Espritmodel.com Telemetry Radio
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Old May 06, 2010, 04:08 AM
PLD
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More pictures from the LOM

Did a lot of work today on the LOM here in the house... couple of interesting things;

1; my noise floor is about 1uV, so that ultimately stops your resolution, there is a 0.22uV 24-bit ADC that Linear produces which I might try next but it'll need a complete board respin.

2; I'm maxed out at 19 bits resolution

3; I've had to increase the test current on the low ohms range (0~10R) to 20mA, it was the only way to get another bit of resolution

Here's some piccies of various results.... note how 0R resistors are worse than wire links!
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Old May 06, 2010, 07:50 PM
ancora imparo
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Hey, Paul, careful you don't get a case of "rapture of the deep". You know - where guys spend so much time finessing their Windows desktop they never actually do any real work.

I would be REALLY happy if you made me a 0.1mOhm version before you saddle up to go chasing that last half microVolt and 20th bit.

John
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I adjusted the firmware a bit more and made the uC go to sleep during the time that the ADC is doing its magic (about 140mS/sample), it's cut down the noise a little bit and I suspect once I mount this board into a metalised chassis and load up the front panel with BNC connectors with common grounds etc etc it should become useful.
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Old May 06, 2010, 08:43 PM
PLD
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John,

Oooh, but the abyss looks sooooo enticing, it's calling me

As for the 0.1mOhm version, that's what I've got now - since I've raised the test current up to 20mA for 0~10R it works a charm and is quite stable. In short, what is sitting on my desk right now would essentially be what you're wanting. I'd be happy to send you the source code too so that you can meddle with it.

Paul.
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Old May 06, 2010, 08:44 PM
PLD
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As a bonus, it makes a nice thermometer too
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Old May 07, 2010, 02:38 AM
ancora imparo
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OK, I'm a buyer.

On the second thing you must be out of your mind. Me meddling with source code....probably turn it into a nuclear detonator by mistake.

John
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In short, what is sitting on my desk right now would essentially be what you're wanting. I'd be happy to send you the source code too so that you can meddle with it.

Paul.
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Old May 07, 2010, 04:09 AM
PLD
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John,

A come now, the firmware is actually rather simple really, some nice little tricks in the math but nothing exotic.

Do you want me to finish this unit up complete with the BNC connectors, slimline benchtop casing and external power connector?
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Old May 07, 2010, 07:42 AM
ancora imparo
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Yes please.
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John,

Do you want me to finish this unit up complete with the BNC connectors, slimline benchtop casing and external power connector?
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Old May 08, 2010, 12:08 AM
Fly more, build less
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Paul,
I really enjoyed watching your build up to this point. They say when it is 80% built, . . . . you should start selling it. You were at 80%, a while ago.
. . . . . .Nice job.
I am building a similiar project reading a .01 ohm resistor, to measure current flow.
I will keep following up on your build.
thanks,
Don
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Old May 13, 2010, 03:00 AM
PLD
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John,

BNC posts arrived today along with the DC jacks (2.5mm afaik) so I can begin the final chassis build.

Don,

Well, you're fairly right about the 80% thing, though in this case the LOM has been a fair bit of a "highly experimental" device and while it'd be nice to push it to sale I'm also not worried if I never go into full production with this. There's a lot of physical labour involved in assembling this unit, most of which isn't the electronics.

Paul.
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Old May 13, 2010, 06:11 AM
PLD
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John,

Did a terrible job casing up the LOM but here it is (I'd personally prefer to put this into a bench-case, then at least it'd be a lot easier to line up everything.

I couldn't test the DC-jack power option because much to my amusement I couldn't find a 2.5mm version, everything I had was 2.1mm.

Anyhow, as you can see here, I'm testing 3R 1% resistor which has a surprisingly good compliance.
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Old May 14, 2010, 01:01 AM
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Paul,
I hope that you take this to the next manufacturing level. I may be looking for something like this and I will be looking at yours.
Thanks for getting it out there.
Don
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Old May 28, 2010, 08:33 AM
PLD
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Well, Mr John has his prototype unit now... It'll be interesting to see how he finds it.

I've received some new 24-bit ADC converters which offer me another 2-bits of real resolution at the levels I'm working at, however I'm suspecting that it might not be practical because already I'm experiencing changes due to air currents moving over solder junctions due to thermocouple effects.

It may be that to push for 0.05mR resolution I'll have to go with increased testing currents. That won't be an issue for motor windings etc which should be okay at 40mA or more, however it'd be somewhat disasterous if used on sensitive electronics (I suppose some auto detection here would work well, anything over 100mR would drop back to 20mA test current).

Paul.
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Old May 30, 2010, 06:38 AM
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Well, as the owner of the only Paul Daniels LOM in the world (for now), I felt some obligation to give it a more respectable home. Don't have access to acrylic laser cutting and the like but a label knocked up in the colour printer turned it from a prototype into SN:0001
This thing has some obvious uses. Great for those lengths of Nichrome you forgot the resitance of, measuring balance lead and wiring harness resistance without all the hassle of setting up a voltage drop rig, and very obviously - motor windings. See attached. The Kelvin leads were a great find.

Paul, I think the firmware upgrade to get rid of the hash at startup (which sometimes seems to never go away), and a switchable higher current range for very low resistance would be nice .

John
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Well, Mr John has his prototype unit now... It'll be interesting to see how he finds it.
Paul.
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Old May 30, 2010, 06:55 AM
PLD
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Oh very nicely done there John - very nice indeed. I see the weather is a bit cooler down there than up here.

At the moment it's running two separate current modes already to get the fine (0~9.9999R) and the coarse (10~100R) ranges. Modifying them at this point would require changing the actual current limiting resistors on board.

I'll spin out a new firmware version to fix that startup issue and make the under/overrange behaviour a bit more on the 'friendly' side. You've done well with the box supplied. I might have to import a box of 100 of those kelvin leads I think.
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Old May 30, 2010, 07:36 AM
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Thanks. On reflection, I'm not sure either the higher current range or the extra bits in the DAC are worth the effort.

I'll have a play around with various motors to see if I have anything with really low resistance but as it is the reading is just stable and I wonder if most folks would have the skills to make use of better resolution? Certainly 0.1mV is more than enough for winding comparisons and anything I would sensibly use this thing for.

Remember the saying in the IT business. For percentage up time, the last 9 costs 99 times as much as the one before.

John
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Oh very nicely done there John - very nice indeed. I see the weather is a bit cooler down there than up here.

At the moment it's running two separate current modes already to get the fine (0~9.9999R) and the coarse (10~100R) ranges. Modifying them at this point would require changing the actual current limiting resistors on board.

I'll spin out a new firmware version to fix that startup issue and make the under/overrange behaviour a bit more on the 'friendly' side. You've done well with the box supplied. I might have to import a box of 100 of those kelvin leads I think.
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