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# how should you design milliohm meter using dvm display

I wanted see if could use it to find dead shorts on circuit boards. Thanks for any help guys.
What resistance is a "dead short"? Are you looking for the actual resistance of the junction or just a pass/fail?

What kind of circuits are you measuring? What kind of chips are on it?

If anyone is interested, here's a milli-ohmmeter that my friend Michael made:

He used a 200mV module, and two independent supplies.

I think you will need excellent probes (4 wire Kelvin type) to null out the wire resistance.

This will measure down to 10 microohms at 1mV

http://tinyurl.com/ymzjsyru simulation

I think you will need excellent probes (4 wire Kelvin type) to null out the wire resistance.

This will measure down to 10 microohms at 1mV

View attachment 144196

http://tinyurl.com/ymzjsyru simulation
The probes in 4-wire ohm meters are great because no current flows through the measurement probes - they only detect the potential difference. Therefore, resistance can be quite large on the measurement probes. Zero current x high resistance is still zero voltage drop. Likewise, the constant current supply of the other two probes is, in fact, constant current, so the resistance of the probes is again unimportant because the current in all parts of a serial circuit are the same (just trying to dispel the rumors or "audiophile"-like aura around 4-point probe quality. The key is both measurement probes are attached to non-inverting inputs of two different op amps in an in-amp configuration (actually, just an in-amp) and zero current flowing through those and all current flowing through the supply probes.

This is why I asked the yet-to-be-answered questions above concerning how the OP plans to use the device and the importance of a measurable value from the device or simply a pass/fail for a "dead short" as described in the original post.

The 200mv meter can use 5v for power and yellow and black for volts measure. So that is not a problem and don't need 3v lipo connection. But I would like to use one of the 10 dvm that use a microcontroller as my DVM. These seem to be very accurate as a digital volt meter. And I like the small size. Also even after I took the 200mv meter apart to make unit smaller the circuit board still to big for my plastic case to put 9v battery. So this why I would like to use DVM display with 5v regulator which will fit in to plastic case. But I don't know how use this DVM as resistance meter so I can measure very low resistances with kelvin clips. I need to know how set up resistances so I can measure 1 ohm or less with my DVM? I looking for a way to do with out a split transformer. I did try using mcp602 op amp but need use as resistance meter that can be read by DVM microcontroller. The plastic box is 3 3/4"L X2 1/2"WX1"H That's why I want use the small DVM microcontroller display for this project. So can some one tell how set up the correct resistance value to do this? Any help would be appreicated. If I need more components for this project let me know.

Hi Tony:

I was going to make it pass or fail device, but after working on it for while I am now going to make it measureable device. A value to read 1 ohm or less on display. I will be buying kelvin probes. How do I make constant current source and read mv as 1 to xxx mohms to the display on DVM microcontroller using a 4 digit display? I will still be using a 5v supply which works very nice with this type of display. Their will be no transformers in my design of this tester. Any thing else is ok.

Use an extra 5v regulator and your 9-v battery to make a super easy constant current source. The resistor next to the regulator determines the constant current source.
I set two options as two different circuits so you'll see one is 0 to 200 mOhms read as one mOhm per mV.

The bottom lets you read 0 to 2 Ohms. It simply cuts the current supply to 10% of the other. Much easier than changing the value of two resistors on the differential amp.

See the four wires at each resistor marked "load". Two are from the constant current source (LM7805) and two from the differential amplifier circuit.

You can make additional ranges of higher resistance by increasing the amperage of the cc source (smaller resistor).

Hi Tony:

I was going to make it pass or fail device, but after working on it for while I am now going to make it measureable device. A value to read 1 ohm or less on display. I will be buying kelvin probes. How do I make constant current source and read mv as 1 to xxx mohms to the display on DVM microcontroller using a 4 digit display? I will still be using a 5v supply which works very nice with this type of display. Their will be no transformers in my design of this tester. Any thing else is ok.
I posted you a complete project above - why not watch that?.

I posted you a complete project above - why not watch that?.
He did watch it, and he did answer why he's not using it. Did you not read his post? Or did you not watch your friend's very boring video?

He did watch it, and he did answer why he's not using it. Did you not read his post? Or did you not watch your friend's very boring video?
Not wanting to do what needs to be done doesn't magically make the problem disappear - if you want to use one of the cheap voltmeter modules you need to isolate the supply in some way. However, there are some nice little isolation modules available from Banggood etc. - they presumably contain a tiny switch-mode PSU and an isolation transformer, I bought a couple for just such an occasion.

