3-in-1 Power Tester – 150W AC + Variable DC + 12V

Why I Built This — Humanix Tech Lab

I don't buy new components if a working second-hand one is sitting in a scrap market for a tenth of the price.

That's not a slogan — it's just how I build. In local scrap and second-hand markets near me, real, working parts show up constantly: SMPS units, laptop and mobile chargers, routers, motors, speakers, buzzers, FM radios, extension boards, switches, sound boxes. A 10–20A 12V SMPS that costs ₹1000–2000 (~$12–23) new turns up here for ₹50–100 (~$0.60–1.20). A laptop or mobile charger for ₹50 (~$0.60) or less. The only problem is nobody there can tell you if it actually works. You buy blind, or you don't buy at all.

So I (Humanix Tech Lab) built something to close that gap: a pocketable device that lets me test almost anything on the spot, before I pay for it — AC devices, low-voltage salvage, and standard 12V electronics, all from one 55Wh battery pack I built myself.

This isn't a fancy microcontroller-driven project, and I want to be upfront about that. There's no code, no display beyond a simple voltmeter, no app. What it is, is a finished, working tool that I use every week, that saved me real money, and that turned a stack of untested scrap into a running collection of laptop chargers, SMPS units, routers, and motors — instead of a pile of gambles.

The Three-Output Logic

Almost everything I encounter in the second-hand market falls into one of three buckets:

• Devices that run on mains AC — laptop chargers, SMPS units, soldering irons, serial-light controllers.
• Devices that need a specific, non-fixed voltage — small motors, broken speakers, toys, LEDs, and random salvaged parts with no label.
• Devices that expect a steady ~12V — routers, Wi-Fi gear, TVs, set-top boxes. This is the single biggest category of consumer electronics, and a ±0.5V tolerance is fine for nearly all of it.

One fixed-voltage power bank can't cover all three — that's the entire reason this is a 3-in-1 device rather than a simpler single-rail one. Every output rail exists to answer a real category of device.

The 55Wh pack is sized deliberately modest rather than maximized for runtime, because the actual use pattern is short bursts: power on, check if the LED lights up or the motor spins, power off, move to the next item. Each test is 30–60 seconds. A 55Wh pack comfortably covers a full day of that kind of walk-and-test use.

How It Works

Battery pack & BMS: Three 21700 Li-ion cells (5000mAh each) are wired in series (3S) into a 3S/20A BMS board. 21700 cells were chosen over the more common 18650 for two reasons: they hold more capacity in the same form factor, and — more importantly for driving the inverter — they deliver up to 3C continuous discharge versus roughly 1C for a typical 18650. The 150W inverter branch pulls real current in bursts, and that extra discharge headroom keeps the pack stable under an AC load instead of sagging or heating up. BMS tap points are labeled by stage voltage (0V pack negative / 4.2V top of cell 1 / 8.4V top of cell 1+2 / 12.6V full pack positive) so the build can be verified with a multimeter as it comes together. A 5-pin JST-XH-style balance connector is broken out from the same taps for external balance charging.

Battery indicator: An MH-DL18S module drives the 4-segment battery-percentage LED gauge, reading pack voltage directly off the main rail. Its jumper pins (S1–S8) need to be shorted to match pack configuration — 3S pins for this build, 4S pins if replicating with a 4S pack — which is what makes the same module reusable across battery sizes.

Main power rail (switch-first topology): Pack positive routes through a 10A inline fuse into the BMS output rail, then splits into three branches. Each branch gets its own rocker switch placed immediately after the BMS, before the module it feeds — not after the module. This matters: if switches sat after each module instead, an "off" inverter or buck converter would still sit there drawing quiet standby current all day. Switching at the BMS side means an off branch is genuinely disconnected — zero draw, not just disabled. Pack negative forms a common ground bus returning to all three modules and the 12V IN/OUT jack.

AC output branch (150W): BMS output → AC ON/OFF rocker switch (LED indicator via 1KΩ resistor) → 150W inverter module (12V→220V) → AC socket. This branch is what lets untested laptop chargers, SMPS units, and soldering irons get plugged in directly, watching for a charging LED or fan spin-up to confirm a unit is alive before handing over money. It also works as a standalone 150W travel inverter. Carries lethal 220V — insulate fully before enclosure closeup, never work on it while live.

Variable DC branch: BMS output → Variable DC rocker switch (green LED via 1KΩ resistor) → XL4015 buck converter → 3-digit digital voltmeter (live readout) → variable DC barrel jack. The external multi-turn potentiometer gives fine control across roughly 1.25V–9V depending on input headroom, covering unlabeled salvage like bare motors, broken speaker drivers, LED strips, and toys. Crocodile clips off this jack make it easy to probe a part with no connector at all.

12V direct IN/OUT branch: BMS output → 12V IN/OUT rocker switch → DC barrel jack, bypassing both the inverter and buck converter entirely. This is the single most-used port on the device, since ~12V is what most consumer electronics run on — routers, modems, set-top boxes, old solar setups, TVs. It doubles as a charging input for topping up the pack from an external 12V source.

