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Old 2nd Feb 2016, 10:10 pm   #21
Pamphonica
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Default Re: A precision voltage calibrator

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Originally Posted by Skywave View Post
Jeremy: you refer to many instruments which you own and imply that you are content with the validity of their readings. So what voltage source (or sources) did you use to give you the necessary re-assurance that those readings were accurate & reliable? Or did you rely on a simple probability / statistical basis?
I checked the dc performance of a number of my DMMs vs an in-cal Keysight 34661A DMM using a DC calibration source and keep the results. The ac performance is of less interest. I have 0.01% resistance standards for the resistance ranges. For counters I run a Rubidium 10MHz standard and am completing a GPSDO to correct the Rubidium. For sig-gen ac levels at RF I use a relatively recently calibrated RF power meter and heads. I am also friends with a cal-lab owner who does reasonable cost calibration verification if I need it. Any more and people might start thinking I am obsessive!

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Old 3rd Feb 2016, 9:36 am   #22
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Default Re: A precision voltage calibrator

I too built the 10V "Precision Voltage Reference" circuit (EPE Magazine - May 2011) mentioned by David G4EBT, in my case using the Analog Devices' AD588BQ device. It works very well and can be used in a four lead "kelvin" sensing arrangement if required.

Later, I also built the "Precision Digital Potentiometer" (EPE June 2012), which makes an excellent accessory for the above, allowing any voltage between 0 and 10V to be entered via a keypad and displayed on a LCD. This has a typical resolution of 1 millivolt.

As has been discussed, it's all too easy to get wrapped up in this precision stuff with no real benefit (especially to the hobbyist). But I did it because a) I enjoy making things like this and b) I'm fascinated by all aspects of measurement. I like using test instruments and consider this a major aspect of my hobby. Also, it's very satisfying when precision "lab grade" equipment can be made on a budget in your shed.

Practical uses for the two devices:-

1) The precision divider is very useful for the accurate calibration of analogue synthesizers, and related equipment, which are often based on, for example, 1V per octave scaling.

2) They let me compare results between my ever growing collection of analogue and digital multimeters. Well, you just can't have too many meters in my opinion.

Granted, when fault finding, the twitch of an AVO needle or the buzz of the continuity range will suffice a lot of the time - without even looking at the numbers. But hey, it's all good fun !


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Old 3rd Feb 2016, 1:31 pm   #23
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Arrow Re: A precision voltage calibrator

Thank you, everyone, for all your responses so far: very informative & useful.

Guided by what I have learnt here, I found a Chinese supplier who has for sale simple pcb assemblies using AD584 device. Price: £3.99 incl. p & p. Now we all know that when you do buy something 'on sight', there is always a bit of a gamble involved, but at £3.99, that's a gamble, which in this case, I'm prepared to take - so I've bought one!

So at this point, and right now, I can't think of anything else I can (or need to) add to this thread. However, when this item has arrived and a completed & cased project finished, I'll present a write-up in the 'Homebrew Equipment' section. Perhaps meet you all again, there and then.

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Old 3rd Feb 2016, 3:10 pm   #24
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Default Re: A precision voltage calibrator

Hi Jerry,
I`m very interested in so an "Digi Poti Project", than I don`t have an accsses to EPE-Mag scripts; is it possyble please that you send me pm the relevant article in pdf? Best thanks in advance !
Sincerely, Karl
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Old 3rd Feb 2016, 4:08 pm   #25
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Default Re: A precision voltage calibrator

Hi Karl,

I'm afraid I don't have the article in electronic format. It may be worth taking a look at the EPE web site and see if there are any remaining back copies of this issue. If I remember rightly I bought that copy as a back issue for something else but ended up making the digital pot instead ! Also, you should be able to download the pcb artwork and the firmware required for programming the 16f88 PIC from the web site.

If you are intending to build the project, I would check that you can get hold of all the parts - in particular the relays. Be very careful to check for the correct size and contact arrangement/spacing as there a few variations available.

