how should you design milliohm meter using dvm display

Nigel Goodwin said:

No, I've never suggested transformers - Michael used them, but the point is you need separate and unconnected supplies - two batteries would work just as well, or some kind of isolating inverter from one battery, to provide a separate isolated supply.

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You told the OP to use the design your friend posted (which uses a center-tapped transformer), then you said to use the schematic shown in the magazine that your friend referenced which uses batteries and two precision virtual ground chips since the ancient opamp used in the design is a TL071 since that opamp cannot have inputs at the same voltage as its negative supply so virtual ground must be used and the circuit shown goes over the top to create a virtual ground by using that chip. Then you said you have no idea what a virtual ground chip is and you have not offered an alternative design for our non-expert OP. Read the schematic and you'll hopefully be able to see what a virtual ground does. The video's creator never shows the "improved" schematic and neither have you.

I think you need to understand the video you recommend and listen to the constraints the OP gives.

ZipZapOuch said:

You told the OP to use the design your friend posted (which uses a center-tapped transformer), then you said to use the schematic shown in the magazine that your friend referenced which uses batteries and two precision virtual ground chips since the ancient opamp used in the design is a TL071 since that opamp cannot have inputs at the same voltage as its negative supply so virtual ground must be used and the circuit shown goes over the top to create a virtual ground by using that chip. Then you said you have no idea what a virtual ground chip is and you have not offered an alternative design for our non-expert OP. Read the schematic and you'll hopefully be able to see what a virtual ground does. The video's creator never shows the "improved" schematic and neither have you.

I think you need to understand the video you recommend and listen to the constraints the OP gives.

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You're still rambling on about 'virtual grounds' when no such thing is used, have you been taken off your medication?. The precision voltage source is used as a precision reference for the comparator in the constant current generator, and there's no need for a 'virtual earth' or indeed the opamp to approach the supply rail.

He never shows a schematic because there's nothing to show - it's already there, his only change is a simple dual power supply.

Nigel Goodwin said:

You're still rambling on about 'virtual grounds' when no such thing is used,

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I'm sorry, my eyes aren't working well. Did I say, virtual ground? I meant virtual ground.

Nigel Goodwin said:

You're still rambling on about 'virtual grounds' when no such thing is used,

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Yes.

Either recommend a video that has a schematic or magazine that uses reasonable components to our beginner OP. Why would you recommend such a bad design with unnecessary components.

ZipZapOuch said:

I'm sorry, my eyes aren't working well. Did I say, virtual ground? I meant virtual ground.

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Yawn - and still not used as a virtual ground - see my previous post, or just look at the circuit you just posted!.

ZipZapOuch said:

Either recommend a video that has a schematic or magazine that uses reasonable components to our beginner OP. Why would you recommend such a bad design with unnecessary components.

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The video explains the issues with using a DVM module, and how to over come them, pretty simple if you ask me?.

Nigel Goodwin said:

Yawn - and still not used as a virtual ground - see my previous post, or just look at the circuit you just posted!.



The video explains the issues with using a DVM module, and how to over come them, pretty simple if you ask me?.

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And yet, the beginner level OP doesn't understand it. Go figure.

Flashdrive34 said:

HI TO ALL MEMBERS:


I was working on milliohm tester and needed a 200mv meter. So I purchased one of these 3 digit digital voltmeters and wanted to change it to 200mv meter or 300mv meter. Which ever would work to get it to 200mv or 300mv meter. With one meter I purchased it is 3 wire red is vcc black vdd and yellow is measure. You read from 0 to 30v full scale. Beyond 30v will destroy dvm. I also ordered 200mv meter from china which should be here in a month or less. I ordered few more dvm from ebay and case I could make work for my tester. It seems like these meters either use a 14 pin ic chip as adc or mcu for this meter. Does any one know how I can convert this meter to 200mv or 300mv full scale? Parameter

Dimensions(L * W * H): 30 * 11.7 * 9.2 mm
Opening Size: 23 * 10 mm
Display Type: 0.28" Red LED Tube
Measurement Range: 0-100V DC
Measurement Rate: ≥200ms/time
Power supply: DC 3-30V
Its a nice little meter thats why I would like to use it for my tester. Thanks for any help you can give me. Since I am 75 years old now guess I should give up on making testers. I wanted see if could use it to find dead shorts on circuit boards. Thanks for any help guys.

