Portable Bench Power Supply (1.25V–10V) — Fits in Your Backpack ⚡

What if you had a fully functional electronics lab inside your backpack?

As electronics hobbyists, we always need a specific voltage for testing — but most bench power supplies are too bulky to carry, and a fixed 5V USB charger just doesn't cut it. I wanted something better. So I built this compact, battery-backed, adjustable bench power supply from scratch.

My 4 main design goals were:

1. Support different output voltages (adjustable 1.25V – 10V)
2. Make it easy to connect any device — banana jacks, USB, and DC output all on the front panel
3. Work even during power cuts — built-in 4S lithium battery pack gives 2+ hours of runtime
4. Stay genuinely compact — fits in a backpack, not just on a workbench

This project requires no programming — it is fully hardware-based and beginner-friendly. You get a real, useful lab tool at the end, not just a demo circuit.

What you can do with it:

1. Test electronic components at variable voltages
2. Power Arduino, ESP32, or Raspberry Pi projects in the field
3. Use it as a mobile repair workbench
4. Control motor speed for mini drills and fans
5. Charge batteries using the banana terminals

No more carrying a heavy bench supply. Let's build it! ⚡

🔗 Check out more of my DIY electronics builds, including the ESP8266 WiFi Car and the EyeLink eye-controlled wheelchair, on my GitHub and YouTube channel — link in my profile.

Supplies

All components are available on Amazon, AliExpress, or your local electronics market.

Approximate total cost: ₹1,000–₹1,500 (India) / $12–$18 (international)

Cost breakdown (approximate, India pricing):

1. 18650 Lithium-ion Cells (2500mAh), 4 pcs — ₹400–₹500 total
2. 4S 40A BMS — ₹120–₹150
3. 12V DC Female Connector (input) — ₹20
4. 10A Fuse + Holder — ₹20
5. ON/OFF Switch, 2 pcs — ₹40
6. DC-DC Step-Up Module (6009/6019) — ₹80–₹100
7. DC-DC Step-Down Module (4015) — ₹80–₹100
8. 10K Multi-turn Potentiometer — ₹50
9. 12V DC Female Connector (output) — ₹20
10. USB Female Connector — ₹20
11. Banana Connectors (Red & Black), 1 pair — ₹30
12. 7-Segment Voltmeter Display — ₹80–₹100
13. 5mm LEDs (Red & Green), 2 pcs — ₹10
14. 1K Ohm Resistors, 2 pcs — ₹5
15. PVC Sheet / Project Box, hot glue, wire, solder — ₹50–₹100 (varies by what you already have)

Note: prices vary by seller and region — these are rough estimates to help you budget, not fixed quotes.

Power & Battery

1. 18650 Lithium-ion Cells (2500mAh) — 4 pcs
2. 4S 40A BMS (Battery Management System) — 1 pc

⚡ Input & Charging

1. 12V DC Female Connector (input) — 1 pc
2. 10A Fuse + Holder — 1 set
3. ON/OFF Switch — 2 pcs

Power Conversion

1. DC-DC Step-Up Module (6009 / 6019) — 1 pc
2. DC-DC Step-Down Module (4015) — 1 pc
3. 10K Multi-turn Potentiometer — 1 pc

Output Terminals

1. 12V DC Female Connector (output) — 1 pc
2. USB Female Connector — 1 pc
3. Banana Connectors (Red & Black) — 1 pair

Indicators & Display

1. 7-Segment Voltmeter Display — 1 pc
2. 5mm LEDs (Red & Green) — 2 pcs
3. 1K Ohm Resistors — 2 pcs

Enclosure & Tools

1. PVC Sheet / Project Box — as needed
2. Multi-strand Copper Wire — assorted gauges
3. Hot Glue Gun — 1 pc
4. Soldering Iron + Solder — 1 set
5. Crocodile Clip Test Wires — 1 set
6. Multimeter — 1 pc (essential for calibration)

⚠️ Important: Do NOT use recycled or no-brand 18650 cells. Use genuine Samsung 25R, LG HG2, or Panasonic NCR18650B cells. Fake cells reduce runtime and can be dangerous.