Jerry
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Old 3rd Feb 2016, 5:55 pm   #26
David G4EBT
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Default Re: A precision voltage calibrator

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Originally Posted by karesz* View Post
I`m very interested in so an "Digi Poti Project", than I don`t have an accsses to EPE-Mag scripts; is it possyble please that you send me pm the relevant article in pdf? Best thanks in advance !l
The project in question was actually in the July 2012 issue of the magazine - not June, the contents of which are stated as:

Quote:

July 2012 Back Issue (Reference #0712)
Projects & Circuits: Lab-Standard 16-Bit Digital Potentiometer;
Intelligent 12V Fan Controller;
Dual Tracking 0V to 19V Power Supply Part 2.

Unquote.

Back issues are available in both hard copy for £4.20GBP (plus Post), or in PDF format, for £2.00 GBP at the EPE online shop at this link:

http://www.epemag.wimborne.co.uk/aca...F_Version.html

The PDF version has the merit that the artwork can be printed directly onto acetate for a PCB mask, but for anyone who doesn't have DIY PCB facilities, the PCB for the digital potentiometer is also available for £11.16GBP + 20% VAT from the online shop.

A PDF version of the magazine is available by annual subscription for £19.99 for 12 issues - especially handy for overseas customers are it obviates delays and costs due to postage. I only buy it selectively if something takes my interest because so many of the articles - as with the likes of Elektor - are beyond my wit or interest, using PICs, complex PTH PCBs and the like. (A good proportion of EPE articles are from Silicon Chip magazine in Oz).

Incidentally, though not relevant to this thread, there is quite a list of free EPE Magazine PDFs for older projects from earlier years which can be downloaded from here, which might interest forum members:

http://www.epemag3.com/lib/free-projects.html

Hope that helps.
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Old 3rd Feb 2016, 6:58 pm   #27
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Default Re: A precision voltage calibrator

Hi Jerry, David,
Best thanks for your promt answers, yes, yuo both helped me with that.
I`m very sorry for my poor english and wors conjugations...
Best wishes - Karl
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Old 3rd Feb 2016, 9:33 pm   #28
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Default Re: A precision voltage calibrator

Thanks for the hint Al, I've just ordered a similar AD584 based board, I have a genuine AD584 if the IC proves to be out of spec.
Interesting to note that some versions claim to use the AD584L which was removed from the data sheet in 2012.

A while ago I bought an old Time 2003S which I calibrated at work using an Agilent 34401A, but it is always nice to have something at home to check it against.

I've got a feeling that I might be at the start of a slippery slope

Jim

I already have GPS disciplined frequency standard, now what to check that against?
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Old 4th Feb 2016, 9:54 am   #29
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Default Re: A precision voltage calibrator

Thanks David,

You are of course, quite correct. The Digital Potentiometer project was indeed featured in the July 2012 issue of Everyday Practical Electronics. Also, this project uses a PIC 16f877A and not a 16f88 as I incorrectly stated earlier. My apologies for the errors.

I'd edit the posts but I don't know how to (or if I'm allowed to !).

Jerry
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Old 4th Feb 2016, 11:20 pm   #30
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Default Re: A precision voltage calibrator

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I'd edit the posts but I don't know how to (or if I'm allowed to !).
We only have a short 'window' to edit our posts Jerry - about ten minutes I think. However, moderators can edit them, so if you write the correct text that you wish to substitute and request that a moderator updates your post, I'm sure they'll oblige.
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Old 5th Feb 2016, 9:48 pm   #31
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Default Re: A precision voltage calibrator

Me - well I use a Time Electronics Ltd DC Voltage Potentiometer 2003N. Goes from 1uV to 9.9999V.
As well as an accurate DC voltage source, I also use it along with a Solartron 7040 DMM & an RS LCR meter to accurately determine FSD values of MC meters.
The 2003N came from a local electronics calibration firm, and was in full spec when I got it buck-sheesh. Just needed the null edge meter changing.

