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Old Dec 10, 2009, 06:43 AM
PLD
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Mark,

Ah yes, those lovely ohm-standards resistors... how I've often considered having one in my collection

I actually ordered today a 50R 0.01% resistor, so that's 50R +/-0.005R, not too bad a reference to use especially if you're calibrating against near full-scale measurement (which I will be, as full scale will be ~64R).

What I also have on hand to help with the process is a nice set of calibrated voltage references (1.25, 2.50, 4.096 and 5.000V @ 0.02%) and a multimeter good for uV measurements, collectively I do hope to come up with some good reference standards for my own workshop.

For the kelvin leads on this LOM, I'm thinking of offering separate leads with alligator clips and also parrot-clips. You can use the alligator clips for the current-carrying pair and the parrot-clips for the actual resistance testing.

Paul.
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Old Dec 10, 2009, 12:06 PM
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Kelvin Clips

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Originally Posted by pldaniels View Post
For the kelvin leads on this LOM, I'm thinking of offering separate leads with alligator clips and also parrot-clips. You can use the alligator clips for the current-carrying pair and the parrot-clips for the actual resistance testing.Paul.
That would seeem a great idea, I checked Kelvin clips on ebay, they are there
Will be in NSW fron 25.12. until 3.01 but will not make it as far as your workshop, this time!
CU Mark
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Old Dec 11, 2009, 02:44 AM
PLD
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No progress again today. My 50R reference didn't arrive so I don't (yet) have anything to do calibrations with.

Just as a design note, I actually created a 4-wire setup on the reference resistor I'm using on the ADC by taking a normal 0805 sized SMD resistor (the 10R 0.1%) and splitting its pads. The resistor is on the 'lowside' of the current loop, so one side goes direct to ground and the ADC measures between ground and the +ve side. I did previously have this such that I just relied on the very low resistance of the flooded ground-plane but I figured that it's probably better to use the 4-pad method if we want to get serious. You can see the 4-pad resistor at the bottom-center of the image, below the ADC chip (the larger 16-pin device).

Also note the perimeter trace around the ADC chip, this is to isolate the ADC ground and the digital ground as much as possible, there's also a high frequency choke on the +ve line to filter out noise from the microcontroller clock (which I'll actually be powering down during the ADC conversion as much as possible in order to reduce the noise even further).

All these little design changes help towards making the LOM that bit better.

Paul.
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Old Dec 28, 2009, 07:31 PM
PLD
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Sincerely hoping that the new LOM boards will turn up today or tomorrow. It would be nice to have the first production unit ready to use before the new-year, would be a great way to launch into 2010 (for me ).

At this point I'm just sitting around with everything else ready to go. I am excited to see how well the precision matches up with the "theory" :cough: , especially now that I have the 50R 0.01% reference resistor on hand.

I've also picked up a supplier of prefitted alligator and hook test leads, though I'll likely let people purchase which pair they want to use (or both sorts) depending on what tasks they're using the LOM for (eg, for thick cables you'll likely use two sets of alligator, where's for fine work you'll use perhaps alligator + hooks.

One question a couple of people have asked is how to go about using this for short-circuit detection, my suggestion is to use a hook or alligator lead for the current-carrying and plug them on the track with the short, then use a pair of probes to trace along the track like a normal multimeter.
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Old Jan 04, 2010, 02:37 AM
PLD
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At long, long LONG last, here's the boards... and here's a quick size-up fitting of the banana sockets and LCD panel.

No parts on the boards yet and I still have to decide on the whole on/off and mode switch selection button placements.
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Old Jan 05, 2010, 04:52 AM
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Okay, the PCB is finally populated and it appears to be working. Initial results have yielded some nice results with my 50R 0.01% reference resistor being very useful.

Initially with a 10.000R setting in the firmware, I got 50.008R on the measurement, with an adjustment to 9999R (which is within the 0.1% 10R tolerance) I was able to get 50.003R which I consider to be appropriate.

Note that I'm assuming the 50R is more absolutely accurate, since it is 0.01% as apposed to the 0.1% of my onboard 10R reference resistor.

