Using Electronic Modules: 1-24V Adjustable USB Power Supply (February 2025)
Using Electronic Modules: 1-24V Adjustable USB Power Supply (February 2025)
1-24V Adjustable USB Power Supply
Using Electronic Modules with Jim Rowe
1-24V Adjustable USB
Power Supply
The “Zk-DP” is a surprisingly
inexpensive supply module that
converts 5V DC from a USB port
into any DC voltage between 1V and 24V at up
to 3W. It features a three-digit LED display showing the output voltage, plus
easy adjustment of the output voltage with a built-in multi-turn potentiometer.
A
lot of small electronic devices
now run from low-voltage DC.
Luckily, many can run from a 5V DC
supply, so they can be powered from
a USB port on your computer, a standard 5V USB mains power supply or a
portable battery bank.
But things are not so easy if a device
needs a supply of 9V, 12V, 15V, or 24V
DC (or another ‘oddball’ value). Usually, you must provide a separate power
supply or plugpack to deliver the required voltage.
In those cases, it would be handy to
have a small, low-cost power conversion device that could take the power
from a standard 5V USB power source
and convert it into one of those other
voltages.
That’s precisely the function of the
module we’re looking at this month.
It plugs directly into a USB-A socket
providing 5V DC and can then power
a device at any voltage between 1V
and 24V DC. Despite its small size, it
can supply in excess of 3W of power
at any of those output voltages, eg,
250mA <at> 12V.
Setting the desired output voltage
is very easy, using a built-in multiturn potentiometer with an attached
knob and a tiny three-digit LED display that indicates the current output
voltage.
From the legend on the PCB, it is
called the Zk-DP Desk Power module,
although it would also be correct to
call it a DC/DC voltage converter. It’s
currently available from several online
marketplaces at prices ranging from
about £2 to £8 plus delivery.
We obtained the unit shown in the
photos via AliExpress from a supplier
called AGUHAJSU Global Purchase
Store for £2.11 plus shipping (a total
of just over £3). We noticed that the
Fig.1: a block diagram for the Zk-DP power supply module. Note that we
have not included values for the resistors and capacitors.
Practical Electronics | February | 2025
same unit is also available from eBay.
The Zk-DP module is 70mm long,
26mm wide and 14mm tall (not including the spindle of the voltage adjustment pot). All the components are
mounted on a small PCB that’s 52.5mm
long and 21.5mm wide. The USB-A
input plug is at one end of the PCB,
while the voltage adjustment pot and
small 2-way output screw terminal
block are at the other.
All the electronics are housed in a
snap-together clear blue plastic case,
which allows the 3-digit output voltage indication to be easily read through
the case.
How it works
Some searching on the internet
didn’t reveal any circuit details of the
Zk-DP. Still, I was able to remove the
PCB from the case and glean enough
information to produce the block diagram (Fig.1). I was not able to determine the type of microcontroller
used as the ID marking on the top of
its 20-pin SSOP package had been
removed.
The five-pin SIL onboard programming header suggested it might be a
Microchip product. However, when
we compared numerous AVR and PIC
microcontrollers in that package to
the pinout used on the board, none
matched, so it’s probably something
else. Luckily, the SX1308 voltage converter chip still had its ID on top of its
6-pin SOT-23 package.
This device, shown just above the
centre of Fig.1, is designed as a boost
converter. However, it is being used
in a slightly different configuration
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Low-cost Electronic Modules
These photos show the rear end and general view of the module with the
supplied blue plastic case. Note that there is not a cut-out for the 3-digit segment
display. Both photos are shown enlarged for clarity.
here, known as a SEPIC converter
(single-ended primary-inductor converter). This has a similar function to
a buck-boost converter but requires
just one switching element instead
of two. The operation is described at
https://w.wiki/9DjN
An ordinary boost converter (eg,
as shown in the SX1308 data sheet)
would have a series diode from pin 1
of U2 directly to the output. However, that would mean the output voltage could never go below 5V because
there would be a direct path for current to flow from USB +5V through L2
and that diode to the output.
Basically, the series capacitor AC
couples the switching waveform to
diode D1 so that there is no longer
a constant path for current to flow,
allowing the output voltage to be
regulated below the input as well
as above it.
The other inductor, L1, keeps the
load current flowing when the internal
switch in U2 is closed and no current
flows through the series capacitor to
the output. That means the output filter
capacitor does not have to supply the
entire load current during this time,
significantly reducing the output voltage ripple.