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Old 8th Feb 2016, 12:13 pm   #32
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Default Re: A precision voltage calibrator

If I can add a little to the debate, I have several voltage standards for various usages including a Time Electronics 2003, several old Bradley units and a couple of small stand alone units from the US supplier mentioned above. The point I was coming to was to endorse one of the small Chinese units which I found to be a good basic standard covering 4 voltages and was very close to the more expensive units.
See pictures attached.
The voltage on a Keithley Meter that was recently in calibration was 10.00316V DC against a claimed 10.00328, the other 3 voltages were equally in the same range. Not sure which is right but it was close enough to check a standard 4 digit DMM. Just search for voltage reference and have a look and buy from someone with good feedback. Whilst on the subject of checking I would also suggest getting a few 0.1% resistors or better of values in the middle of the range of you DMM to keep a check on the resistance ranges.

Chris
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Old 8th Feb 2016, 4:38 pm   #33
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Default Re: A precision voltage calibrator

Seems that many are being drawn toward the black-hole of voltnut-dom;
Spaghettification is inevitable !

Strange how things have changed over the years; when I could afford to buy my first digital meter, from Maplins, I felt like a pools-winner.

Nowadays, many novices feel they wont be able to do any worthwhile electronics unless they own a Fluke 87 V, and seem terrified of actually doing anything until they've calibrated everything in sight.

The nearest I've got, is acquiring a Weston standard cell, which I checked on a calibrated meter from work, and then on my own DVMs.
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Old 10th Feb 2016, 12:45 am   #34
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Default Re: A precision voltage calibrator

I hope that photo was taken with the standard cell glassware vertical. They are quickly ruined if they lie on their sides for long. And of course you must draw essentially zero current from them (originally used with a potentiometer). Even extended use with a 10Mohm input DMM is not recommended.

Nowadays, you can get a cheap voltage reference just as reliable and accurate as a Weston cell, so they are (in my experience) not very good buys secondhand, but are nevertheless fascinating.
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Old 12th Feb 2016, 2:39 pm   #35
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Default Re: A precision voltage calibrator

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That as may be, but surely you will also have access to test kit that falls into the 'other' category: "To be re-calibrated annually" (or some other regular interval). In government R & D and industry, that has certainly been my professional experience.
Yes we do. But this test gear goes for special UKAS accredited calibration and I didn't think this level of checking/cal was relevant. So I only mentioned how well the sub class (uncalibrated and marked for indication only) test gear holds its cal for many years.

However, on Thursday I gathered up 5 Agilent 34401A meters that were UKAS calibrated recently. These are destined for use in ATE racks.

I borrowed them all for part of the day and compared them against each other on a cheapo voltage reference. 3 of them read 5.00028V and one read 5.00029V and one read 5.00026V after a 2 hour warmup. The readings were stable well before the 2 hours were up. I did find a newer Agilent 34410A bench meter that was spare and this had less warmup time but it read 5.00031V after about an hour but read 5.00028V on a cold start.

Some people may find this info interesting. My Keithley 2015 measured the reference at 5.00025V and it hasn't been calibrated since 2011 according to its internal data as you can get it to tell you this via the front panel. My very old Fluke 45 read 5.000V on the normal range but 4.9993V on the slow range. I don't think this meter has been calibrated in a very long time.

Note that the most demanding thing I think I've ever done here at home was measure some YIG reference voltages in my old TR4172 analyser. I had to measure 10V and 5V within +/-3mV. Besides this, I really haven't 'needed' accuracy like this in many years.

In my experience DVM technology is very reliable as long as you buy something fairly reasonable. Even my 30 year old Maplin gold meter has proven to be reliable and so have a couple of meters bought at rallies. Eg a cheap looking TMK DMM made in the far east and also a very old Data Tech Model 30A. These last two meters cost about £1 each at a rally. But both read 10.00V against my other DMMs year after year after year...
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Old 12th Feb 2016, 3:05 pm   #36
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Default Re: A precision voltage calibrator

One other thing that might be worth mentioning is that you can permanently affect the output voltage of a reference during normal soldering because the heat can age the reference. So if you do solder it try and use a heat shunt and be quick with the iron.

