A small ammeter rated for up to 5 A

I ordered this meter quite a while ago from a different store, but while browsing AliExpress I found a listing for what looks exactly like the same device. Since I'm almost certain they all come from the same basement are made by the same large manufacturer, here's a link to another store.
I'll say right away that I wasn't impressed with it, which is partly why I decided to write this mini-review.


I've covered various small meters several times before, mostly voltmeters and universal meters. Voltmeters are more popular, but ammeters are useful too, from time to time.

The ammeter arrived in a small bag with the main specifications printed on it:
1. Supply voltage: 4.5–28 V
2. Current measurement range: 0–5 A
3. Display color: red (green and blue are also available)
4. Model: C20D


The ammeter is just a small board with a display; no enclosure is included.


The wires are about 20 cm long, but quite stiff.


The power wires are soldered directly to the board, while power is connected through a connector. I bought the version with a red display. The display measures 23 × 14 mm, and the digits are 9 mm tall (0.36 inches).


All the components except the display are mounted on one side of the board. You can see the controller with its markings removed, an op-amp (also with its markings removed) for the shunt signal, the 5 mΩ shunt itself, a 3.3 V regulator, and a few other small components.
The good thing is that it uses a low-resistance shunt: at 5 A, the voltage drop across it is just 25 mV. Including the board traces, the total resistance is 6.5 mΩ, so the total voltage drop across the meter is about 32 mV.


The specifications say the meter's minimum supply voltage is 4.5 V. In practice, I found that:
1. It starts up at 2.4 V.
2. At 3 V it works and draws about 7 mA, but I wouldn't trust its readings at that voltage.
3. Once the voltage is raised to 4.3 V, the current draw stops increasing and the regulator reaches its normal operating range.
4. The current draw doesn't change between 4.3 and 28 V.


The first test was done with a separate power supply set to 12 V, connected according to the diagram.


But the test results weren't very encouraging, which is why I wouldn't recommend this device.
At currents below 1 A, the readings are much too low, especially in the 0–600 mA range. They improve noticeably above that, but at 3.5–5 A the meter starts reading high.
The calibration being off wouldn't be such a big deal on its own; there's a trimmer on the board for calibration. The problem is that readings are low below 1 A and high above 3.5 A, so adjusting the trimmer is no use.


Although the specifications list a maximum current of 5 A, you can actually measure more. But even my adjustable power supply is more accurate in setting and measuring current than this “number display,” unfortunately.


The second test was just for curiosity. This time, both the meter and the load were powered from the same source. I used a 10 Ω resistor as the load and varied the voltage from about 4.4 to 25 V.


The result is roughly in line with the previous test.


While putting this review together, I was looking for other places that sell this ammeter and stumbled across a website featuring antique instruments. I couldn't resist showing them here too; I really liked them.


The previous ones were selling for around $35–150, but this one costs two orders of magnitude more: $3,500. I was a little shocked by the price. I even wondered if they had raised it from the Titanic :)


The instrument is in pretty poor condition, of course, but apparently that's part of the appeal.


That's about it. I wasn't very impressed with the test results. In practice, the meter is usable in the 1–4 A range: below that it reads low, above it reads high. The trimmer won't fix this, and I don't know how to apply software correction.
I probably won't use it; most likely I'll just toss it in a drawer, just in case. Or perhaps for something very basic, like displaying the charging current on a car battery charger. I have one lying around at home, and that application doesn't need much accuracy; the measurement range is just right, too.