Using Cheap Asian Electronic Modules
By Jim Rowe
Mini Digital Volt/
Amp Panel Meters
There are many low-cost digital
panel meters available which
can display voltage and current
at the same time. Quite a few
have popped up on the market
in the last year or so. So let’s
take a look at some of the more
popular models, see what’s
inside them and whether they’re
easy to use.
T
here are a surprising number
of these low-cost digital panel meters currently available.
Many are quite similar to each other,
but a few are noticeably different.
This article will focus on a few of
the more popular and useful models.
We’ll be looking at the meters designed
to measure DC parameters this month
(ie, DC voltage and current), with a follow-up article to describe those which
make AC measurements.
The first is the DSN-VC288 from the
Chinese firm Geekcreit (we’ll be seeing
more of their products in later articles).
It is available in two versions: one
with a 0-10A current range using an
internal current shunt, and the other
with a 0-50A current range using an
external current shunt. Both versions
have a 0-100V voltage range.
The 10A version comes with two
plug-in connection leads for around
£2 from aliexpress.com, while the
50A version comes with both the leads
and an external 50A current shunt for
a similar price from aliexpress.com
The DSN-VC288 is quite small, at
48mm wide, 29mm tall and 22mm
deep. Some the suppliers describe it as
having a 0.56-inch dual LED display,
but that is misleading.
The three-digit seven-segment displays used for both voltage (red) and
36
current (blue) are each only 7mm or
0.28in high. Despite this, the displays
are quite readable. The display ‘window’ is 35 × 18mm.
Both versions of the DSN-VC288 can
be powered from a supply voltage of
4-30V DC, usually drawing less than
20mA. So if they are to measure voltages in this range, they can be powered
from the same voltage source.
The only thing to bear in mind is
that the DSN-VC288 can only measure voltages which are positive with
respect to its negative rail. That also
applies to current measurements.
Inside the DSN-VC288
The circuit of the DSN-VC288 is
shown in Fig.1. It’s all based on IC1,
an STMicro STM8S103F3 8-bit microcontroller. This runs firmware which
directs it to take voltage and current
measurements every 300ms or so, then
show them on volts display DS1 and
current display DS2.
Three-pin connector J3 at upper left
is used for both the meter’s supply input (V+ and V−) and its voltage measurement input (VIN). The V+ supply
input connects to the anode of diode
D1 and then to the input of REG1, an
ME6203 LDO (low drop-out) regulator,
which provides a regulated 3.3V supply for the rest of the circuit.
On the other hand, the VIN input
from J3 goes to the AN4/PD3 input
(pin 20) of IC3 via a 270kΩ/8.2kΩ resistive voltage divider, together with
VR1 (the voltage calibration trimpot)
and a 100nF filter capacitor across the
8.2kΩ resistor.
The meter’s ‘current’ input is via
two-pin connector J4, at lower left.
Here pin 1 (−) is connected straight to
the meter’s negative rail, while pin 2
(+) connects to the non-inverting input
of IC2b, via a low-pass filter formed by
a 330Ω resistor and 100nF capacitor.
IC2b is connected as a DC amplifier
with an adjustable gain between 23
and 25 using trimpot VR2, to calibrate
the current range.
Resistor RS connected across the current input pins of J4, shown in red, is
the internal current shunt. For the DSNVC288 version with the 10A current
range, RS is a 7.5mΩ (milliohm) resistor. In contrast, the DSN-VC288 version
with a 50A current range has no internal resistor RS, as the current shunt is
external, with a value of 1.5mΩ.
The only other thing to note about
Fig.1 is that ‘connectors’ J1 and J2 are
not physical connectors, but actually
a row of test points in the case of J1,
with the purpose of J2 unexplained.
Presumably, J1 is also used to program
IC1 at the factory.
Practical Electronics | December | 2021
Fig.1: circuit diagram for the DSN-VC288 digital
panel meter. The internal current shunt RS is
only fitted on the 0-10A current range version, the
alternative model with a current range of 0-50A uses
an external shunt.