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Although with some great effort for you to build such a conversion , I recommend a simpler solution using a low cost lab supply now about $50 with digital control of CV CC and displays V I and Pwr on 3 displays. Shopping from Banggood or similar will locate this .

Then set the voltage to say 0.100 V and CC to 0.100 you can measure with a resolution of 1 mohm with adequate probes. The 0 ohm reference would be shorting the probes and then it would display the voltage at 0.100 A and power less than 0.01 watt which is excellent.

These tend to be rated for 30 V or >1 A and some power limit and are very useful for many other uses from charging Li Ion batteries to powering LEDs etc

others may press like if they agree

Tony Stewart said:

Although with some great effort for you to build such a conversion , I recommend a simpler solution using a low cost lab supply now about $50 with digital control of CV CC and displays V I and Pwr on 3 displays. Shopping from Banggood or similar will locate this .

Then set the voltage to say 0.100 V and CC to 0.100 you can measure with a resolution of 1 mohm with adequate probes. The 0 ohm reference would be shorting the probes and then it would display the voltage at 0.100 A and power less than 0.01 watt which is excellent.

These tend to be rated for 30 V or >1 A and some power limit and are very useful for many other uses from charging Li Ion batteries to powering LEDs etc

others may press like if they agree

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Or you could just buy a low-ohm meter from AliExpress.

Which is what I did

Something like this one:

But with the PSU method I recommended you can raise the current to 2.5 A at 0.1V and dump 0.25W or more V if you want to fuse open a micro/sliver short but risks unknown to cct.

Hi all members:

zipzapouch give me schematic for 2 opamp circuits, but I can use 9v battery he shows in his circuit because my mcp6021 can only use 5.5v max supply per data notes power supply range 2.5v to 5.5v max. If I use 9v battery in schematic it should destroy my opamp? Also what should I see with load 33 ohm resistor on 200mv display? All I see is 1 on display no resistance. Using DVM it only shows voltage.If I use one of my dvm microcontrller units. I pick up 10 these little dvm display and can not even use them for this project a bummer. Also I can find any supplyer that sell 200mv that small. If I short pin 5 to pin3 it reads 17.6 mv or ohms I do not know what stands for?

I hope you have two or more of the 5v regulators...


On the regulator that I added, look up the datasheet and add capacitors as recommended. The battery supply should be very stable without them considering the battery source and the stable circuit.

Nigel Goodwin said:

You're still rambling on about 'virtual grounds' when no such thing is used, have you been taken off your medication?. The precision voltage source is used as a precision reference for the comparator in the constant current generator, and there's no need for a 'virtual earth' or indeed the opamp to approach the supply rail.

He never shows a schematic because there's nothing to show - it's already there, his only change is a simple dual power supply.

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Nigel Goodwin This is the 1st time I have seen this TLE2425 "precision virtual ground" IC , the one used in your friend's video. It is simply a very accurate 2.5V reference buffered to provide a precise 5V/2 ground reference for a single supply amplifier required for precise 0 to 5V full scale ADC measurements where 2.5V becomes "Ground" = 0V measured.

"Ground" always means a 0V reference for anything even for PE protective Earth, floating batteries and circuit measurements for 0V.

"Virtual Ground" is a very common definition in electronics and means the "null" reference can be shifted to any voltage defined as the "reference" which is always the positive input to an Op Amp (OA) while the inverting input tracks the VIn+ using high gain negative feedback to create a null differential input , the "virtual null". This is always used when converting bipolar measurements for a single supply. Nigel Goodwin might have known this if he had remembered his formal training in the 60's or 70's but was not commonly used by skilled repair techs.

As ZipZapOuch said the "precision virtual ground" IC in the video is not necessary with a rail to rail (R2R) Op Amps (OA) and all R2R OA's are CMOS yet not all CMOS OA's are R2R or limited to a 5V supply.

This 200mV meter operates from 30 down to 3V which means it uses a precision regulator inside which could be tapped to power your external circuit without having to add an LDO regulator such as the CMOS IC you have that ZZ suggested.

