#433 Adjustable Power Supply Kit
Building and testing a popular AC-powered 30V/3A adjustable power supply kit, with enclosure, fan, voltage and current display.
Notes
I purchased “0-30V 2mA-3A Adjustable DC Regulated Power Supply DIY Kit Short Circuit Current Limiting Protection For School Education Lab” (aliexpress seller listing) for US$4.54 (Sep-2017).
NB: it is no longer offered by this seller, bu the same product “WON 0-30V 2Ma-3A Adjustable DC Regulated Power Supply DIY Kit Short With Protection” (aliexpress seller listing) is currently offered for SG$5.75 (Jan-2026).
The kit is widely available from most hobby electronics markets (aliexpress, Banggood, eBay). It features continuously variable output voltage, and a variable current limit with overcurrent indicator/shutdown.
It appears this design may have originated from SmartKit in Greece, improved by various people, and at some point the “canonical design” was picked up for mass production (instantly identifiable by the red PCB and tall cap).
I’m certainly not the first to build the kit - it’s a common project, just search YouTube for 0-30V power supply - for example this build by Boris Dusnoki:
Kit Specifications
- Material: PCB
- Color: Red
- Input Voltage: 24V AC
- Input Current: Max 3A
- Output Voltage: 0-30V (continuously adjustable)
- Output Current: 2mA-3A (continuously adjustable)
- Output Voltage Ripple: Max 0.01%
- Assembled Item Size: 9x8.5x3.5cm / 3.5x3.4x1.4in
- Package Size: 15x11x2cm / 5.9x4.3x0.8in
- Package Weight: 86g / 3oz
See the installation_manual.pdf.
Parts
| Qty | Refs | Description | Note |
|---|---|---|---|
| 1 | 0.47Ω 5w | ||
| 1 | 33Ω 1/4w | ||
| 1 | 82Ω 1/4w | ||
| 1 | 220Ω 1/4w | ||
| 2 | 1kΩ 1/4w | ||
| 1 | 1.5kΩ 1/4w | ||
| 2 | 2.2kΩ 1/4w | ||
| 1 | 2.2kΩ 1w | ||
| 1 | 3.9kΩ 1/4w | ||
| 1 | 4.7kΩ 1/4w | ||
| 5 | 10kΩ 1/4w | ||
| 2 | 27kΩ 1/4w | ||
| 2 | 56kΩ 1/4w | ||
| 1 | 270kΩ 1/4w | ||
| 2 | B10kΩ variable pot | ||
| 1 | 100kΩ trim pot | ||
| 2 | 100pF | ||
| 1 | 330pF | ||
| 1 | 100nF | ||
| 1 | 220nF | ||
| 1 | 10µF 50V | ||
| 2 | 47µF 50V | ||
| 1 | 3300µF 50V | ||
| 1 | 1N4004 | ||
| 4 | 1N4148 | extras supplied | |
| 4 | 1N5408 | ||
| 2 | 5V1 | ||
| 1 | 3mm white LED | replaced this with a red LED in the final build | |
| 1 | L7824 24V 1A Positive Regulator | ||
| 1 | 9014 NPN Small Signal Transistor | ||
| 1 | 9015 PNP Small Signal Transistor | ||
| 1 | D882 NPN Power Transistor | ||
| 1 | D1047 High power NPN bipolar transistor | ||
| 3 | TL081 JFET Input Op Amp | ||
| 1 | 2-pin terminal block | ||
| 1 | 3-pin terminal block | ||
| 1 | heatsink | ||
| 2 | XH2.54 3P female plug and wire | ||
| 2 | XH2.54 3P male PCB socket | ||
| 1 | 2 pin header | ||
| 4 | M3 screw | ||
| 22pF | extra 3 in the kit for some reason! |
Construction
The board is an easy build. At this point I tested it to verify operation. Note that it requires an AC supply to operate correctly.
How It Works
More information can be found in various places, such as:
Here’s a partial schematic of the main power supply circuit. I’ll probably tidy and complete this later (e.g. it does not include the 24V fan power supply).
