How do I check the calibration of my DVM

[blfinche]

The title says it all. I want to make sure my charger is peaking at the correct voltage and I know my DVM needs to be accurate. What do you use as a reference source?

Thanks,
Bobby

[Gary Warner]

Originally posted by blfinche
The title says it all. I want to make sure my charger is peaking at the correct voltage and I know my DVM needs to be accurate. What do you use as a reference source?

Thanks,
Bobby

You could send it to Micro Metrology in Chatsworth, CA. and for $95 they will calibrate it for you (NOT).

You can build a circuit that has a highly accurate calibration voltage (too much trouble).

Buy a new Fluke for $399 that's guaranteed to .05% (or .01% - I forget) for two years (no wait, that's what I did...).

Get another meter from a friend and compare them on the same reference voltage at the same time. Any steady voltage will do. Design accuracy will be a factor in just how close they agree, but if one of them is far off, it will be obvious. If they agree, your in business.

[Ollie]

Zinc-air hearing aid batteries (available in most drugstores) have a very constant no-load voltage of 1.4 volts per cell over their life. You can use them as a voltage reference to check a DC voltmeter. Allow several minutes after removing the sealing tab for the cell to activate itself fully.

[blfinche]

Thanks for the reply guys.

[Discharger]

Does anyone have a suitable circuit diagram for a voltage reference? I seem to recall (in the dim dark past) a magazine article using zenner diodes of around 5.1v with low temp coef. as the basis of just such a device.

[AndyOne]

Discharger,

A Zener diode will not be accurate enough, they usually come with 5% accuracy but there are some at 2%. This isn't really accurate enough to check a DVM especially if you intend to measure Li-poly charging voltages.
Comparing it with a friend's DVM is a good method, better though would to be to compare it with two others.

Andy.

[Discharger]

AndyOne, thanks for your comments. I realize that the actual voltage of zenners will vary, however my thoughts are as we are looking for a constant voltage over time rather than a specific value for checking the meter, then perhaps this might be worth a try. The reason for the 5.1v or 5.6v is as mentioned previously because or their low temp coef. and drift. As you suggested, this circuit would need to be calibrated probably with as many other DVMs that could be rustled up! Also what about two voltage regulators in series..... Are you the Andy from CSM?

[Mr.RC-CAM]

Typical zener diodes will not offer the temp stability that you need. I suggest that you use a precision voltage reference diode like the National LM236 or LM285 series. They have them in various tolerances, including 1%.

The LM236AH-5.0 is a 5V 1% reference shunt that may provide the performance you need: http://www.digikey.com/scripts/us/dksus.dll?Detail?Ref=13689&Row=200432

RC-CAM

[AndyOne]

Mr RCCAM

You beat me to it I was going to suggest precision band-gap type reference which are available from around 1.2V to 10V with stability down to a few 10s of ppm per deg.C.

Andy.

ppm=parts per million