Ralph if you want to add a resistor to Null your probe short voltage, or improve your probe using the Kelvin 4 wire to two probes, I think your project is almost done unless you have more questions.



Which 4 digit or 3.5 digit display is this one below? This can be used for 0.1 mOhm resolution = 0.1mV but does not have leading zero blanking. hmm.

Tony Stewart said:

Nigel Goodwin This is the 1st time I have seen this TLE2425 "precision virtual ground" IC , the one used in your friend's video. It is simply a very accurate 2.5V reference buffered to provide a precise 5V/2 ground reference for a single supply amplifier required for precise 0 to 5V full scale ADC measurements where 2.5V becomes "Ground" = 0V measured.

"Ground" always means a 0V reference for anything even for PE protective Earth, floating batteries and circuit measurements for 0V.

"Virtual Ground" is a very common definition in electronics and means the "null" reference can be shifted to any voltage defined as the "reference" which is always the positive input to an Op Amp (OA) while the inverting input tracks the VIn+ using high gain negative feedback to create a null differential input , the "virtual null". This is always used when converting bipolar measurements for a single supply.

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I know exactly what a virtual ground is, however in this case the chip is used as an accurate reference voltage for the constant current generator, not as a virtual ground. If it was used as a virtual ground then the 'ground' would be used for something, it's not, and it's not even half of the supply voltage.

Nigel Goodwin said:

I know exactly what a virtual ground is, however in this case the chip is used as an accurate reference voltage for the constant current generator, not as a virtual ground. If it was used as a virtual ground then the 'ground' would be used for something, it's not, and it's not even half of the supply voltage.

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Your understanding contradicts the definition I gave and it certainly is both a Virt.gnd and a Vref.

Tony Stewart said:

Your understanding contradicts the definition I gave and it certainly is both a Virt.gnd and a Vref.

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We'll have to agree to disagree then

Hi zipzapouch:

This is what found so far on this project. The constant current source 96.8 ua. The display 3 digit only shows 1 and decmal point on 200mv scale. On resistance meters means overrange until you take reading. The load I used is 33 ohm and should the display show something other then a 1 and decimal point? I set up to 78L05 regulators as you told me to do. I tryed for 3 hours why the display does not show any resistance value? I rechecked my wiring and can find any mistakes. But still lost of what I am doing wrong on this circuit? Any suggestions on what I should check ?

Short the 33 ohm resistor out, 33 ohms is pretty high for a low ohm meter - shorted out it should read zero, or close to it. The voltmeter will read over-voltage if the resistor is too high, or if there's no resistor at all. If it still reads over-volts when shorted out, then you have a fault.

100uA (or so) constant current seems rather low for a low ohm meter?.

I am using 51K circuit. I shorted 33 ohm out and no change. So somewhere a fault. 5v 51k should show up around 100ua? I will change opamp maybe defective?

Hi all members:

Changed out mcp602 opamp and their still is an over range condition. So opamp is not the problem. Any ideas how to find problem in this circuit? I traced every component from your schematic always looks correct. Checked both 7805 regulators 5v out. Spit supply looks ok with dmm? Also I made the circuit I took from youtube 4th or 5th post down in my thread why does this circuit work ok and the one you gave does not ? Maybe the simple design is easyer to work? Its not very accurate but does function. I just don't know how fix your circuit? I will try subing your 51k constant current source in the youtube circuit and see how generator works? I run out ideas how to shoot this trouble on your circuit? I not saying your design is bad but do not know how shoot this trouble on this circuit? Give me some ideas on what to do?

Hi members:

I used 78L05 to92 which handles 100ma each regulator is that problem for this circuit design?

Also here are pictures of bread board the way lay it out for circuit. Should I redo the on circuit board instead of bread board? Or should it work on bread board ok? I rewire without large jumper wires to make cleaner circuit. I though since I 2 78l05 regulators one for generator and one for 200mv meter they would work ok? The larger 78L05 does not fit on circuit board holes.

Hi members:

This constant circuit source works some what. So here's the screen shot attached. I still don't know why the one you gave does not I will try analize why does not act like one took from youtube? Its the green ckt bd using the same 200mv meter that used for your ckt. Any help you can give me would be apprieated. thanks Ralph