The SEPIC configuration is related to the Ćuk converter (https://w.
wiki/9Db2), except that the positions
of the diode and second inductor (L1
here) are swapped. Thus, SEPIC gives
a non-inverted output voltage compared to the input. In contrast, the Ćuk
converter produces a negative output
voltage from a positive input.
The SX1308 runs at a fixed switching frequency of 1.2MHz and uses an
internal power Mosfet (with its drain
connected to pin 1) as a low-loss switch.
The output voltage is adjusted by varying the voltage divider ratio to send a
proportion of the output voltage back
to pin 3 of the SX1308, its FB (feedback) input.
U2 varies the Mosfet duty cycle in
response to changes in the feedback
voltage. With a 50% duty cycle, the
output voltage is similar to the input
voltage of 5V. Higher duty cycles allow
the output voltage to go above 5V, while
lower duty cycles result in an output
below 5V.
The conversion efficiency is quite
high because the power Mosfet inside
the SX1308 has an on-resistance of
only 80mW (80 milliohms). For example, when configured as a boost converter and converting between a 5V
input and a 12V output, its efficiency
for load currents between 100mA and
400mA is better than 92%.
The microcontroller’s main job in the
Zk-DP module is to measure the output
voltage and show it on the small 3-digit
LED display. The LED digits are 6mm
high and are quite readable.
Trying it out
After connecting the Zk-DP module
to a bench power supply capable of
providing well over 3W, I also fired
up my bench DMMs and connected
them to the module’s output. I used
one to measure the module’s output
voltage, while the other measured the
current it delivered to a programmable
DC load. I used a third DMM to monitor the input voltage to the module.
Using this setup, I could test the
module’s performance at various output
voltages for a range of output currents
at each voltage level. The results are
summarised in Fig.2.
The red horizontal lines show the
module’s output current at the nine
voltage settings I used for testing: 24V,
18V, 15V, 12V, 9V, 7.5V, 5V, 3.3V and
2.5V. The dashed pink curve shows
the module’s rated maximum output
power of 3W.
An example photo
showing what the
voltage display
looks like when
powered on, here it
is supplying 15.0V.
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Practical Electronics | February | 2025
1-24V Adjustable USB Power Supply
The output voltage at each setting
remained essentially constant for current levels beyond that corresponding
to 3W of output power; there was no
‘drooping’ on any of the voltage plots.
The voltage level at the 24V setting remained within 30mV up to a load of
200mA (4.8W!), while the level at the
18V setting was within 45mV up to
300mA (5.4W).
The voltage at the 15V setting remained within 3mV at loads up to
300mA (4.5W); at the 12V setting, it
remained within 5mV at loads up to
300mA (3.6W); at the 9V setting, it
remained within 5mV at loads up to
400mA (3.6W); and at the 7.5V setting,
it remained within 25mV at loads up
to 500mA (3.75W).
Its output voltage held up similarly
well at the 5V and lower voltage settings, so you can see why the plots in
Fig.2 are all shown simply as horizontal lines.
Although I tested the module’s performance beyond the 3W limit, that
was only for brief periods. I would
not recommend using the module to
deliver more than 3W for more than
short periods to prevent it from overheating and possibly being damaged.
The next test I ran on the module
was to check the accuracy of its LED
voltage display at various output voltage settings. Here again, it performed
well, as shown in Fig.3.
The readout error was highest at
2.5V, at +1.2%, then varied between
-0.2% and +0.8% before rising to
+0.5% at 12V, then falling to -0.3%,
-0.1% at 18V and 20V, and then to
-0.6% at 24V. So, using the module’s
LED display to set its output voltage
gets you pretty close.
The error percentages provided are
best-case values, an additional error
of up to 100mV is possible due to display rounding.
Fig.2: this graph shows how the Zk-DP power supply module performed at
various voltages for different output currents.
Fig.3: this graph shows the difference between the selected output voltage and
the voltage displayed on the 3-digit segment display.
Conclusion
There is little more to say about this
tiny low-voltage voltage conversion
module. It is nicely made, performs
surprisingly well and carries a very
small price tag.
You could use it to power a small
breadboard during development from
a conveniently nearby computer, or
any other time you need a stable DC
voltage at a modest current level.
Adding it to a USB power bank makes
a handy portable, adjustable DC voltage source.
PE
Practical Electronics | February | 2025
There’s nothing of importance on the underside of the module, although this is
the only place that the output polarity is clearly indicated.
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