Not wanting to do what needs to be done doesn't magically make the problem disappear - if you want to use one of the cheap voltmeter modules you need to isolate the supply in some way. However, there are some nice little isolation modules available from Banggood etc. - they presumably contain a tiny switch-mode PSU and an isolation transformer, I bought a couple for just such an occasion.

i can't seem to find the schematic used in the project in your friends video that our OP can use. Please post it.

i can't seem to find the schematic used in the project in your friends video that our OP can use. Please post it.
I don't have it, but it's shown in the video - it originally came from a magazine article (which used a multimeter as the display), I think he even mentions the magazine?.

I don't have it, but it's shown in the video - it originally came from a magazine article (which used a multimeter as the display), I think he even mentions the magazine?.
If you want the OP to build a project based on a DMM, than please read the OPs request to use a small, 0-200mV meter. The project in the magazine uses a separate DMM. Your friend spends most of the video describing WHY the project in the schematic in the magazine is BAD.

Why would you recommend that the OP look at a video where the presenter specifically says, (paraphrased) "don't ask for a schematic, I'm not going to take the time to draw it out." Look at 8:30 in the video.

If you want the OP to build a project based on a DMM, than please read the OPs request to use a small, 0-200mV meter. The project in the magazine uses a separate DMM. Your friend spends most of the video describing WHY the project in the schematic in the magazine is BAD.

Why would you recommend that the OP look at a video where the presenter specifically says, (paraphrased) "don't ask for a schematic, I'm not going to take the time to draw it out." Look at 8:30 in the video.
What's to draw out? - it's dead simple - replace the DVM with the module, power them both separately.

Tony Stewart thanks for information on op amps. The op amp I will be using has dual op amp and you have watch how you correctly ground or tie unused op amp positive. So which is the correct way to terminate the unused op amp to positive or negative rail and should it be through a resistor either way? If through a resistor what value should the resistor be? I was under impression they are not connected internally which may be wrong? So give me your opinion on how I should do it? And should it be inputs or outputs of unused op amp?
These might help :

Regards, Dana.

What's to draw out? - it's dead simple - replace the DVM with the module, power them both separately.
So you would keep the precision virtual ground chips in the schematic? It doesn't look like your friend has any of those on his board.

So you would keep the precision virtual ground chips in the schematic? It doesn't look like your friend has any of those on his board.
I've no idea what you're on about?, what 'precision virtual ground'?. I presume you mean the magazine schematic at 5:25? - he's got everything from that schematic except the DVM - and there's no 'precision virtual ground' (whatever that's supposed to be?

Hi zipzapouch:

Your schematic shows 50k, 500k ,90, 1meg but all values for resistors 51k, 510k, 91, and 1meg. So do I have put resistors in series make these values or use values I just ordered? Or do have buy some trim pots linear to adjust to you values in schematic? Nigel Goodwin wants me use transformers and I do not want to use transformers in my unit. I mention that few times in my post. I did mention my plastic box is only 3x2x1 in size. And want to only use the DVM displays I purchased because they very nicely fit in the plastic box have. Btw Zipzapouch gave me really nice schematic to work with. Which I really appreciate. Thank you.

Hi zipzapouch:

Your schematic shows 50k, 500k ,90, 1meg but all values for resistors 51k, 510k, 91, and 1meg. So do I have put resistors in series make these values or use values I just ordered? Or do have buy some trim pots linear to adjust to you values in schematic? Nigel Goodwin wants me use transformers and I do not want to use transformers in my unit. I mention that few times in my post. I did mention my plastic box is only 3x2x1 in size. And want to only use the DVM displays I purchased because they very nicely fit in the plastic box have. Btw Zipzapouch gave me really nice schematic to work with. Which I really appreciate. Thank you.
Use the resistors you have. The accuracy will be off by 2-3% but, since you were considering a pass/fail design as an alternative, using the resistors you have will be perfectly fine. Also, if the 2-3% error concerns you (it shouldn't), the inexpensive 200mV meters have significant error and drift so, make sure you know the performance of your meter before investing time and money into changing the resistors.

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Hi zipzapouch:

Your schematic shows 50k, 500k ,90, 1meg but all values for resistors 51k, 510k, 91, and 1meg. So do I have put resistors in series make these values or use values I just ordered? Or do have buy some trim pots linear to adjust to you values in schematic? Nigel Goodwin wants me use transformers and I do not want to use transformers in my unit. I mention that few times in my post. I did mention my plastic box is only 3x2x1 in size. And want to only use the DVM displays I purchased because they very nicely fit in the plastic box have. Btw Zipzapouch gave me really nice schematic to work with. Which I really appreciate. Thank you.
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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