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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 $4.12 to $15.50 plus delivery. We obtained the unit shown in the photos via AliExpress from a supplier called AGUHAJSU Global Purchase Store for $4.12 plus shipping (a total of just over $6). 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. siliconchip.com.au Australia's electronics magazine 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 October 2024  63 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. 64 Silicon Chip Australia's electronics magazine siliconchip.com.au 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. SC siliconchip.com.au There’s nothing of importance on the underside of the module, although this is the only place that the output polarity is clearly indicated. Australia's electronics magazine October 2024  65