Also, I've equipped the unit with an auto-ranging mode, which lets it get up to 99.999R.

As an additional feature, though I've not yet enabled it, I've got an audiable short-circuit locator mode, where the closer you are to 0.000R the shorter the gap between the pip's from the speaker, until it's a continuous tone.

Paul.
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Old Jan 05, 2010, 05:59 AM
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OK Paul, some of us are getting excited by all this suspense!

Any idea when you might be ready to ship them? I actually need to measure the resistance of something.

John
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Originally Posted by pldaniels View Post
Okay, the PCB is finally populated and it appears to be working. Initial results have yielded some nice results with my 50R 0.01% reference resistor being very useful.

Initially with a 10.000R setting in the firmware, I got 50.008R on the measurement, with an adjustment to 9999R (which is within the 0.1% 10R tolerance) I was able to get 50.003R which I consider to be appropriate.

Note that I'm assuming the 50R is more absolutely accurate, since it is 0.01% as apposed to the 0.1% of my onboard 10R reference resistor.

Also, I've equipped the unit with an auto-ranging mode, which lets it get up to 99.999R.

As an additional feature, though I've not yet enabled it, I've got an audiable short-circuit locator mode, where the closer you are to 0.000R the shorter the gap between the pip's from the speaker, until it's a continuous tone.

Paul.
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Old Jan 05, 2010, 06:30 AM
PLD
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John,

Just working on the firmware right now in the 'comfort' of my office with proper airconditioning.

Paul
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Old Jan 05, 2010, 11:00 AM
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All going good

Here's some pics of the final results of tonight's efforts.

I even managed to get the audiable shorts-locator going but it's going to need some tweaking to get the most range (making a hack of a logarithm function ), right now I'm doing about 0~300mR.

Pictures show me testing a 1R (1%) and 50.000R (0.01%) resistor.
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Old Jan 05, 2010, 01:20 PM
ancora imparo
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Hadn't realised how small and neat that case was. Good find.

John
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Old Jan 05, 2010, 05:26 PM
PLD
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John,

The real trick is going to be the faceplate cover... always a pricey little number getting those lexan/polycarbonate things done in small quantities.

Paul.
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Old Jan 05, 2010, 07:37 PM
PLD
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Here's a flipside shot of the board... probably will get myself into strife posting this.

It's fairly sparsely populated but a lot of the placements are critical to minimise noise.
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Old Apr 07, 2010, 05:06 AM
PLD
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I've decided to create two varieties of the LOM... the current version with the 16-bit measurement will be aimed for R/C etc but I've just submitted a design for a 24-bit enhanced version with tempco and various other "high precision" adjustments, however this won't be ending up in the usual R/C environment, rather with its 0.1~0.25mR precision it'll be something you keep locked away in a lab and have sent off for periodic recalibration against national standards.

There won't be much of a market for the 24-bit unit but it's nice to know that it'll be available. Likely I'll case it in a benchtop casing.
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Old Apr 08, 2010, 02:32 AM
ancora imparo
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Paul, not too sure about that. For R/C use most folks won't be interested in a LOM at all I would think. When they are, it's likely to be for measuring the drop in connector wires, plugs, balance leads, motor windings etc. - often sub 5-10 mOhm items. 0.1 Ohm precision doesn't look all that much in that space. For general electronic use, measuring 1-5 Ohm stuff it's a different story.

Just a thought.

I know someone who is interested in 24 bit unit.
John
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Originally Posted by pldaniels View Post

There won't be much of a market for the 24-bit unit but it's nice to know that it'll be available. Likely I'll case it in a benchtop casing.
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Old Apr 08, 2010, 02:48 AM
PLD
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John,

I can produce the 24-bit units with the current boards that I have, the trouble is I won't have tempco on there (unless I install a daughterboard). Will have to get a few of them in stock before I build of course . Without the tempco there'll be a swing of about 10ppm/'K from 25'C ( so, at 35'C, there will be about a 0.01% error just from the reference, ignoring other temperature related cumulative effects - however those should be nulled out due to the nature of the way I'm doing the testing ).

Paul.



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