how should you design milliohm meter using dvm display

[Nigel Goodwin]

How about posting actual schematics of what you're trying to use?, so far there's been nothing but partial diagrams and vague partial simulation diagrams - you need to post EXACTLY what you're constructing.

 

[Tony Stewart]

Ralph's last description sounds like he was trying ZZO's LDO solution.
A 3 terminal current source is the easiest solution. namely the LM317ADJ

TO-220 devices can be used in a breadboard by soldering the pins to short resistor wires or a DIP header.

LDO's are thermally protected and useful for current sources and will not dissipate much heat if you limit the excess voltage drop from the battery.

Resolution:

Resolution on R increases with current and tracing shorts is a transient condition so it won't drain the battery long. So you could use 100 mA from a Lithium Ion cell or only use 1 mA and a x100 amplifier, then you have 2 Ohm full scale for 200 mV.

Heat:
100 mA drop from 3.6V cell = 360 mW. The current sense resistor will have 1.25V across Vo-ADJ as it always has in regulation for this style of regulator. thus 1/8W is fine for a 1/4W resistor. To achieve this use 12.5 Ohms = 1.25V/100 mA

This thread will be solved when Ralph defines all the assumptions we do not know. Design Specs: have Inclusions , exclusions, "must use" components (if nec.) and a real-purpose to satisfy this effort.

If something does not work, take measurements with a DMM of everything and report that.

 

[Flashdrive34]

Hi Tony Stewart:

I have unbranded LM317L T092 100ma regualtor can I use that in place of LM317 To220? I like this because it fits in my bread board. And to Nigel Goodwin that was Zipzapouchs schemtic that I tryed to use not my design I hope that clears up question on this problem. Tony thanks for your comments Ralph

 

[Tony Stewart]

If you use the battery I specified ok.
If you used the 9V that ZipZapOuch specified with a TO-92, not OK due to Pd max * Rja but if using Op AMps , they must be rated for low voltage CMOS. that are Rail-2-Rail in and out.

 

[ZipZapOuch]

With the LM317L T092, you need a minimum of 1.5 to 2.5 mA output to maintain regulation. The advantage of the TL7805 regulator is that it can regulate down to 0mA output.

 

[Nigel Goodwin]

Hi Tony Stewart:

I have unbranded LM317L T092 100ma regualtor can I use that in place of LM317 To220? I like this because it fits in my bread board. And to Nigel Goodwin that was Zipzapouchs schemtic that I tryed to use not my design I hope that clears up question on this problem. Tony thanks for your comments Ralph

So still no schematic, the schematic was only a partial one for simulation, not a fully working design - did you add the required capacitors round the regulators, as he mentioned?.

 

[Tony Stewart]

With the LM317L T092, you need a minimum of 1.5 to 2.5 mA output to maintain regulation. The advantage of the TL7805 regulator is that it can regulate down to 0mA output.

I hope everyone realized my design meets the minimum current load with the max 100mA CC limit and voltage regulation is not needed for no load. Also maximum thermal limit is best suited to minimum voltage with 3.6V Li ion or perhaps a 3V Li cell for 200mV full scale. https://www.electro-tech-online.com...m-meter-using-dvm-display.165639/post-1447978 This gives the necessary resolution with simplicity with no amplification, unless minimal power drain was specified.

 

[Flashdrive34]

Hi Tony Stewart and all members:


Tonys last schematic works perfectly and I will include snap shots of display. I am using 9v battery to 78l05 regulator to power 200mv display. For current geneator using lm317 I am using 2 AA batteries and generator appears to working correctly. As 3v supply. These are readings with no geneator all 000 on display. With geneator turn on 1 and decimal point. Overrange. With shorted 5.6 on display and measuring 1 ohm resistor reads 119.4 . 5.6- 119.4 =113.8 milli ohm. Am reading the milli ohm value correctly? Thanks for help. I don't know why could not get the op amp to work as geneator? Now I have to make the 3 digit dvms mcu work in place of the 200mv meter. Ralph

 

[Flashdrive34]

Hi Tony and all members:


If I measure 1 ohm then I should see 1000 m ohm on digital display. Which I do not I see 1138. Which means my error is 138 m ohm error reading for 1 ohm. I ordered kelvin clips this may help so will see when they arrive. I need trim my resistor value to exactly 12.5 ohms . Right now I have 12 ohms. Also the decimal point is not correct so I will not use it when reading milli ohm reading.

 

[ZipZapOuch]

Hi Tony and all members:


If I measure 1 ohm then I should see 1000 m ohm on digital display. Which I do not I see 1138. Which means my error is 138 m ohm error reading for 1 ohm. I ordered kelvin clips this may help so will see when they arrive. I need trim my resistor value to exactly 12.5 ohms . Right now I have 12 ohms. Also the decimal point is not correct so I will not use it when reading milli ohm reading.

Try measuring your constant current output with your ammeter. Current should be constant even with poor clips.

 

[Flashdrive34]

Hi Zipzapouch:


I measured 91.1 ma constant current rock solid. Its not 100ma but it close. What would you like me to try next? Ralph

 

[Nigel Goodwin]

Hi Zipzapouch:


I measured 91.1 ma constant current rock solid. Its not 100ma but it close. What would you like me to try next? Ralph

91.1 isn't 'close' to 100 - it's no where near. First job, get your constant current set correctly - EVERYTHING depends on the constant current been accurate.

 

[Tony Stewart]

Good for measuring shorts which are 0 Ohms +/-100% (LOL)

you are out by approx 10% which is due to resistor tolerances.

Keep in mind your DMM may have errors. The LM317 is very stable but the Vref has a range. 1.2min 1.25 nominal 1.3 Vmax guaranteed.


Ticky Tacky details

So 12.5 Ohms not ideal , just nominal. So if 91.1 mA * 12 Ohms = 1.1V which means your DMM is less accurate than the LM317 spec.

You can tweak that by adding another R that is ~10x higher value in parallel with the current sense Rs. Adjust to taste.

1.2 to 1.4 / R= I so reducing R increases I . So don't look for a 12.5 Ohm resistor. I hope your DMM reads < 0.1 Ohms with probes shorted otherwise subtract the result from the R reading.

Sorry about not telling you about tolerances.
3V is the minimum voltage, so a Li Ion battery which I recommended is better than two AA cells. That means the current will drop as the batteries drop below 3V. OK? Also you should have gotten the 1A IC not the 100 mA. Now I think you current errors are due to the choice of batteries and not the resistor which should be closer to 12.5 You can add and subtract R's but the battery MUST BE 3V min.

If you needed 1% accuracy, It would be a different design.
Banana plugs get oxidized and may need WD40 to prevent poor contact after a while.

 

[Flashdrive34]

Hi to all members:

Well circuit is working, but can't I use 78L05 for the Lm317 current source instead of the 3.6v li rechargeable battery. Using chargeable 9v battery 280ma to power 78L05 regulator instead of tonys chargeable li battery. Using 2 78L05 regulators one for current source and one for the display. I will see how well this works? The 3v AA do not work well.