Another "handy little gadget": a compact voltmeter with a measurement range up to 99.9 V

Continuing the August 25 shopping roundup, I have to mention a rather useful and inexpensive item: a small voltmeter. A friend of mine picked it out, but I ordered one for myself too, just to have around.

As I wrote in the introduction, I ordered the voltmeter just because. Sometimes you need a tiny voltmeter, and offline shops usually charge a pretty hefty price for them. You may not need one often, but it still feels bad to overpay.
Anyway, here's another mini-review :)

I bought them for $1.2 each. The seller is running a sale now and they're $1.25, but I got the impression that “sale” is more of a state of mind for this guy.

The seller didn't bother with packaging: he tossed two little bags of voltmeters into a regular thin plastic mailer and took them to the post office. There wasn't a hint of bubble wrap or anything like that.
But surprisingly, everything arrived intact—or at least it looked that way.


According to the sticker, they were made on August 22, three days before the sale.
Honestly, I can just picture it: the middle of the night, a Chinese guy sitting there soldering voltmeters, when his lovely wife comes up behind him and says—
-Honey, let's go to bed. It's late.
-No, I still have 100 more to solder. There's a sale in three days; maybe someone will buy something.
Or maybe (dreamily) someone will write about my product on Muska :)

They will, they will, don't worry :)))

By the way, the seller shipped them almost immediately, so credit where it's due.


When I unpacked them, my first impression was—what a mess how tiny they are: a couple of small boards with displays.


The boards are 35 mm long, and the digits are 9 mm high.


Just in case, I made a little dimensioned drawing; the proportions are accurate.
There are a couple of holes on the board, about 2.8 mm in diameter. The board is about 9 mm tall overall.


I got the three-wire version of the voltmeter, with separate power and measurement connections.
The thing is, these voltmeters are designed to measure up to 100 volts, and models with a separate power supply are more common in that range.
But there's nothing stopping you from connecting the power wire to the measurement input; the tradeoff is that the upper limit drops to 30 volts.


There's nothing interesting on the underside. The soldering is neat, so no complaints there. The wire connections are labeled, and even if the wires break off, it's easy to solder on new ones.
The RX and TX pads are visible on the left; they're probably used for calibration.
I didn't desolder the display, since it's already clear that there's a microcontroller underneath, a 3.3 V regulator (presumably), and a handful of resistors.


Since I don't have much else to say, let's move on to testing.
To start, I connected the measurement lead to the power lead, since I was testing in the range up to 30 V.
The voltmeter starts up at around 3.7 V, but reads high at that point.
That's normal, really.
To make the readings easier to see in the photo, I added a red filter.
I used two voltmeters in the test, and their readings were identical. When the voltage was changed slowly, the difference between when they switched was about 0.01 V or less.
The test showed that both voltmeters start working properly when the supply voltage is above 4.5 V. It was also immediately clear that both read slightly low, by about 0.07–0.1 V.


I tested them up to 27.8 V. Across the entire range, both read low by about the same amount, around 0.1 V.


While I was at it, I measured the voltmeters' current consumption and was pleasantly surprised. Depending on how many segments were lit, the two together drew 20–26 mA, or 10–13 mA each.
That's a pretty good result, if you ask me.


Since the voltmeters are rated to measure up to 100 V (99.9 V in practice), I didn't stop at 27 V and decided to test them further.
This time, one power supply powered the voltmeters while I measured the output voltage of another.
Above about 35 V, things changed: both voltmeters started showing exactly the same reading as the multimeter.


The power supply whose voltage I was measuring tops out at 60 V, so I stopped there.
At 60 V, the readings also matched the multimeter exactly.
This suggests that the problem in the lower range is probably not with the voltmeter's divider, but with calibration accuracy.
As far as I know (though I could be wrong about such tiny devices), these things are calibrated at two voltages, and then the microcontroller works out the correction factors itself.
It's quite possible that during calibration of the lower range, the calibration voltage was 0.1 V too high, so the calibration ended up offset by that amount, while the voltage used for the upper range was correct.


A quick summary.
Pros of the product: good build quality, low power consumption, satisfactory performance, and a low price.
Pros of the seller: fast shipping.

Cons of the product: readings are about 0.07–0.1 V low in the 4–35 V range.
Cons of the seller: basic packaging and no tracking number (though that's understandable at this price).

I think these are decent, inexpensive voltmeters. I still don't know where I'll use mine; it'll sit around waiting its turn. I'd say they're worth buying and will come in handy. Another little useful gadget, all in all :)

I hope this mini-review was useful. I think that even if the seller raises the price, it probably won't stay up for long.