Enclosure and Additional Hardware
To trick this out as a finished bench power supply, I opted for the following:
A 6.7” x 5.1” x 3.1” blue metal project case: “Cheapest 6.7” x 5.1” x 3.1” Blue Metal Enclosure Project Case DIY Junction Box” (aliexpress seller listing) purchased for US$7.82 (Apr-2019)
Mains power supply via fused socket and switch: “High quality Black Red 10A AC 250V 3 Terminal Power Socket with Fuse Holder Rocker Switch NEW AC Power Cord Inlet Plug Socket” (aliexpress seller listing) purchased for US$0.84/2 (Nov-2018).
A 15W, 24VAC step-down transformer
A Volt/Ammeter display for the front panel. I’ve covered this component in detail in LEAP#289 DC Voltmeter/Ammeter Modules It is powered from the 24V fan power supply, in parallel with the fan.
I’ve mounted a fan on the side of the unit, without any additional heat-sink on the main power transistor: “40mm 4cm 2 Pin 9 Blades Computer Chipset Cooling Fan DC 24V” (aliexpress seller listing), purchased for US$0.98/2 (Jul-2018).
It turned out to be a noisy bugger. My power requirements are minimal for now, I will probably re-assess in the future and make some temperature measurements under load.
For the front panel power connectors, I’ve used a nice dual banana binding post: “High Quality Speaker Amplifier Dual Binding Post Banana Plug Socket Connectors 5pcs” (aliexpress seller listing), purchased 5 pieces for US$3.83/lot (Nov-2018).
Finally a metal bezel for mounting the current-limit LED on the front panel: “10pcs 3mm LED Emitting Diode Metal Bezel Holder Round Chrome LED Lamp Base with 10pcs Plastic Cover” (aliexpress seller listing) purchased for US $0.62/10 (Sep-2018).
Final Build
Controls:
- Output voltage setting: red knob
- Current limit setting: blue knob
Calibration
There are three calibration points for my build: zero volt offset for the regulator, and voltage/current adjustment for the panel meter.
The VR1 (blue pot) on the PCB is to zero the voltage offset of the power supply:
The meter has adjustments for voltage and current on the rear of the unit. With a 50Ω load (actually 49Ω according to my DMM), I’m reading 0.19A at 11.8V according to the panel meter. The voltage is spot on, but the current is off from the expected ~0.23A, and I later made the adjustment to get this within 10mA of actual.
Over-current Test
The current-limit setting (blue knob in my build) works as expected. When current limit is exceeded, the output shuts off:
Final Comments
This is a nice kit and a fun build, but a couple of draw-backs:
- the voltage output is only as accurate as your meter, and fine adjustment is tricky. I’ve seen build variations that include course and fine voltage adjustments; this would be a good variation on the build (two pots or large and small resistance in series would work fine)
- the current-limiting feature works well, but suffers from the fact that there is no feedback on what the setting is (without tripping it to test.
- the fan is a very noisy heat-management solution - my own fault for selecting the fan I did. I would be interested in seeing how much power the unit could handle with heatsink alone.
All in all, a good general purpose power supply for the bench, especially where voltage/current limits do not have to be extremely precise. I think I still prefer my DPS-3005-based power supply as a more accurate and featured solution - see LEAP#407 DPS3005 Bench Power Supply for more on that project.
Credits and References
- “0-30V 2mA-3A Adjustable DC Regulated Power Supply DIY Kit Short Circuit Current Limiting Protection For School Education Lab” (aliexpress seller listing)
- Originally purchased for US$4.54 (Sep-2017).
- No longer offered by this seller
- “WON 0-30V 2Ma-3A Adjustable DC Regulated Power Supply DIY Kit Short With Protection” (aliexpress seller listing)
- same product currently offered for SG$5.75 (Jan-2026)
- same/similar kit from banggood
- original (?) design from SmartKit
- example pre-cursor design
- LEAP#289 DC Voltmeter/Ammeter Modules
- LEAP#407 DPS3005 Bench Power Supply
- D882 datasheet
- D1047 datasheet
- ..as mentioned on my blog