Resistor Ohm Meter

[TheMadMan]

Hi Trent.

The more obvious solution would be to database the entire range of resistors. When a reading is taken, the system would basically look for a closest match and display the results.

There will have to be a database (array) present to get it working. (use a PIC with a big enough program area)
Once the tested value is known, it should display ALL the "standard" values where the tested value is between the upper and lower of said "standard" value.

I guess one possible solution would be to have roundup & round down buttons. Pressing these buttons would effectively remove the tolerance from the reading by either adding or subtracting 0.25 to 10% (user selectable) - of the resistor's value. From memory there's about 180 different values in the 5 band 1% range. This is the E? something range (totally forgotten) Lot of resistors to catalog though.

Problem is that a 10% may only be 5% off target. If you subtract 10% you wouldnt get the correct value anyway.
But your post gave me another idea: A rotary selector for the tolerance value to use when the PIC has to figure out what to show. The lower you set the tolerance, the less overlapping there will be.

You forgot the "E? range"! I didnt even know!

BTW. What are the standard tolerance ranges?
I've got 0.05% (grey), 0.1% (violet), 0.25% (blue), 0.5% (green), 1% (brown), 2% (red), 5% (gold) and 10% (silver).

If that is it, we'll need 3 pins to drive the tolerance rotary selector.
Rotary selectors are EXPENSIVE, maybe a dipswitch would be less expensive, especially in an environment where you only deal with one specific tolerance.

All I've ever used is 5% (grey) which sometimes looks like silver.

I swear I spend more time figuring out what the value of a resistor is than I do soldering the thing into place. Even then I often get it messed up as I simply cant make out the colors properly on some of the small ones.

Did you know that a 3mm led makes an audible "pop" when it goes?

All the best,

[Luciano]

Hi,

Are you a resistors manufacturer?

From page 3: (THEORY of OPERATION)
http://www.harris-irt.com/anonymousftp/irtman5012xp.pdf

The 16F877 Microprocessor based circuit includes a dual limit comparator which compares
the measured resistance with upper and lower tolerance values entered on front panel thumbwheels.
Red and green front panel light emitting diodes indicate whether or not the resistance is within tolerance.
An output relay permits the resistance tester to be converted to a rejection device.
For “fail-safe” operation the contacts are normally open. The contacts close if the part is
within tolerance. The Microprocessor based circuit also provides for electronic range switching
and system control.



Best regards,

Luciano

[T.Jackson]

Problem is that a 10% may only be 5% off target. If you subtract 10% you wouldnt get the correct value anyway.

Yes, that did dawn on me shortly after I posted that. A half-baked thought that didn't have a real world scenario attached to it.

[T.Jackson]

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2074&stc=1&d=119177155 3" border="1">
</br>
If I was going to do a project like this, this is probably the road I'd take.
</br>

[Luciano]

Hi Trent,

(Maybe one stage of your circuit).

Resistance measurement with Current Source

http://www.delabs-circuits.com/cirdi.../del20015.html
(Click on the link "PDF circuit" and then click on the Acrobat Reader save button).

Best regards,

Luciano

[T.Jackson]

Hi Luciano,

Have you built & tested that circuit? ... (looks comprehensive at first glance)

I usually do most of my projects from scratch. But I do get ideas from other people's circuitry. I know that there's a lot of people around that just slap together odds & ends that they find in magazines, from books & on the internet. Some of them even try claiming it as their own. This is one of the reasons why I procrastinate about doing anymore projects for publication.

[Luciano]

Hi Trent,

Here is a circuit which was published in a magazine back in 1994.
The circuit was connected to the serial port of a PC via a parallel
to serial interface circuit. A MS-DOS program (see picture) was
used to display the measured resistance value and show the color
code of the measured resistor.

10mA was used for resistors from 1 to 100 ohm.
1mA was used for resistors from 100 to 1k ohm.
100µA was used for resistors from 1k to 10k ohm.
10µA was used for resistors from 10k to 100k ohm.
1µA was used for resistors from 100k to 1 Mega ohm.

TS27M2CN datasheet:
http://www.st.com/stonline/books/pdf/docs/2306.pdf

LM334 datasheet:
http://www.national.com/ds/LM/LM134.pdf

Here is the schematic. (I have added the values).

(Click to enlarge)

How the current source is used:

(Click to enlarge)

The DOS program:

(Click to enlarge)


Best regards,

Luciano

[T.Jackson]

That's top info Luciano!

National Semiconductor's LM334 is quite good. Seems a bit of a waste to have one for each range though. I think a lot of magazines tend to step & repeat sections sometimes, it is a much easier read. And I also think the project is a bit over the top.

Who would use it and how would they benefit from it?

That's the first question I tend to ask myself these days.