Electronics > Metrology

An Experimental AC Voltage Calibrator - Part 2

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enut11:
Experimental AC Voltage Calibrator
I debated whether this project fits in the ‘Projects, Designs and Technical Stuff’ thread or the ‘Metrology’ one. Since I love precision electronics, here goes.

After spending a brief session with a Fluke 5200A AC Voltage Calibrator, I realised what a handy instrument it is. The 5200A offers a wide range of accurate high resolution voltage at frequency settings. Biggest drawbacks for me: 1) Bulk: The 5200 is a huge and heavy box that needs lots of bench space, 2) Cost: around $2000 USD or more for a good one.

So, whenever I come across an idea for a DIY AC Voltage Calibrator, it gets my attention. Recently, a friend suggested an interim solution. Not a ‘proper’ calibrator but a stable, variable amplitude/frequency source of AC volts. Such an  instrument, together with an accurate DMM, could be used to ‘transfer’ an AC voltage/frequency combo to another instrument.

The concept
Take a stable low distortion audio frequency generator and feed the signal into a HiFi audio amplifier. This alone will give you up to 20vRMS in the range of 20Hz-20KHz and possibly well beyond. For higher output voltages, connecting a 100v/8ohm audio line transformer, back-to-front, to the amplifier will give you up to 200vAC.

A simple concept, but does it work? How stable would the output be? These are the questions I needed to answer for myself. More to come…
enut11

Jester:
I made something  like this quite a while ago before I had an ac calibrator. I used HP sig generator as source driving audio amp and conventional transformer running at about half its voltage rating. It’s a lot better than using MAINS voltage and capturing and correlating two sets of measurements, at least the measurements are relatively stable.

The bulky size, cost and cost of the conventional ac calibrators was also a deterrent for me. I prefer to have all the instruments that I use regularly within reach at my bench. Bottom row 2nd from right.

Clarke Hess (smaller less expensive ac calibrator) show up on eBay once in a while, I picked one up in non working order for next to nothing and fixed it, pretty decent little calibrator if super accuracy is not required. I tried to get a schematic from Clarke Hess, they were uncooperative. Troubleshooting a bad one without the schematic is a bit tricky. Mine just had a leaky diode.

mzzj:
I did take a part in DMM intercomparison (8½ digit Fluke 8508A) with shoestring budget equipment some years ago.
Laptop soundcard, audio amplifier and bunch of transformers to get up to 700Vac  >:D
Results looked good with approx 0,03% uncertainty up to amplifier max voltage.
transformer coupling increased the uncertainty to 0.06% at 700 Vac (3458A as reference is not that great here either)

Laptop sound card was much better choice than either of tested (digital) function generators.

mawyatt:
Interesting idea. How stable is the audio amp gain and how flat over frequency?

A very simple and useful low voltage AC calibrator we used some time ago is based on a CD4060 divider chip and a 4.096MHz crystal. The VDD voltage for the CD4060 is created with a stable 5 volt reference, since the CD4060 is CMOS, the selected frequency divided output will swing from ground to ~VDD without any load (typical DMM isn't much of a load). The output voltage is an accurate 5.000 Vpp 50% duty cycle squarewave with an AC rms and DC average value of VDD/2 or 2.500 Volts and stable.

Of course the squarewave has harmonics and the frequency range must be taken into consideration, but works quite well on the various DMMs we have, KS34465A, HP34401A, AG34401A, SDM3065X, and DMM6500 just to verify low voltage AC performance.

Edit: Just plugged in the reference and let it run for a few minutes, it's AC reading on DMM6500 is 2.499264V RMS with 30uv SD. Also reads 2.500049V DC with 10uv SD.

Edit: Found the calibrator thread.
https://www.eevblog.com/forum/projects/diy-dvm-simple-dvm-cal-device/msg3157650/#msg3157650

Best,

bdunham7:

--- Quote from: enut11 on November 28, 2021, 09:08:25 am ---The concept
Take a stable low distortion audio frequency generator and feed the signal into a HiFi audio amplifier. This alone will give you up to 20vRMS in the range of 20Hz-20KHz and possibly well beyond. For higher output voltages, connecting a 100v/8ohm audio line transformer, back-to-front, to the amplifier will give you up to 200vAC.

A simple concept, but does it work? How stable would the output be? These are the questions I needed to answer for myself. More to come…

--- End quote ---

Having done both (but not the transformer part) I can tell you that it works fine as long as you have a known good audio amplifier, an ordinary AWG and a good reference meter.  The accuracy limits of this setup will likely be determined by the reference meter.  A transformer will likely work as long as you stay well within its saturation limits.  I use one or two (bridged) Marantz monoblock amplifiers and I can do better than 80Vrms @ 50kHz.

The 5200A has significant limitations and having had/fixed/used them I don't miss having one.  The frequency controls have fine resolution, but poor accuracy--you need to use an external AWG to control them if you want precise frequency.  They only go to 120Vrms on their own, and that only up to 80kHz or so.  The frequency range goes to 1.2MHz, but that is only at a low voltage.  You can get those low-cost add-on amplifiers for your AWG that will do nearly as well if you need to go beyond what your audio amp can do.

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