Practical case: Coil as a filter in Arduino power supply

Materials

• 1 × Arduino Uno (or similar 5 V board).
• 1 × 5 V switching power supply (USB charger or similar; ideally something noisy).
• 1 × Inductor (coil) of 100 µH (1 to 470 µH also works for experimenting).
• 1 × 100 µF / 10 V or higher electrolytic capacitor (for the filter).
• 1 × 100 nF (0.1 µF) ceramic capacitor (in parallel with the electrolytic, optional but recommended).
• 1 × Breadboard.
• 6–8 × Jumper wires.
• 1 × Digital multimeter.
• 1 × Oscilloscope (recommended; if you don’t have one, you can still build the circuit and use the multimeter).
• 1 × Optional noisy load: small DC motor or 5 V relay module.
• 1 × Flyback diode (for example 1N4007) if you use a motor or relay (basic protection).

Wiring guide

These connections define exactly the schematic you will see in the next section.

• Primary (unfiltered) supply
• • Connect the positive terminal of the 5 V supply to the node called 5V_IN.
• • Connect the negative terminal of the 5 V supply to the GND node (common ground for the circuit and the Arduino).

• LC filter

• • Connect inductor [L1] 100 µH between node 5V_IN and node 5V_FILT (filtered output).
• • Connect electrolytic capacitor [C1] 100 µF between node 5V_FILT and GND (observe polarity: + to 5V_FILT, − to GND).
• • Also connect ceramic capacitor [C2] 100 nF between node 5V_FILT and GND.

• Powering the Arduino with filtered voltage

• • Connect the Arduino “5V” pin to node 5V_FILT.
• • Connect an Arduino “GND” pin to node GND.

• Noisy load (optional, to see the effect of the filter)

• • Connect one terminal of the motor or relay to node 5V_IN.
• • Connect the other terminal of the motor or relay to node GND.
• • Place diode [D1] 1N4007 in parallel with the noisy load: anode to the load node that goes to GND, cathode to the load node that goes to 5V_IN.

• Oscilloscope measurement points

• • Channel 1 (CH1) of the oscilloscope: probe tip to node 5V_IN, ground clip to GND.
• • Channel 2 (CH2) of the oscilloscope: probe tip to node 5V_FILT, ground clip to GND.

Schematic

             +5V fuente
             |
             o 5V_IN
             |
           [L1] 100µH
             |
             o 5V_FILT
             | \
             |  \
          [C1] 100µF
             |    \
             |    [C2] 100nF
             |      |
             |      |
            GND    GND


      Arduino Uno
      -----------
      5V pin  o----o 5V_FILT
      GND pin o----o GND


   Carga ruidosa (motor o relé, opcional)

          5V_IN o----( Motor / Relé )----o GND
                          |           |
                         [D1] 1N4007
                          |           |
                       ánodo       cátodo
                        a GND     a 5V_IN
                            ---

Measurements and tests

• Basic voltage check with a multimeter

  • Measure the voltage between 5V_IN and GND with the multimeter in VDC mode: you should read around 5 V (the actual voltage of your supply).
  • Measure the voltage between 5V_FILT and GND: it should be practically the same (very small loss in the coil).
  • If the voltage drop between 5V_IN and 5V_FILT is > 0.2–0.3 V without a heavy load, check the connections or whether the inductor is heating up.

• Measuring noise on the input (V_IN) with an oscilloscope

  • Set the oscilloscope to AC coupling (if your model allows it) on channel CH1 and connect it to 5V_IN (tip) and GND (ground).
  • Set the vertical scale to something like 20 mV/div or 50 mV/div to see small ripples.
  • Observe the waveform: the noise/ripple will be small “waves” or sawteeth superimposed on the 5 V voltage.
  • Estimate the peak-to-peak ripple value: for example, 100 mVpp (millivolts peak-to-peak). We will call this ripple measurement “Vpp_IN”.
  • Vpp_IN means “peak-to-peak voltage on the unfiltered input”, that is, the difference between the maximum and minimum value of the noise on the 5 V line.

• Measuring noise on the filtered output (V_FILT) with an oscilloscope

  • Configure channel CH2 of the oscilloscope and connect it as before, but to 5V_FILT and GND.
  • Use the same vertical scale as on CH1 so you can compare directly.
  • Observe the waveform: it should be noticeably “flatter” than that of 5V_IN.
  • Measure the peak-to-peak ripple at the filtered node (we call it “Vpp_FILT”).
  • Vpp_FILT means “peak-to-peak voltage at the filtered output”; compare it with Vpp_IN. Success criterion: Vpp_FILT < 0.5 × Vpp_IN (at least 50% less ripple).

• Test with a noisy load (motor or relay)

  • With the motor or relay connected to 5V_IN, turn the load on/off (for example, with a simple switch in series with the motor).
  • Watch on CH1 how larger spikes appear when connecting and disconnecting the load.
  • Watch on CH2 how those spikes are attenuated thanks to the LC filter.
  • Check that the Arduino keeps running (a blink sketch on pin 13, for example) without resets when the load switches.
  • If you have a sketch that reads an analog pin, watch in the serial monitor how the readings are more stable when powered from 5V_FILT than if it were powered from 5V_IN (you can first test “without filter” and then with the filter connected).

Basic explanation: what is the coil doing?

• The coil (inductor) [L1]:
• Opposes rapid changes in current (it doesn’t like the current to change abruptly).
• When noise tries to vary the current in the 5 V line, the coil “smooths out” that change.
• Capacitor [C1] (and [C2]):
• Provides an easy path to ground for fast voltage variations (high-frequency noise).
• For noise, the capacitor behaves almost like a short circuit to GND, “shunting” the noise.
• With the series coil and capacitor to ground, a low-pass LC filter is formed:
• High-frequency variations (noise) are attenuated.
• The low-frequency component (5 V DC voltage) passes practically unchanged.

Common mistakes

• Connecting the electrolytic capacitor backwards:
• The “−” terminal must go to GND and the “+” to node 5V_FILT.
• If reversed, the capacitor may heat up or get damaged.
• Using an inductor with too low allowable current:
• If you power many devices from 5V_FILT, make sure the inductor supports that current (check its datasheet).
• Leaving the Arduino GND separate from the supply GND:
• There must be a common ground; otherwise, measurements will be wrong and the setup may not work.
• Measuring ripple with the oscilloscope improperly set:
• If the vertical scale is at 2 V/div you will not see millivolts of noise.
• Make sure you work with scales of 10–50 mV/div and AC coupling if your oscilloscope allows it.

Possible improvements and variants

• Try different L and C values:
• Inductors of 47 µH, 220 µH, 470 µH.
• Capacitors of 47 µF, 220 µF.
• Observe how Vpp_IN and Vpp_FILT change and the response when the load is switched on/off.
• Add a small series resistor (for example 1–2.2 Ω) along with L and C to form a more damped “pi” (RLC) filter.
• Filter only the analog part:
• Use 5V_FILT only for sensors and AREF.
• Leave 5V_IN for logic and digital loads (modules, relays, etc.).
• Add more ceramic capacitors near the Arduino power pins and noisy modules:
• Improves local decoupling and further reduces high-frequency noise.

With this hands-on case you have seen how to use a coil as a simple supply filter and how to check its effect with objective noise measurements.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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