Preface: Understand, I know just enough about electronics to be dangerous... so pretend I know nothing as it will be close to reality.

I am trying to figure out how to build a simple Ohmmeter. The goal is to measure the resistance of a heating wire on a small scale.

The circuit requirements are 3vdc and to measure 0-10 Ohm of resistance accurate to .1 possibly .01. The output will go to a LED diplay and all will be mounted in a AA battery box.
Here are the basic components... the suppliers are just who I found the parts fastest at for reference purposes.


What I want it to do is basically the same funtcion as
http://www.ebay.com/itm/280826388767?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649


Project form factor:
http://www.amazon.com/Cells-Battery-Plastic-Storage-Holder/dp/B009OZYY7W/ref=sr_1_14?ie=UTF8&qid=1377875883&sr=8-14&keywords=3+x+aa+battery+holder

The output:
http://www.ebay.com/itm/7-segment-3-digit-Common-Anode-Led-Display-1D2-/151107740032?pt=LH_DefaultDomain_0&hash=item232eb92180

I was thinking that I would need an operational amplifier. Then set it up to show on the display the inverted voltage results. So when the resistance is added to the circuit the display would scale upward/downward based on the wire resistance.

I have seen several schematics that are very close to what I want, but my needs are far simpler than most of the options.

I was thinking about the LM741 as control. I understand the basics, but I am lost in the circuit details.

Any help would be GREATLY appreciated.

 

Buy a $5 multimeter and a 0.1Ω (for example) resistor, 1% tolerance. Put the resistor in series with your wire. Set the meter to its 200mV scale, measure voltage across the resistor and then the wire. Calculate resistance of the wire: Rw = Vw/Vr*Rr

Note that this strategy allows an in-circuit measurement. That won't be possible with a normal ohmmeter.

You don't really want to make your own ohmmeter if you can get your data some other way, right?

 

Actually quite the contrary.. I have about a half dozen meters. This is for a very simple project box. The simplicity is the key. I can make one using the Ohmmeter panel listed above, but I want it all to fit into a battery holder.

This is going to be for rebuilding e-cig atomizers and I want a fast way to read the coil resistance.

 

...I want a fast way to read the coil resistance.

And none of your meters are suitable? I think I'd just buy the right meter. That sounds simple to me.

 

Look here. This is in the Completed Projects forum.

http://forum.allaboutcircuits.com/showthread.php?t=54992

 

From your link

Resolution: ±10Ω

Your are asking for a simple meter that can do four digit accuracy to .01Ω?

Have you considered the implications of this?

You need four-terminal measurements for that.

Wayne's idea of a series low value resistor and a voltmeter, or there is a more conventional parallel resistor and currentmeter method may do you.

 

I am perfectly fine with .1 accuracy

 

oi.. now I see where my posts went..lol

 

For tiny amounts of resistance, the device in post#5 can give you a constant current so you can use the "Kelvin" style of connections to get a good measurement with a volt meter.

 

Hello,

You could make a constant current source.
Here is an example made by one of our members:
Project: Battery-operated 1 amp current source

By feeding the resistor with the constant current source, you can measure the voltage accross the resistor and determine the resistance.

Bertus

 

That's the one I pointed to in post #5.

 

I am waiting on a mod to release at least one of my replies.

I am looking for a fully self contained device much like I posted in the original post. I like the battery box form factor as it lends itself to easy use.

 

You will have to design the box yourself.
Here's a nice drawing:

 

I don't need complex... I have held this device in my hands and it is simple and elegant. I can make one more bulky using a panel....

For some reason my posts with links to what I am trying to describe are not being permitted....fine

Someone tell me how this works
DROK AC/DC LCD Digital Megger High Resistance 0-2M Ohms Meter Panel Impedance Electric Tester 3 1/2 Digit

I bought several off Amazon

 

. I have held this device in my hands and it is simple and elegant.

That's exactly what I said about the project that I modified for you. Except for the batteries, it uses up about 2 cubic inches plus whatever meter face you want to put on it. It doesn't get much more elegant than achieving 1% accuracy with 10 parts.

That DROK thing you found is a display, but it doesn't display lower than 1000 ohms. I think you wanted something lower than that.

Here's the page:http://www.amazon.com/DROK-Digital-...ce+Electric+Tester+3+1/2+Digit#productDetails

 

That's exactly what I said about the project that I modified for you. Except for the batteries, it uses up about 2 cubic inches plus whatever meter face you want to put on it. It doesn't get much more elegant than achieving 1% accuracy with 10 parts.

That DROK thing you found is a display, but it doesn't display lower than 1000 ohms. I think you wanted something lower than that.

I do think your solution will work.I tested one of the coils on an old crappy meter and it read 3.2Ω I also tested in on the battery mod I use that reads Ohms and it reads 3.2Ω. This same coil I placed on a the device I want to emulate and it read 3.2Ω

If it works this time...here is the actual circuit using a .56 size led segment in the device I like:

Backside

and...the full device inside a 3 X AA box powered by 2 X AA and circuit occupies the center.

and a picture of the disassembled pieces

 

Actually, it's not my solution. It is in the Completed Projects forum, originally designed and built by "someonesdad". I also believe it will work because it has been built by several, and reviewed by dozens of, well educated people that infest allaboutcircuits.com. All I did was copy the original design, then illustrate how to make a Kelvin connection and attach a digital panel meter. You are welcome to build one. That's why it was published on the allaboutcircuits website.

 

btw,
#12 I really do appreciate your help. I don't want to give you the wrong impression. I am planning on building up your circuit once I get components in.

I am curious how they managed to run the entire circuit off a single power source though.

 

someonesdad did not make this design to run from one power source. It has (2) two batteries in it. The 9V battery is plenty for the brains and an LCD panel meter, but it takes a C cell to provide enough current to get less than an ohm to register well on most available meters. The C cell is working rather hard to get an amp flowing (compared to its internal limitations), thus, the push button to connect the brains. If you just "push to test", this device will last for years on most work benches.

 

what is your opinion of this circuit?