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Old 31st Jan 2016, 9:39 pm   #5
David G4EBT
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Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,737
Default Re: A precision voltage calibrator

Simple ‘Precision Voltage Reference’ projects have appeared twice in recent years in Everyday Practical Electronics Magazine, namely in May 2011 and June 2015, based on a custom IC - the AD588/AD587. I built the one that featured in May 2011. It’s a precision voltage reference, which - without any calibration or adjustment - will provide a 10.000V DC source within +/- 3mV, (an accuracy of 0.03%). I can't think of any applications for which we - as hobbyists - would require a higher degree of accuracy.

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 the 12V AC input. As with all EPE projects, the PCB artwork can be downloaded from their website for a small fee or the PCB can be purchased.

Not having a precision Voltage Reference hadn’t 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 fitted 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 £7.32 inc VAT, so for anyone without DIY 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. I designed a front panel for it which I printed off and fixed with clear library film.

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 exactly 10V. All my other multimeters are similarly accurate, including Maplin 'two for a fiver' ones - some reading just under 10.000V, some a little over. I’ve since built a homebrew FET AC/DC Millivoltmeter which I calibrated using the 10V Voltage Reference. (The last pic is of that meter, but it's just checking the voltage of a 9V battery - not connected to the Voltage Reference).

Frankly, for anyone not wanting to go to the trouble of making a precision voltage reference unit - and most won't - fresh mercury cells have a Voltage at 20C of exactly 1.3566V, which - if not used - falls slowly to about 1.3524V (99.7%) after a year or so. Silver Oxide cells have a voltage of 1.550V, so both types of cell provide a useful voltage reference source, more than adequate for hobbyist needs.

A few pics of the PSU and Voltage Reference are attached. The first pic is of my homebrew 12 V AC PSU, the second is the inside of the Voltage Reference, the third is unit boxed up, the fourth is the excellent 'Toolzone' £10.00 multimeter on test, the last is the homebrew millivoltmeter I built, which I calibrated using the Voltage Reference.

The May EPE 2015 Magazine, covering the 10V Precision Voltage Reference is here:

http://www.epemag.com/proj/0515.html

The PCB is £7.44 inc VAT from here:

http://www.epemag.wimborne.co.uk/acatalog/May_2015.html

The earlier June 2011 EPE Magazine project that I built is here:

http://www.epemag.com/proj/0611.html

The PCB is £7.32 inc VAT from here:

http://www.epemag.wimborne.co.uk/aca...June_2011.html

Hope that's of interest.
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