Small voltmeters up to 30–35 V

I already have a review of a small voltmeter that I did about three years ago, and now another similar little thing has landed in my hands. The previous voltmeter was more expensive, but it had a separate measurement input and could measure up to 99.9 V. This one is much simpler, but I decided that even these little devices might be useful to someone.


I didn't buy these for myself; they only ended up with me because I also ordered a few other small items from this seller.
I ordered something for myself at the same time because this seller charges 75 cents for shipping to my country, but the fee is shared across the whole package. So if you order three items, shipping for each one works out to 25 cents. It seems like a small thing, but some particularly crafty sellers add the shipping cost to every item in the package.

The title lists the price for one unit, but the price depends on the color. There are four options: red, green, yellow, and blue. Red is the cheapest, blue is the most expensive, and that's the one I'll be reviewing today.

The voltmeters came in individual antistatic bags, which were packed in a regular yellow envelope. They look like they made it through just fine, though.


Since these weren't ordered for me, I won't unpack all of them. I'll just use one, especially since I don't expect any major differences between the others.


I have a similar little device at home, but even that looks big next to the one I'm reviewing, which measures 30 (23 without the tabs) x 11.2 x 9.5 mm.


It has a two-wire connection, so the voltmeter is powered by the circuit being measured. I don't think it would be difficult to modify it for a three-wire connection, with the power supply and measurement input separated. The wires are about 18 cm long.


The board has an ME6203A voltage regulator with a 3.3 V output, but the specifications list an input range of 2.5–30 V.
The product title also mentions 40 V, but you can't apply that much: according to the datasheet, the regulator's maximum input voltage is 36 V, and its operating range goes up to 30 V. There's also another regulator with similar markings, but its input voltage only goes up to 8 V.
A separate chip handles the measurements, but its markings have been erased. There's also a trimmer on the board for calibration.


The display is soldered on crookedly, which you can see even in the photo. There are also various resistors underneath it.


The display is bright; in some situations it could even be too bright. Readability without a filter is so-so, and in photos it looks completely washed out because the flash lights up the inactive segments.


I tried a few different filters. With two red ones, you can barely see anything, which makes sense. The green one works, but it's a bit dim. I was already wondering what to do, since taking photos like this is almost impossible, but then I found a blue filter at home. It works perfectly, although a darker one might have been even better.


And of course, a few tests. First, I'll check the current draw and the voltage at which it stops increasing—in other words, the voltage at which the regulator starts regulating.
The voltmeter really does start up at about the specified 2.5 V, but because this one has a blue display, it only starts to light up at 2.7–2.8 V, and even then it's barely visible. As the input voltage rises, the brightness and current draw increase. At 3.9 V, the increase stops, and from then on the current draw simply depends on how many segments are lit.


Measurement accuracy check.
A test at 2.8035 V showed that the voltmeter reads high. I also tested it up to the maximum of 35 V, but in practice it's better not to use it above 30 V.


Since this has “hardware” calibration, I tried adjusting it a little. Surprisingly, it was quite convenient, because the adjustment is very smooth. Turning it clockwise lowers the reading; counterclockwise raises it. That's a little counterintuitive, but it doesn't really matter.
So I applied 30 V to the input and turned the trimmer until the voltmeter display showed the same reading. That was it.
It's best to calibrate it at around 20–30 V; it's easier to adjust that way.

It's almost perfect in the 15–30 V range. At lower voltages it reads a little low, but not critically so.


A few quirks showed up during testing and calibration.
1, 2, 3. The display shows 3.71, 3.72, and 3.73 V, even though the input voltage stays the same. In this case, it isn't the last digit drifting; it's something else. The voltmeter does have a floating-point display and supposedly measures to hundredths of a volt at voltages up to 10 V, but in reality it doesn't. As the voltage changes gradually, the readings switch between 3.50, 3.60, 3.70, and so on. Sometimes the last digit can change by 1–3 counts, but then the readings look like 3.51, 3.61, 3.71, and so on. So in practice, the voltmeter only has one digit after the decimal point; the second can show anything except the actual value. I never saw the digits 4–9 in the last place at all.
4, 5, 6. On top of that, the voltmeter has noticeable hysteresis: you can get a reading of 5.01 with 5.00 V at the input, or 4.91 over the 4.97–5.05 V range, depending on whether the voltage is rising or falling.

The last thing I wasn't too happy with is the low measurement speed, about once per second, though that doesn't matter for most applications.


To sum up, the voltmeters themselves are pretty good: they're inexpensive, draw little power, and are easy to calibrate. But their actual accuracy is only 1–2 digits after the decimal point; the hundredths digit they display is just an illusion.

That's about it for this little review. I hope you find it useful.