Using the DSN-VC288
It’s easy to put the DSN-VC288 module to use, as shown in Fig.2. The first
two diagrams show the connections
for the version with the internal 10A
current shunt, with (A) showing the
connections when the module has a
separate power supply, and (B) showing the connections when it shares its
power supply with the load. (B) can
only be used when the load supply is
below 30V.
The other two diagrams show the
connections for the DSN-VC288 version with an external 50A shunt. (C)
shows the connections when the module has a separate power supply, while
(D) shows the connections for a shared
power supply. Again, it must be less
than 30V.
The two short (150mm) connecting leads which come with the DSNVC288 are distinguished by both their
size and their insulation.
The wires attached to the 3-pin
connector that plugs into J3 are thin,
while the two wires attached to the
larger 2-pin connector that plugs into
J4 are thicker.
But these four connection options
are not the only way that the DSNVC288 modules can be used. For example, if you want to measure lower
Practical Electronics | December | 2021
DSN-VC288 Mini Volt/Amp Meter
currents than their nominal 10A or
50A, you can do that.
Bear in mind that the current range
of the DSN-VC288 is really just a
0-75mV voltage range, with the firmware scaling this range to show the
current passing through the shunt.
So you can get a lower current range
by changing the shunt resistor value.
This is easier with the version using
an external 50A shunt, but it’s also
possible with the other version if
you’re careful.
For example, if you’d like to use
the 50A version to measure currents
between 0 and 50mA, replace the big
50A shunt with a 1.5Ω 0.1% resistor.
The meter’s scaling will then simply
provide current readings from 0-50mA
instead of 0-50A.
The same approach could be used
to give the meter current ranges
of 0-500mA or 0-5A, although the
decimal point will be in the wrong
position. If that doesn’t worry you
greatly, the shunt values to use would
be 150mΩ for 0-500mA, or 15mΩ
for 0-5A.
If you have the internal shunt version of the DSN-VC288, to change its
current range, you’ll need to remove
the internal 10A shunt. This is a stout
The underside of the 50A current range version of the DSN-VC288 module.
37
FIGURE 2
U-shaped wire soldered to the meter’s
PCB just to the right of J4, looking from
the rear. This is what you need to desolder to change the meter’s current range.
Since the internal 10A shunt has a
resistance of 7.5mΩ (providing 75mV
when 10A is flowing through it), the
scaling firmware in this version will
turn 75mV into a reading of ‘10.0’.
So you can change its current range
to 0-10mA by replacing the internal
shunt with a 7.5Ω resistor (ideally
with 0.1% tolerance).
Or again, you could give it a range
of 0-100mA by using a 750mΩ shunt,
or a range of 0-1A by using a 75mΩ
shunt. But in both cases, the decimal
point will be in the wrong position.
Testing
I ordered a couple of 50A versions of
the DSN-VC288 from Banggood and put
them through their paces. Both worked
exactly as claimed, with an operating
current of 20mA, a voltage measurement accuracy within ±0.1% and a
current measurement accuracy of ±1%.
In both cases, the readings could
be made ‘spot on’ compared with my
reference instruments using little trimpots VR1 and VR2.
So bearing in mind that the DSNVC288 is very compact and has relatively small readouts, it is very practical and useful, as well as being great
value for money!
The PZEM-051 meter module
The PZEM-051 is one of a range of
measurement modules made in China
by Ningbo Peacefair Electronic Technology, based in Ningbo City, Zhejiang Province.
It’s available from various suppliers
via online markets like AliExpress,
eBay and Amazon for between £8 and
£12, depending on whether you want
the 50A version or the 100A version.
There is a similar module with a 20A
current range available from Banggood
for around £12, designated the PZEM031 (https://bit.ly/pe-dec21-bg1).
