Calibrating an auto-ranging meter

I only have a cheap Radio Shack 3.75-digit auto-ranging multimeter (22-803), and I want to calibrate the DC voltage reading. The ranges break at 400mV,

4V, 40V, 400V and 1000V. On opening the case I find two pots labeled VR1 and VR2, and I would bet money that VR1 is the one to adjust. But I should be able to confirm that by connecting a 1-meg resistor from the wiper to one end of VR1, and see if that changes the reading.

Anyway, I don't understand how auto-ranging meters work. In particular, if I find a really accurate 3.6V source, and calibrate to that, does that say anything about how accurate a 12V source would be read, which would be in a different range? Would it be better to calibrate to the 12V source, and would that automatically make the 3.6V source read correctly, or are the two ranges completely independent? Well, I know they aren't *completely* independent since I assume VR1 is most likely *the* V-ref for all purposes for the IC on the other side of the PC board. But I would guess that each range has its own resistors which would make that range subject to the accuracy of the resistors. And there certianly aren't pots for each range in this meter. So it may be that you can only make the meter accurate within one range. But as I say, I don't really know how auto-ranging meters work, so the stuff about resistors may be all wrong.

If anyone would like to explain how these auto-rangers work, and what my calibration options are, I would appreciate it.

I have a Digikey order coming up, and see that I could add a TI LM4132 3.3V reference for about $3, and/or the 4.096V version, which should put me into the next range. These are rated .05%.

Reply to
Peabody
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(That looks really cheap.) Unless you have a lot more time than money I'd get a "real" DMM... I think for ~$50 you can get something decent. Perhaps check out Dave's eevblog for recommendations.

George H.

Reply to
George Herold

You only need one reference, but if there's multiple ranges, you want an attenuator with good calibration (like, an old Wheatstone bridge, they are relatively cheap nowadays). Also, a voltage-difference amplifier or (old-school) a sensitive galvanometer. Autozero op amp should go on your DigiKey list.

A precision potentiometer is basically a high-linearity resistor divider with a four-digit readout on the adjust knob.

These come with a bunch of accessories (galvanometer, standard cell, etc.) that you probably don't want to employ.

Reply to
whit3rd

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Dan

Reply to
dcaster

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** Use your 3.3 voltage reference to calibrate the 4V range - this is the b est you can do with such a meter and one reference.

The second trim pot is likely for AC volts. The ohms ranges need no trim ca libration, readings depend only on the reference resistors inside the meter .

Since you cannot adjust each voltage range, calibration involves making a t able of the percentage errors found on each range. Comparing voltage readin gs with a known accuracy meter is the easiest way. A few precision ( 0.1%) resistors will do for the ohms ranges.

.... Phil

Reply to
Phil Allison

Thanks very much. Can you explain why it's better to use the 3.3V reference than the 4.1V reference? Is is just that you need accuracy more in the 4V range than the 40V range?

That's what I suspected. I don't think I'm going to mess with AC since usually all I need to know is whether the mains are live or not, so accuracy isn't important.

As luck would have it, in going through my stuff I discovered that my old analog meter has died. Actually, the Eveready 504 15V battery is finally dead after about

20 years. But that battery costs $10, and there are a couple ranges on the meter that no longer work. So instead I spent $10.85 on Ebay for an old (but still "new in cosmoline") military analog meter, which uses normal batteries. I still prefer to use an analog meter for some things. So that means any precision reistors in my old meter can now be salvaged.

Just kidding about the cosmoline, but the new meter is still in its original sealed package, and, you know, voltage and resistance haven't changed much since 1986, and I doubt there are any electrolytics in the meter. So it should work fine. Requires a 9V and 2 AAs. Lots of useful ranges, including 50 uA. I'll post a review when I receive it.

Reply to
Peabody

Peabody-

On manually-switched meters, calibration is done on the most sensitive scale. Accuracy of less sensitive scales is determined by precision resistors in a voltage divider. If you have a reference voltage in the second range, that may be less accurate, but is the best you can do.

Fred

Reply to
Fred McKenzie

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