⚠️ Why a 40A BMS for 2500mAh cells? A 2500mAh 18650 typically discharges safely up to 5–10A continuous. The 40A BMS is oversized for this current draw on purpose — it gives a comfortable safety margin, keeps the BMS running cool, and adds headroom if you ever upgrade to higher-drain cells later. You don't need to match BMS rating exactly to your load; oversizing on protection circuitry is safer than undersizing.

I designed a compact enclosure using PVC sheet. This keeps the build lightweight and easy to cut at home.

Front panel holes:

1. Voltmeter display
2. USB connector
3. Banana jacks (Red & Black)
4. Potentiometer knob
5. Output switch
6. LEDs (Power & Charging)

Back panel holes:

1. 12V DC input jack
2. Main ON/OFF switch
3. Ventilation slots

After cutting, sand all edges smooth. Apply primer and 2 coats of spray paint for a clean finish. Let it dry completely before installing any components.

Battery pack: Connect 4 × 18650 cells in series (4S configuration).

1. Nominal voltage: 14.8V
2. Full charge voltage: 16.8V
3. Connect BMS balancing wires to each cell junction exactly as per BMS datasheet
4. Always double-check polarity before soldering

Charging circuit:

1. Power the 6009 Step-Up module separately first
2. Adjust the onboard trimmer until output reads exactly 16.2V
3. Then connect: DC Input Jack → 10A Fuse → Switch → Step-Up input
4. Step-Up output → BMS charge input (C+ and C-)

⚠️ Use a 12V SMPS charger only (laptop brick style). Standard 12V wall adapters will not charge properly.

Follow this connection order:

1. DC Input Jack → 10A Fuse → Main Switch → BMS input
2. BMS output (P+ / P-) → main power rail
3. Step-Up 6009 input → BMS output | Step-Up output → BMS charge input
4. Step-Down 4015 input → BMS output
5. Step-Down 4015 output → Banana jacks + DC output jack
6. USB module input → BMS output (gives fixed 5V)
7. Voltmeter → Step-Down output terminals
8. LEDs → via 1K resistor → BMS output

For clarity, here's the same wiring order grouped by function:

Main power path: DC Input → Fuse → Switch → BMS input → BMS output (P+/P-) becomes your main power rail.

Charging path: Step-Up 6009 takes power from the input side, boosts it to 16.2V, and feeds it into the BMS charge input.

Variable output path: Step-Down 4015 takes power from the BMS output and feeds the banana jacks + DC output jack — this is the adjustable 1.25V–10V output, controlled by the potentiometer.

Fixed 5V path: USB module taps directly off the BMS output for stable 5V USB charging.

Indicators: Voltmeter reads the Step-Down output terminals; LEDs run off the BMS output through a 1K resistor each.

⚙️ Before closing the case:

1. Trace every wire with a multimeter in continuity mode
2. Confirm fuse is in-line
3. Confirm no bare wires touching the enclosure
4. Fix all modules firmly with hot glue

Voltage range test: Sweep the potentiometer from min to max — confirm 1.25V to 10V on the voltmeter.

COB light test (6V 5W):

1. Light starts turning on at ~4.5V
2. Stable brightness at 6V — no voltage sag

Mini drill test:

1. Connected via banana jacks
2. Speed varies smoothly with voltage adjustment
3. Works great for different material types

🔋 Battery charging test:

1. Connected 18650 cells in parallel to banana jacks
2. Set voltage to 4.2V
3. Current drops as cells reach full charge — working correctly

🔌 USB test:

1. Connected phone to USB output
2. Confirmed stable 5V charging

Working principle:

12V SMPS → Fuse → Switch → Step-Up (16.2V) → BMS charge input

Battery Pack ↔ BMS (protection & balancing)

BMS output → Step-Down → Variable output (1.25V–10V) → Banana / DC jack

BMS output → USB module → 5V USB output

The BMS handles overcharge, over-discharge, short-circuit, and cell balancing automatically. The multi-turn potentiometer gives smooth, precise voltage control.

Future upgrade ideas:

1. Add current meter for full power monitoring
2. Add USB-C PD output
3. 3D print a custom enclosure
4. Add CC/CV current limiting module

📺 Watch the full build video on YouTube: ▶ Click here to watch the complete build

Full circuit diagram and component links are in the video description.

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