It isn't something that would worry me too much (I'd rather solder it than use a socket) but I think you can knock it out of the datasheet specs if you get clumsy/slow with the soldering and you use an unregulated iron that runs too hot.
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Old 12th Feb 2016, 3:31 pm   #37
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Default Re: A precision voltage calibrator

An odd philosophical point...

Calibrating your test equipment tells you whether all the work you did with it in the previous year was OK or not. It stops a fault that occurred last year from wrecking the next year's work, but it gives no protection from errors that arise after the calibration date.

David
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Old 12th Feb 2016, 6:55 pm   #38
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Yes, I tend to view the annual calibrations at work as being a single point health check on the date they are checked. So this only adds limited value.

In the case of the DVM tests on my old Advantest analyser I suspect that the absolute accuracy wasn't as important as making sure that all the references agreed within 3mV when tested with the same meter. So I probably didn't 'need' absolute accuracy even in this one off event
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Old 12th Feb 2016, 7:01 pm   #39
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Default Re: A precision voltage calibrator

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Calibrating your test equipment tells you whether all the work you did with it in the previous year was OK or not. It stops a fault that occurred last year from wrecking the next year's work, but it gives no protection from errors that arise after the calibration date.
Very true. It is a point often overlooked.

I've resisted the temptation to become in this thread as I am involved in professional calibration for part of my living.

The device which began this lengthy thread is not, of course, a "Voltage Calibrator" precision or otherwise. It may be a precision voltage source but even that is a moot point.

Unless the full specification is known and understood, including the uncertainty budget, it isn't much use for calibration to the highest standard.

However, does that matter as far as most of us are concerned? I would suggest that in most cases it doesn't matter in the least if our bench meter is within +/- 5% or even 10%.

If we are fault finding, then what was the tolerance on the voltages printed on the circuit diagram? I bet there isn't one stated. It probably doesn't matter very much, all that is needed is an indication that the circuit is doing something.

In my own home workshop I do have some instruments which I maintain to a high standard of calibration, because I sometimes need them. The kit I use for general work (Philips PM 2517E which is probably at least 40 years old and a number of AVOs of various vintages) are close enough for what they are used for.

It is worth considering that a full calibration in a UKAS accredited calibration laboratory is not cheap. In fact you could buy a decent DMM for about what we'd charge for a full calibration.

As with many things in life, it is horses for courses. If I'm fault finding on a piece of vintage equipment I don't need the same accuracy of calibration as I might if I am setting up a piece of equipment which someone's life may depend on.
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Old 12th Feb 2016, 11:50 pm   #40
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Default Re: A precision voltage calibrator

Essentially, this is a vintage radio restoration forum. Probably the largest proportion owned/collected by members are from the 30's & 40's. But those were the days of moving iron and not too reliable moving coil test meters. Plus body/tip/ spot resistors only offering 20% error tolerances. Those of us who bang on occasionally about beloved AVO VCM's must admit that Manual tabulations and voltage selection switches offer pretty variable tolerances at best.
Again, age profiles of members probably reveal a high percentage of retired guys. Some of whome have limited pension funds.
Really, only Forum guys who collect, renovate, calibrate - test equipment need some items of high accuracy parameter sources. An AVO8 Mk5 or 6, a Fluke or a Newlec DMM would be accurate for most guys, quite frankly.
Yes, I've some decent "Lab Grade" items, but they were luckily bucksheesh & saved from a skip. I couldn't have afforded to buy them on eBay or from internet dealers.
So if folk just have a faithful old AVO8 MM, don't feel left out of the game if you haven't got or cannot afford NPL standard test equipment.
Being such a friendly Forum - if you need some old metering or T/Eq testing &/or calibrating - there is bound to be someone nearby who will help. Just ask.

Regards, David
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