How I can make a Milli ohm meter???

[ikelectro]

I want to make an milli ohm meter which can measure the low ohms like under 1 or 2 ohm with 1000 resolution... is that possible to make as a electronics projects??? or is there any other equation to measure the resistance other than R=V/I with more simple!!!!

If you guys help me here It'll be great for me..

 

[tcmtech]

The most basic concept is to just make a good stable constant current source that will give you a equivalent voltage drop relative to the resistances involved.

The down side is that going into accurate milliohm ranges with a reasonable repeatability requires a fairly high amp constant current source.

In the case of milli ohm measurements you may need a constant current source that has to run at 10 to 20 amps or more which will make the design some what power hungry.

 

[ikelectro]

The most basic concept is to just make a good stable constant current source that will give you a equivalent voltage drop relative to the resistances involved.

The down side is that going into accurate milliohm ranges with a reasonable repeatability requires a fairly high amp constant current source.

In the case of milli ohm measurements you may need a constant current source that has to run at 10 to 20 amps or more which will make the design some what power hungry.

Is that mean, if I'm going to make a micro-ohm meter I have to make sure about more current source???? like more than 20Amp??!!!!

 

[Mike odom]

If you run that much power through a resistor you will change the resistance due to heating. Another method would be to create an r/c oscillator, and measure the frequency produced. You need a stable low leakage capacitor, but it's value can be calculated using a high precision known resistance source.

Another method would be to use the resistor under test in the gain circuit of an opamp, again calibrating the amp with a known precision resistor.

A third method would be to use the resistor under test *IN* the current source, sourcing this current across a highly precise known resistance and measuring the voltage drop across this resistor.

 

[ikelectro]

A third method would be to use the resistor under test *IN* the current source, sourcing this current across a highly precise known resistance and measuring the voltage drop across this resistor.

can you please elaborate it more.. for my better understanding.....

 

[Mike odom]

here is the schematic of a current source. make R1 the resistor under test. Supply a voltage to P1. Connect your precision resistor from P2 to ground. Measure voltage across precision resistor. See LM234 data sheet for setting up your current source for values of R1/R2.

 

[tcmtech]

If you run that much power through a resistor you will change the resistance due to heating. Another method would be to create an r/c oscillator, and measure the frequency produced. You need a stable low leakage capacitor, but it's value can be calculated using a high precision known resistance source.

Milli ohm resistors are typically only used on high current applications where even 20 amps is a negligible current to work with. A piece of 10 gauge copper wire one foot long is approximately 1 Milli ohm and at a 20 amp current would exibit a mear .02 volt drop while disipating only .4 watts which would exibit an unpercetable amount of heating effects.

Second how do you propose to make a RC oscillator that works with R values in the few thousandths (.001) of and ohm range?

Here is a good example of a high end unit. It tests at up to 35 amps. https://www.hypatia.com/307SpecsV3.2.html

 

[Mike odom]

https://www.nutsvolts.com/index.php?/blog/post/build_an_audible_milliohmmeter

 

[MrAl]

Hi,


To measure low resistance values often a bridge circuit is used where the ratio of two resistors allows you to calculate the lower value resistor.

But there is another way too. We are lucky here in that we can use DC current to measure resistance. That means we can use low bandwidth op amps that have very very low input offset. Using this op amp we can amplify the voltage across the voltage across the resistor ten or even 100 times, and this helps relieve the current requirement needed to measure low ohm values.
For example, for 10mOhms at 10 amps would produce 0.1 volts, which could easily be measured down to 1 mv, but 10 amps is still a little extreme. Using 1 amp we would only see 10mv, but amplifying this 10 times we're back up to 0.1 volts which again we can easily measure.
The trick is to pick an op amp with an auto adjusting input offset voltage. These are typically the chopper stabilized op amp types. You set the op amp up for a gain of 10 and you then calculate everything based on that.
If you want to shoot for a gain of 100 you could do that i guess. That would give you more output voltage vs what was actually being measured.

