Small, simple 60 V 10 A voltmeter/ammeter

I came across a small voltmeter/ammeter board on Taobao and decided to order one and give it a try. Turns out I made the right call—but more on that in the review.


This board is also available on AliExpress, but the $13 price didn't really appeal to me. Even after factoring in the agent's fee and shipping within China, I got two boards for about the price of one.
The Taobao listing offers three options:
1. Board without display — $3.92
2. Display — $0.87
3. Board with display — $4.79
Shipping within China is $1.88, but stays reasonable regardless of quantity, so it's definitely worth ordering at least a couple. Besides, many people already have 1602 displays lying around, or can get one cheaply on AliExpress, so there's no point in ordering one from Taobao.


Here are the specs from the product page, in my own loose translation:
Supply voltage — 4.8–5.3 V
Current consumption with display (measured) — 30–35 mA
Voltage measurement range — 0–60 V
Voltage measurement error — ±0.2% + 0.2% FS
Current measurement range — 0–10 A
Current measurement error — ±0.4% + 0.3% FS
ADC counts — 65,535 (16-bit)
Update rate — 5 Hz; in reality, probably closer to 3 Hz, though I didn't measure it specifically
Dimensions, excluding display — 36 × 50 mm

I'm not used to seeing additive error specified this way (FS). Usually, they just state the possible error in the least significant digit. Here, as I understand it, they mean the same thing, but as a percentage of the full-scale value. Either way, the tests will tell me more.


The seller ships the kits in compact little bags; the larger bag is from the forwarding agent. That's a plus in this case, since it reduces the parcel's overall weight and size—very handy for topping up larger orders. The two boards together weighed just 39 grams.


In my case, each kit consists of the voltmeter/ammeter board and mounting hardware. For some reason, one bag had five screws instead of four—probably a mistake. They also include two standoffs that attach the display to the board.


The dimensions are listed above, but here's an easier way to picture them: the board is exactly the size of a standard matchbox in all three dimensions.


At first glance, the board looks completely basic. That's what I thought when I ordered it, too, but in reality, it turned out to be better than it looks.
Unlike most small voltmeters/ammeters, which are built around just a microcontroller, this one uses a separate 16-bit, dual-channel ADC.

There's also a CJ431 voltage reference on the board (most likely a TL431 equivalent), which I think could reduce measurement accuracy somewhat, and a trimmer for adjusting the display contrast.


It's not exactly the best you could hope for, of course, but it's definitely better than the 10-bit ADCs built into cheap microcontrollers.
Link to the datasheet.


A wire shunt is located near the terminals. The terminals are labeled, so overall it's just like a regular voltmeter/ammeter.


The shunt resistance is very low, around 4 mΩ, measured from the back of the board. There's no op-amp for the shunt, though—the ADC does all the work. That's not ideal and could cause problems when measuring low currents, especially with such a low resistance. On the plus side, this shunt has very little effect on the circuit: even at the maximum current of 10 A, the voltage drop is only a little over 40 mV.


There are no components on the underside, but it does have a brief spec label. Oddly, it says the shunt voltage is the standard 75 mV. That suggests the shunt itself is probably rated for 20 A, though the board was originally designed for a 10 A shunt with a 75 mV drop at its rated current.


Radio amateurs often have one or two 1602 displays lying around at home—bought for some project but never used, salvaged from other devices, and so on. The same thing happened to me, but it turned out my display had a female header soldered on, while this board needs pin headers. Then I remembered there was one in my frequency generator, so I borrowed it for a while.
Keep in mind that some displays have a current-limiting resistor right on the display board. These are usually newer models with side lighting from a single bright LED. Older displays have no resistor on the board and use several LEDs instead (more common with Winstar). This board is designed for the first type of display. AliExpress listings most often show the first type, but it’s best to check.


Once assembled, it makes a fairly rigid structure. It can be attached to the front panel using standoffs like these, which is very convenient.


The photos in the store listings on both Taobao and AliExpress show the old version of the board (V 2.63), which is red. Mine is the newer version (V 3.1), with a blue PCB. I don’t know what the differences are; they look very similar.


When power is applied, a welcome message appears briefly, then the instrument switches to operating mode. If you turn it on while holding down the button, it enters settings mode.


Unfortunately, the manual is only in Chinese. You can try translating it with Yandex Translate, which is why I’ve included this screenshot. But, briefly, here’s what it says:
There are four modes: two display modes and two calibration modes.
Maximum displayed capacity: 999.99 Ah
Maximum displayed time: 99 hours, 59 minutes, 59 seconds.