Enclosure: All modules are mounted in a PVC enclosure, with the AC section physically separated and fully insulated from the low-voltage side. Finished as a simple painted PVC box — functional over decorative, since the priority was a tool that could actually be carried and used, not a showpiece.

Weight: Final build weighs 794g, confirmed on a scale — under an 800g target set deliberately, since the device goes into a bag or gets carried by hand for a full market visit rather than sitting on a bench. Cell choice, enclosure size, and skipping non-essential extras (no display beyond the voltmeter, no unnecessary shielding, no oversized heatsinking) all fed into hitting that weight target.

Field Testing

In practice, a market visit looks like this:

• AC branch — plug in an untested SMPS or laptop charger, watch for the power LED.
• Variable DC branch — probe an unlabeled motor or speaker with crocodile clips, dial in voltage, listen/watch for response.
• 12V branch — plug in a router or set-top box, confirm it boots normally.

Each test takes under a minute. A single visit might mean 15–20 quick tests, and the 55Wh pack handles that comfortably on one charge.

Bench validation before field use: the AC branch held steady powering a bulb, a hot glue gun, and a router. The variable DC branch was dialed across its full range driving a small DC fan, LEDs, and a small light, with smooth response confirming buck converter and voltmeter accuracy. The 12V branch drove an LED COB and a 775 DC motor directly, with instant, steady power confirming it cleanly bypasses the inverter/buck stage.

Where It Gets Used

Indoors:

• A portable workbench power supply for Arduino and general electronics projects — one unit covers variable DC, fixed 12V, and AC.
• A mini UPS for routers, switches, and set-top boxes — the internal 12V pack can power them directly for 2–3 hours with no inverter stage involved.
• A fast AC charger on demand — the 150W inverter branch handles phone chargers or small tools when a wall socket isn't convenient.

Outdoors, in the market:

• A low-power tester for LEDs, small lights, speakers, and small motors via the variable DC branch.
• A mid-power tester for larger motors, bigger lights, and old 12V solar gear.
• A 150W AC tester for old appliances, chargers, and SMPS units — the highest-value, highest-risk category to buy blind.

Wiring Summary

┌── AC ON/OFF switch ──► 150W Inverter ──► AC Socket (220V, 150W)
Battery Pack (3S)        │
   → 10A Fuse             ├── Variable DC switch ──► XL4015 Buck ──► 3-digit Voltmeter ──► Variable DC Jack
   → BMS (3S/20A) ────────┤        (pot-adjustable, ~1.25–9V)
                         │
                         └── 12V IN/OUT switch ──► DC Barrel Jack (12V, 10A fused, bidirectional)

Battery Indicator (MH-DL18S) taps directly off main rail → 4-segment LED gauge

Each branch switch sits directly after the BMS (not after its module), so an "off" branch draws zero standby current rather than just being disabled.

Safety Notes

• Verify polarity at every BMS tap point with a multimeter before first power-up.
• Never omit the 10A main fuse — it's the primary protection against a wiring fault on the main rail.
• The AC side carries lethal 220V. Fully insulate all AC wiring, terminals, and the socket housing before closing the enclosure, and never work on it while the inverter is live.
• If building with a different pack size (e.g. 4S), confirm the BMS, the inverter's input voltage range, and the MH-DL18S jumper setting are all rated for that configuration first.
• Use only protected 21700 cells from a reputable source, and stay within the BMS's rated continuous current (20A here).

Why This Matters (Even Though It's Simple)

This won't look as advanced next to microcontroller-driven builds. That's fine — what's here is something a low-budget or middle-class maker can actually replicate for about ₹1500–1600 (~$17–18), using it to avoid spending 5–10x that on new parts, while pulling working components out of the e-waste stream instead of letting them get scrapped. It's not a prototype sitting on a shelf — it's a tool used in the same market it was built for, every single week.

Frequently Asked Questions

Who built this 3-in-1 power tester?This project was designed and built by Humanix Tech Lab, an independent electronics maker channel focused on DIY power electronics, e-waste rebuilds, and portable tool builds.

Where can I find more projects from Humanix Tech Lab?On the Humanix Tech Lab YouTube channel, GitHub (HumanixTechLab), and Instagram — all linked below.

Is the circuit diagram open source?Yes — the full circuit diagram and wiring notes are available in the Humanix Tech Lab GitHub repository linked below. There's no firmware/code for this build since it's a fully analog/hardware design.

About the Creator — Humanix Tech Lab

Humanix Tech Lab (also written HumanixTechLab) is a DIY electronics and maker channel focused on practical, low-cost builds — power electronics, e-waste rebuilds, portable tools, and field-tested projects for makers on a budget.

• 🎥 YouTube: youtube.com/@HumanixTechLab
• 📺 This build's video: youtu.be/L9B9g3Xih64
• 💻 GitHub: github.com/humanixtechlab
• 📸 Instagram: instagram.com/humanixtechlab

If you build your own version of this Humanix Tech Lab power tester, tag @humanixtechlab or drop a comment on the YouTube video — I'd love to see what modifications you made for your own local market.