Lab 0.1 — Power Supply + Fluke Safety

← Course 2 syllabus · Module 0 · Next: Lab 0.2 »

Goal

Before a single component is powered, learn to operate the two instruments that everything else depends on: the WANPTEK bench power supply and the Fluke 117 multimeter. Specifically, learn to set a current limit that protects your circuit, and to use the DMM without blowing its fuse or your parts. This is the single most important habit in the whole course — a correctly set current limit is the difference between a wiring mistake that does nothing and one that releases smoke. If you internalize constant-voltage vs. constant-current here, every later lab is safer.

Equipment & parts

  • WANPTEK 30 V / 10 A DC bench power supply + its output leads.
  • Fluke 117 DMM + test leads.
  • One resistor from the kit: 100 Ω (and optionally a 1 kΩ), ¼ W.
  • Breadboard + a couple of jumper wires (used only as a load holder).

Safety & don’t-break-it

  • The Fluke’s A (current) jack is a near-short. With a lead in the A jack and the dial in current mode, the meter is ~0 Ω. If you touch those leads across the supply output you create a dead short. The supply’s current limit (set below) is what saves you, but the correct habit is: leads live in the V/Ω/COM jacks; only move to A deliberately, in series, and move back immediately after.
  • Never connect the DMM in current mode in parallel with a source or component. Current is always measured in series (the meter becomes part of the loop).
  • Set the current limit before connecting a load (procedure below). Treat “I forgot to set the limit” as a stop-and-restart event.
  • The 100 Ω resistor across a supply dissipates real power — at 5 V it sinks 50 mA and dissipates 0.25 W, right at a ¼ W part’s rating. Don’t leave it energized at higher voltages; it will get hot.
  • Keep one hand away from the circuit when powered; never let the two output leads touch.

Background

A bench supply has two regulation modes. In constant-voltage (CV) it holds the set voltage and delivers whatever current the load draws, up to a ceiling. When the load tries to draw more than the current limit \(I_\text{lim}\) you set, the supply switches to constant-current (CC): it holds the current at \(I_\text{lim}\) and drops the voltage as far as needed. A dead short in CC mode simply sits at \(I_\text{lim}\) with the output near 0 V — safe.

For a resistor \(R\) across a CV supply at voltage \(V\), Ohm’s law predicts the current and power:

\[I = \frac{V}{R}, \qquad P = VI = \frac{V^2}{R}.\]

For \(V = 5\text{ V}\), \(R = 100\ \Omega\): \(I = 50\text{ mA}\), \(P = 0.25\text{ W}\). If your set current limit is above 50 mA the supply stays in CV; set it below 50 mA and you’ll watch it drop into CC — a deliberate, instructive demonstration.

Procedure

Part A — Set a current limit (do this every session).

  1. Power on the WANPTEK with nothing connected to the outputs.
  2. Set the voltage knob to 5.0 V (read it on the display).
  3. Set the current limit: turn the current knob fully down, then briefly short the output leads together (or use the supply’s built-in method) — the display drops toward 0 V and the CC/constant-current indicator lights. Now dial the current up to ~100 mA. Remove the short. The supply now enforces a 100 mA ceiling.
  4. Confirm: with the short removed, the display should read 5.0 V and 0.00 A (CV, no load).

Part B — Measure the supply voltage with the Fluke.

  1. Put the Fluke leads in COM (black) and VΩ (red). Turn the dial to V⎓ (DC volts).
  2. Touch red to the supply’s + output, black to −. Read ≈ 5.00 V. This confirms both the supply and your DMM voltage function.

Part C — Constant-voltage: measure load current in series.

  1. Power off the supply output. Build a one-resistor circuit: supply + → one leg of the 100 Ω resistor; other leg → back toward supply −, but leave the − return open where the meter will go.
  2. Move the Fluke’s red lead to the A jack and set the dial to A (DC current). Insert the meter in series in that open return: supply − → meter COM, meter A → resistor.
  3. Power on. The Fluke should read ≈ 50 mA; the supply should stay in CV at 5.0 V and show ≈ 0.05 A. The two current readings should agree.
  4. Power off. Move the red lead back to VΩ and the dial back to volts.

Part D — Force constant-current (see the limit work).

  1. Re-set the supply current limit to ~20 mA (Part A method).
  2. Power on the same 100 Ω circuit. The load wants 50 mA but the supply caps it: it enters CC, the CC indicator lights, current holds at ~20 mA, and the voltage sags to \(V = I_\text{lim} R \approx 0.02 \times 100 = 2\text{ V}\).
  3. Confirm with the Fluke (voltage across the resistor ≈ 2 V). Power off.

Deliverable & expected results

A short bench note (docs/lab-0-1.md) recording:

  • The CV reading: 5.00 V, 50 mA, supply in CV.
  • The CC reading: current pinned at your limit (~20 mA), voltage sagged to ≈ 2 V, supply in CC.
  • A one-line statement of why the voltage sagged (the supply gave up voltage to hold current).
Quantity Predicted Measured
Load current (CV, 5 V, 100 Ω) 50 mA …
Power in resistor 0.25 W …
Voltage in CC at 20 mA 2.0 V …

Analysis & reconciliation

Compute \(I = V/R\) and \(P = V^2/R\) by hand and compare to the meter. Expect small discrepancies (a few %): the resistor’s tolerance (often ±5%), the meter’s burden voltage in current mode (the meter’s own small resistance drops a little voltage in series), and the supply’s set-point accuracy. If your CC voltage isn’t exactly \(I_\text{lim}R\), check whether the resistor’s true value (measure it in Lab 0.2) explains it.

Going further

  • Repeat Part C with a 1 kΩ resistor and predict the current first (5 mA). Notice the meter’s burden voltage matters even less at lower currents.
  • Explore the supply’s OCP (over-current protection) trip vs. simple CC foldback, if your unit distinguishes them.
  • Read the Fluke’s capacitance and VoltAlert (non-contact) functions in the manual now; you’ll use capacitance in Lab 0.3.