Precision Voltage Reference

[Yorkie]

Back last summer a little project in June 2011 EPE magazine caught my eye, and I've finally got round to building it. Namely, a precision voltatge reference, which - without any calibration or adjustment, will provide a 10.000V DC source within +/- 3mV, (an accuracy of 0.03%). Many years ago there used to be 'Weston Cells' which - from 1905 - 1972 was the national standard of EMF used in the USA, and was also an international standard. The Weston Cell was developed by Edward Weston of New Jersey, who had improved upon an earlier standard invented in 1873 by an English engineeer, Josiah Latimer Clark. Weston cells had a cadmium-mercury amalgam anode, a pure mercury cathode and a cadmium sulphate electrolyte, and they had a voltage of precisely 1.0183V. They still crop up on e-bay from time to time.

The EPE Voltage reference uses a custom IC - the AD588. The unit requires 12V AC input and supplies + and - 15V DC to the chip. Apart from the chip, there are four diodes, and a few resistors and caps, built on a small PCB, plus the requirement for a 12V AC input. As with all EPE projects, the PCB artwork can be downloaded FOC from their website.

Not having a precison Voltage Reference has not kept me awake at night or left a gap in my life, but as a compulsive builder of bits of simple test gear, I thought why not? I had small 12V mains transformer, so made a little PCB on which to mount it and fit it into an ABS project box along with a fuse in the secondary in case of inadvertent shorts on the output side. I etched a PCB for the device, but a PCB can be bought from EPE for £7.77 so for anyone without PCB facilities it's not a big obstacle. As directed, I built the unit in a metal box - a cheapo aluminium one as a diecast box of the correct dimensions was ludicrously expensive.

Not a lot more to say about it except that I've checked my trusty Toolzone £10.00 multimeter - probably the best buy I've ever made, and it reads 10V, but I'm not sure whether the multimeter is verifying the accuracy of the Voltage Standard or vice-versa! All my other multimeters are smilarly accurate, including Maplin 'two for a fiver' ones - some reading just under 10.000V, some a little over. (Probably come from the same factory as Fluke anyway, but without the Fluke fading display probs).

Frankly, for anyone not wanting to go to the trouble of making such a unit, and most won't, fresh mercury cells have a Voltage at 20C of 1.3566V, which if not used, falls slowly to about 1.3524V after a year or so. Silver Oxide cells also have a voltage of 1.55V. So yes, maybe I need to put my soldering iron away and get out more

A few self expanatory pics of the PSU and Voltage Reference are attached.

Next, I must make a box for the capacitance meter I've just finished from the same kit that Alan Beckett used.

Hope that's of interest to someone.

[pwdrive]

Very neat job Dave, puts my lashups to shame.

Lawrence.

[Yorkie]

Thanks Lawrence - I do my best!

My trouble has always been that my constructional skills exceed my technical skills, and I've always built things as neatly as I can, on the basis that they will work when finished. Experience has taught me that this aproach is very dodgy logic and many is the time that I've built things exactly as directed, checked them over and over to no avail, then months down the line an 'errata' appears saying 'please note, D2 was shown the wrong way round' or 'a wire link is missing which needs to go from R2 to C3'. The result of this over the years is that my spares box has often had several nice looking projects that don't function, and to that extent I guess that there is merit in those who say 'I build it rough then if it works build it neatly' (which I doubt they ever do!).