The Bench Power Supply

The bench DC power supply looks like just a box that makes power — but it's far more than a fancy battery. Its adjustability and built-in protection make it a genuine debugging instrument, not just a source.

What it is

A bench power supply provides adjustable DC power with two key knobs:

• A voltage control — set the exact output voltage (e.g., 3.3V, 5V, 12V).
• A current limit control — set the maximum current it will deliver.

Plus a display showing the actual output voltage and the actual current being drawn. That combination — set the voltage, cap the current, and watch both — is what makes it special.

Voltage you can dial in

Unlike a battery (fixed voltage, slowly sagging as it drains), a bench supply holds a precise, steady voltage you choose. Need to power a 3.3V board? Dial 3.3V. Testing how a circuit behaves at the low end of its range? Dial it down and watch. This precision and adjustability alone make it the right way to power anything on the bench.

But the voltage knob isn't the clever part. The current limit is.

The current limit: a safety valve you control

This is the feature that elevates the supply from "source" to "instrument." You set a maximum current the supply is allowed to deliver. The supply will provide your set voltage as long as the circuit draws less than that limit. If the circuit ever tries to draw more — say, because of a short — the supply refuses, holding the current at the limit and dropping the voltage instead.

Think of it as a valve you set: "deliver up to this much current, and not a drop more." A battery has no such valve — it'll happily dump enormous current into a short (which, with L6's LiFePO4 packs, is exactly why they have a BMS). The bench supply lets you set that ceiling deliberately.

Why both controls matter together

• Voltage set correctly → the circuit gets what it expects.
• Current limited sensibly → if anything is wrong, the damage is capped.
• Both displayed → you see how much current the circuit actually draws, which is itself diagnostic ("this should draw 50mA but it's pulling 2A — something's wrong").

That third point is subtle and powerful: the supply's current display turns "power the board" into "power the board and watch its appetite." A healthy board draws a predictable current; a faulty one often doesn't.

The next slide turns the current limit into an active debugging technique — the single most useful trick a bench supply offers.

Here's the technique that makes seasoned engineers reach for a bench supply when powering anything new or suspect: deliberately limiting the current to protect the board. It's the supply's debugging superpower.

The scenario: powering something for the first time

You've just assembled a new board, or you're reviving a suspect one. The riskiest moment is first power-on. If there's a short — a solder bridge, a backwards component, a wiring error — connecting full power can let a large current surge through, burning components, lifting PCB traces, and turning a small fix into a dead board.

A battery or wall adapter offers no protection: it'll deliver whatever current the fault demands. But a bench supply lets you cap the danger in advance.

The technique

Before powering a new/suspect board:

1. Set the correct voltage (what the board expects, e.g., 5V).
2. Set a LOW current limit — just above what a healthy board should draw (say, a couple hundred mA for a small board).
3. Power on and watch the current display.

Now two things can happen, and both are good outcomes:

• The board draws a normal, small current and runs → great, no short. You can raise the limit if needed for real loads.
• The current immediately pins at your limit and the voltage drops → there's a short or excessive draw. But because you limited the current, the fault current never surged. The supply just saved your board. Power off and investigate (back to the multimeter workflow: hunt the short).

Why this works

The current limit converts a potentially destructive event into a safe diagnostic signal. Instead of a fault announcing itself with smoke, it announces itself with a harmless "current pinned at the limit" reading. You learn there's a problem without paying for it in dead components.

This is why "bring it up on a current-limited bench supply" is standard practice for first power-on. It's not paranoia — it's cheap insurance that also happens to tell you something (whether the board is shorted) in the very first second.

Beyond first power-on

The same idea helps elsewhere:

• Finding a short's location: push a small, limited current into a shorted rail and feel (carefully) for the component that gets warm — the limited current makes this safe.
• Characterising consumption: watch the current as a board does different things (idle vs transmitting vs driving a load) to understand its real power draw — directly useful for L6 power budgeting.

The takeaway

A bench supply isn't just where power comes from — it's a protective, diagnostic instrument. Set the voltage, limit the current, watch the draw. That habit prevents destroyed boards and turns first power-on into a test rather than a gamble. The next slide reads the supply's own status indicator — CC vs CV — which tells you, at a glance, whether your board is behaving.

Every bench supply shows a small but crucial status: whether it's in CV or CC mode. Reading that indicator tells you, at a glance, whether your circuit is healthy or in trouble — and it ties straight back to the CC/CV idea from L6's battery charging.

Two modes, one indicator

A bench supply is always in one of two states, shown by a CV or CC light/label:

• CV — Constant Voltage: the supply is delivering your set voltage, and the load is drawing less than the current limit. The voltage is the thing being held constant.
• CC — Constant Current: the load tried to draw more than the current limit, so the supply is now holding current constant at the limit and dropping the voltage to do so.

It switches automatically based on what the load demands. You don't set the mode — the circuit's behaviour sets it, and the indicator reports it back to you.

What each mode tells you

This indicator is a free, continuous diagnosis:

CV (the healthy norm). Your circuit is getting the full set voltage and drawing a reasonable current under the limit. For a working board, CV is the expected, happy state.

CC (a flag to investigate). The supply has hit its current limit. Why does this matter? Because if you set a sensible limit and the board immediately jumps to CC — especially at near-zero volts — it's drawing the maximum you allowed and the voltage has collapsed. That's the classic signature of a short or excessive load.

The L6 connection

You've seen CC/CV before — it's exactly the charging profile from L6 (the lithium charger does constant-current bulk, then constant-voltage absorption). A bench supply uses the same two regimes, and the mental model carries over: CV = voltage-controlled; CC = current-limited. Recognising it here reinforces that the same principle governs both a charger and a bench supply.

Reading the supply as an instrument

Put the whole lesson together and the bench supply earns its place as a debugging instrument, not just a source:

• Set voltage + low current limit before first power-on (protection).
• Watch the current draw (is the board's appetite normal?).
• Read CV/CC (CV = healthy; unexpected CC = short/fault).

Three habits, and your power source actively helps you find faults. Combined with the multimeter (Lessons 1–2), you can already diagnose the majority of "it doesn't power up" problems with confidence.

But some faults aren't about static voltages and currents at all — they're about signals changing over time: a microcontroller's output, a PWM duty cycle, an I²C burst. For those, neither the multimeter nor the supply can help. You need to see the signal move. That's the oscilloscope — the next lesson.