To enter the settings menu, hold down the button while powering on. After you release it, four options will appear; cycle through them with short presses. Press and hold to select a mode and save it in the first two cases.
1. Display mode: voltage, current, power, and equivalent load resistance. Holding the button in this mode freezes the readings for as long as the button is held down.
2. Voltage, capacity in Ah, current, and time. A short press starts the time and capacity counters; a long press resets the time and readings.
The modes are saved, so the next time you turn it on, it will use the mode that was active before it was switched off.

3. Fine adjustment mode for the voltmeter. This isn’t really calibration, just a small correction to the voltmeter readings. I haven’t tried it; the description mentions 1/10000.
4. Calibration. Here you need to check it in sequence at zero current, then at 5 V, and finally at 3 A. The voltage and current must be set very accurately, since they serve as calibration references.


Even if you don’t know Chinese, you can get a rough idea of the calibration process from the English text.


The wiring diagram is no different from those for other ammeters/voltmeters. It also lists some additional specifications:
Maximum displayed power: 4 kW, although the instrument itself can measure 60 × 10 = 600 W.
Equivalent resistance: 0–10 kΩ.

Interestingly, the diagram specifies a measured voltage of 40 V. Apparently, that was carried over from some old versions.


Voltage measurement accuracy was checked over a range from 47.7 mV to 62.34 V. To save you from looking through all the pictures, I’ll just say that at the upper end, the difference between the actual and measured voltage was 70 mV, or about 0.1%. That’s excellent compared with the specified 0.2%, especially since you can try to correct even this small error without affecting the calibration.
The last photo shows that the instrument can measure above 60 V too. I think its maximum is 65.535 V, corresponding to the ADC resolution.


Current measurements started at values close to zero. At 4–5 mA, the readings are unstable and “bounce around” in a range of roughly 0–6 mA.


At 10 mA and above, the readings are relatively stable, fluctuating by ±1 least significant digit.
The measured current is also slightly higher than the actual value, just like the voltage, but it still falls within 0.2% error. The specified error is 0.4%, so the results here are pretty good too.

Judging by the results, it looks like the voltage reference has drifted slightly, since both the current and voltage readings are high by roughly the same amount.


Here are the test results in a table. The error shown is relative to the measured and actual values, without taking the full measurement range into account. I also haven’t included the additional error (that same 0.2–0.3% FS), so technically I put the instrument through tougher conditions, and the actual result is significantly better.
The table shows that the near-zero reading occurs in the 10–15 V range for voltage and 0.5–1 A for current, although logically it should be around 5 V and 3 A, respectively.


This is less a check of accuracy than a check of how stable the calculated results are. The voltmeter input was connected to the point where the tester probe meets the wire leading to the shunt. In other words, the measurement included the voltage drop across the shunt plus the drop across the wire, and the equivalent circuit resistance was calculated from that. The test was performed at four current levels, and the calculated resistance varied by just 2 mΩ.


The final test checked the accuracy of the capacity calculation. In this case, though, I was checking not so much the calculation itself as the accuracy of the timekeeping, since time is one of the key factors here. Ah = current (amps) multiplied by time (hours).

Over 40 minutes, the ammeter-voltmeter clock counted 39 min 50 sec, or 15 seconds short of an hour. Accordingly, the current measurement error is 0.4%. Put more simply, charging a 1 Ah battery will show 1.004 Ah (not accounting for current measurement error).
That seems acceptable to me, but of course I’d like the clock to be more accurate. 15 seconds per hour is still quite a lot.


I first spotted this ammeter-voltmeter on Taobao about a year ago and even wanted to buy one, but kept forgetting. The product page includes a photo showing examples of how it’s used, which suggests it’s quite popular, but for some reason it’s not very well known here.


To sum up, I really like these little boards. In my opinion, they’re genuinely better than the well-known ammeter-voltmeters, both in accuracy and functionality. Also, if you have a spare 1602 display lying around, you can save a little money. Another bonus is that you can use a larger 1602 display, and there’s also the option of fine adjustment and calibration.
I also like the convenient voltage and current measurement ranges. It should work well with the linear power supply I showed some time ago. That one was rated for up to 20 A, but I doubt many people would build one for that much current; 10 A is much more realistic.
I’m not even sure what to criticize. If I’d bought it on AliExpress, I’d complain about the inflated price, but as it is, the price is excellent.



That’s all. I hope this was useful. As always, I’ll be glad to answer questions and read your comments.