Also, Banggood has another version
called the PZEM-015 (https://bit.ly/
pe-dec21-bg3), with extra displays including a bar chart display and measurements of battery capacity and internal resistance. That one comes with a
50A-300A shunt and costs just over
£18. (all the above are (sometimes) plus
delivery, which is usually inexpensive
when using Chinese suppliers.)
The common PZEM-051 is somewhat larger than the DSN-VC288, at
90mm wide, 50mm high and 25mm
deep. It has a display ‘window’ measuring 50 × 30mm, and the display is
an LCD with blue LED backlighting.
As you can see from the photo, it
offers four-digit displays of both voltage and current, plus two additional
four-digit displays: one for power (in
The external 50A 75mV shunt is in the foreground, with a
similar 100A shunt behind.
38
Practical Electronics | December | 2021
The current shunt story
The stout U-shaped wire (circled in
red) is what needs to be removed to
change the meter’s current range.
either watts or kW) and the other for
energy in either watt-hours (Wh) or
kilowatt-hours (kWh).
Other features include switching
the display backlighting on or off, resetting the energy indication to zero,
setting a voltage alarm level and configuring the PZEM-051 for use with
either a 50A or 100A current shunt.
These functions are changed using the
small pushbutton just to the right of
the display window, via various long
and short button press combinations.
The button is recessed slightly to
prevent accidental presses, and can
only be pressed intentionally using
a small screwdriver or stylus. All of
these settings are stored in non-volatile
memory, and are retained even when
the power is turned off.
The operating voltage range of the
PZEM-051 is 6.5-100V DC, and it can
measure voltages within the same
range. The current measurement range
is either 0-49.99A or 0-99.99A, depending on the version and the current
shunt. The power measurement range
is 0-10kW, with a display format of
0-999.9W for levels below 1kW, or 10009999W otherwise. Similarly, the energy
measurement range is from 0-9999Wh
for levels below 10kWh, or 10-9999kWh
for levels of 10kWh and above.
In the not-too-distant past, voltages and currents were measured
using ‘moving needle’ analogue
meters (ie, moving-iron and moving-coil meters). The current shunt
was developed to allow these meters to measure currents that were
higher than their basic sensitivity.
For example, if a meter needed
1mA to give a full-scale reading
(ie, 1mA FSD), it could be used
to measure currents up to say 1A
by connecting a low resistance
‘shunt’ across its terminals. The
resistance was chosen so that it
would carry 99.9% of the current,
leaving just 0.1% to flow through
the meter itself. This effectively
converted the 0-1mA meter into
a 0-1A meter.
Similarly, the meter could be
used to measure currents up to
10A by shunting it with an even
lower value resistor which would
carry 99.99% of the current, leaving just 0.01% to flow through the
meter itself. The current shunt
would conduct all of the current
at 10A, except the 1mA needed
for the meter to achieve full-scale
deflection (FSD).
The name ‘shunt’ comes from
railways, where a train is shunted
onto a parallel section of track,
just like how the current shunt
parallels the pre-existing current
path through the meter.
Working out the required resistance of the current shunt
was fairly easy, once you knew
the resistance of the meter itself,
and the fraction of the current
which needed to be diverted
past it. For example, if the shunt
needed to take 999 times the
meter current (999mA/1mA), it
would need to have a resistance of only 1/999 that of the
meter itself.
So if the meter had a resistance of 100Ω, the shunt would
need a resistance of 0.1001Ω or
100.1mΩ (100Ω ÷ 999).
In the same way, to take 9999
times the meter current, the shunt
would need to have a resistance
of 10.001mΩ (100Ω ÷ 9999).
So that was the purpose of current shunts back in the old ‘analogue’ days. But things changed
with the advent of digital meters.
Since these essentially respond
to voltage rather than current, the
role of current shunts needed to
change as well. Instead of just
taking the major proportion of the
current, they became a currentto-voltage converter.
Their resistance value is chosen to cause minimal disturbance to the circuit in which
the current is flowing, while still
providing enough voltage drop
to allow accurate measurement.
