-online.com/custompdfs/2012/11/lmp8358.pdf
 
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The LMP8358 has an output reference too. This is common on a differential amplifier.

RefF or (Reference Force) and RefS (or Reference Sense) is what you want the output referenced too.

So, RefF can be put at power supply ground and RefS can be placed at the actual ground terminal that feeds the meter.

So (+IN minus -IN = Vout minus RefS; RefS~=RefF

That's how the amp works. It's a kinda neat amp.
 
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The LMP8358 has an output reference too. This is common on a differential amplifier.

That's how the amp works. It's a kinda neat amp.

Here's the connector pins I'm using (they're industrial connectors so they are kinda big and, low resistance); the two pieces shall plug together
**broken link removed**
I'm using white wires for current and blue for voltage.

The discrepancy I'm been complaining about so far is between the 0.4 mΩ I got by clipping the the aligator clips(two wires of each clip are connected to the current source and volt meter respectively) of a Keithley current-source&voltagemeter combo (using 4-wire measurement setup )directly to the big metal ends on this connecor, and the 0.6/0.7mΩ I got by using the four wires(with the same Keithley current-source&voltagemeter combo)

So I'm thinking that maybe I shouldn't have soldered the white(current) and blue(voltage sensing) wires together on the connector ends?
Illustrations about 4-terminal sensing I've seen mostly draw volt-sensing wires on the inner side. but I guess they're eventually connected together, like this resistor
**broken link removed**

What you think? shall I seprate the soldered wires?
 
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What Keithley meter are you using?

When soldered, your also measuring the soldered connection resistance as well.

I'm not sure of your application, even with the connector, but for simplicities sake. If there is a PAD on the DUT, then there needs to be two probes to that pad on the DUT.

I spent a LOT of time measuring solar cells professionally for research.

FWIW: Soldering these pins would be difficult. I your going to do a lot of these connections, then you should be using resistance soldering. See: **broken link removed**

I've used American Beauty stations in the past, but I got better tip lifetime by making them. I used Stainless welding rod instead of the supplied tips.
 
Keithley should have a low level measurements book on their site. I'll have to look for you. I'll also look at the meter datasheets. Sometimes you have to worry about inadvertent cross-connections to ground by the measurement instruments.

I could easily explain why it's necessary with a chalkboard.

Here is a lot more Kelvin type clips than I'm used to: **broken link removed** The alligator versions of those clips have wires to each side of the jaw.
 
What were the values of voltages and currents for both measurements?

The soldered connections do contain two dissimilar metal junctions. wire to solder and solder to pin

Clipping the leads to the pin don't.
 
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