So for a reasonably simple circuit you could measure 10mOhms and get a reading of 0.100 volts, where 0.101 volts would mean 10.1mOhm..

A few things you have to be careful about are:
1. Not overheating the part being measured, even with 1 amp.
2. Taking the lead resistances into account, possibly by shorting them first and doing a 'tare' measurement first, then subtracting that from the final measurement.
3. Making sure the 1 amp (or whatever) current source is accurate, which means you'd have to calibrate.

 

[schmitt trigger]

There are many ways to skin a cat, and the posters above have all provided different methods. All are fine, and each has its set of advantages and drawbacks.

My two cents here is that you'll need a 4 wire -or Kelvin- connection to perform the test accurately.

 

[ikelectro]

I should have been more informative in this matter, what I'm actually want do is that >>> I want to measure the resistance of a track in a PCB or want to measure the resistance of a simple wire, like that.. this will be my aim for making a milli ohm meter.... so what do you think, how should I go....?

and of course all the reply are really helpful to me....

 

[fernando_g]

Yes go for it...this is a project which will allow you to learn and practice several electronic techniques.

The final accuracy will of course depend not only on your individual component's accuracy, but also how you lay out and construct the tester.

 

[steveB]

I used to use lock-in amplifiers for difficult optical system measurements, so I thought using AC signals, with a lock-in amp might be better than a DC measurement. This can work if inductance is low (as in the case of PCB traces and simple wires) and using very low frequency AC, which is no problem for a lock-in amp.

I did a quick search, and it looks like someone has tried this. See the following link.

http://cappels.org/dproj/dlmom/dlmom.html

I might have approached it a little differently than this person, using lower frequency and more analog than digital, but there is a wide range of implementations possible with this approach. A key benefit is that low currents can be used, which is crucial for PCB trace resistance measurements on a modern multilayer high density PCB. Those 8 mil traces can't take too much current, especially on the inner layers.

 

[ccurtis]

Two examples/ideas to share:

A simple instrument using current source/voltage measurement: **broken link removed**

A more elaborate method using "lock-in" amplifier, can measure microohms: **broken link removed** Although the title of the article suggests the use of software to operate, software is not necessary.

 

[ikelectro]

Two examples/ideas to share:

A simple instrument using current source/voltage measurement: **broken link removed**

in this circuit what is the purpose of P1 and P2.????? and what is their value means 10 and 100 stated as the symbol of variable resistor????

 

[ccurtis]

in this circuit what is the purpose of P1 and P2.????? and what is their value means 10 and 100 stated as the symbol of variable resistor????

Sorry, I don't see a P1 and P2, or 10 and 100. I think you are referring to post #6, for which mike odom has the privilege to answer to.

 

[steveB]

Sorry, I don't see a P1 and P2, or 10 and 100. I think you are referring to post #6, for which mike odom has the privilege to answer to.

Figure 1 from the article you referenced has two potentiometers P1 and P2 with values 100 and 10.

 

[ccurtis]

Figure 1 from the article you referenced has two potentiometers P1 and P2 with values 100 and 10.

Opps, yes, thank you.

in this circuit what is the purpose of P1 and P2.????? and what is their value means 10 and 100 stated as the symbol of variable resistor????

Hi. Those two pots adjust the constant current level for each of their respective current ranges; P2 (10 ohm pot) for the 100 mA current range, P1 (100 ohm pot) for the 10 mA current range. For lowest resistance values to be measured the higher current may be required to get a high enough voltage to measure. The pots calibrate the current level to a known value through a known resistance value before the intrument is ready to be used. The procedure is given in the text of the article.

 

[ronv]

You can get wire and pcb resistance from the internet. It might be easier and more accurate.

 

[steveB]

You can get wire and pcb resistance from the internet. It might be easier and more accurate.

But, what if you want to verify the integrity of a connection, or check for the location of a short? Then, you will want to check actual values against expected values. Expected values can be calculated or looked up on the internet, but actual values can't.