And the voltage level chosen
was 75mV (millivolts), so most
modern digital meters are designed to have this full-scale
voltage sensitivity on their current ranges.
It is still relatively easy to work
out the resistance value of a
shunt for any particular current
range. For example, if a meter
needs a 0-10A current range,
the shunt value required would
be V/I or 7.5mΩ (0.075V ÷ 10A),
according to Ohm’s famous law.
Or if you wanted to give the same
meter a 0-1A current range, you’d
need a current shunt with a value
of 75mΩ (0.075V ÷ 1A).
So that’s the function of a
current shunt nowadays – to
provide a small but accurately
measurable voltage drop when
a particular current is flowing
through it.
Front and rear views of the PZEM-051 module. As shown by the label on the back, this meter has a voltage range of
6.5-100V DC and a current range from 0-50A or 0-100A depending on the external shunt used (see opposite).
Practical Electronics | December | 2021
39
www.poscope.com/epe
An inside view of the PZEM-051 module. The main controller for this board is a
Mixchips MXM11P62 (U3; lower middle) which is an 8-bit microcontroller.
- USB
- Ethernet
- Web server
- Modbus
- CNC (Mach3/4)
- IO
- PWM
- Encoders
- LCD
- Analog inputs
- Compact PLC
- up to 256
- up to 32
microsteps
microsteps
- 50 V / 6 A
- 30 V / 2.5 A
- USB configuration
- Isolated
PoScope Mega1+
PoScope Mega50
- up to 50MS/s
- resolution up to 12bit
- Lowest power consumption
- Smallest and lightest
- 7 in 1: Oscilloscope, FFT, X/Y,
Recorder, Logic Analyzer, Protocol
decoder, Signal generator
40
I couldn’t find a circuit diagram for
the PZEM-051, but once the 100A version I ordered from AliExpress arrived,
I carefully opened its case to take a
look inside.
As you can see from the internal photo, there is not a great deal in it. At its
heart, there’s a Mixchips MXM11P62
8-bit microcontroller (U3) with 14KB
of one-time programmable ROM, 256
bytes of SRAM, an ADC with 24-bit
resolution, 18 bidirectional I/O pins,
three 8-bit timers and a UART.
There’s also a Holtek HT1621B LCD
interface chip (U2) which links the
MCU to the four 4-digit displays on
the LCD, and a K24C02 (U4) two-wire
serial EEPROM, which is presumably
used to store measurement and display
settings. So the design of the PZEM-051
is quite elegant.
Trying it out
Using the PZEM-051 is just as easy as
the DSN-VC288, as you can see from
Fig.3. The two uppermost screw terminals need to be connected to the
voltage/power source, while the two
lower terminals are connected to the
ends of the current shunt. The two inner terminals must be connected to the
negative side of the power source and
the current shunt, respectively.
Note that the screw terminals are
located at the rear of the PZEM-051, at
the left-hand end. They’re shown at the
front in Fig.3 purely for clarity.
I measured the PZEM-051’s voltage
readings as 0.16% high, while the current readings were just over 2% high.
The latter may be due to the current
shunt tolerance.
There was a pleasant surprise when
I measured the meter’s own current
draw, which was just below 3mA with
the backlight switched on, falling to
around 1mA when it was switched off.
Therefore, despite its extra functions,
the PZEM-051 is much more energy-efficient than the DSN-VC288, due to the
use of an LCD rather than LED screen.
To summarise, the PZEM-051 multifunction DC measurement module can
be described as both extremely useful
and decent value for money.
Coming up
As mentioned earlier, a future follow-up
article will describe some of the newer
AC-measurement meter modules.
Reproduced by arrangement with
SILICON CHIP magazine 2021.
www.siliconchip.com.au
Fig.3: a simple example of how you can use the PZEM-051 meter to measure DC
power, voltage, current and energy consumption.
Practical Electronics | December | 2021
