Practical case: Reducing sparks when activating a switch

Schematic — Practical case: Reducing sparks when activating a switch

Objective and use case

What you will build: A small spark suppression circuit by placing a capacitor in parallel with a switch that controls a 12 V DC resistive load (for example, a lamp or test resistor). You will learn how to measure how the voltage waveform and the spark at the contacts change.

What it is for

  • Reduce the visible sparks when opening and closing a switch that powers a 12 V / 0.3 A lamp.
  • Increase the service life of switches used in models or homemade control boxes by reducing switching stress.
  • Achieve “smoother” switching when turning small 12 V lamps or relays on and off.
  • Reduce small electrical noises that couple into other nearby circuits at the moment of switching.
  • Safely experiment with how a capacitor changes the voltage at the switch terminals and the shape of the transient.

Expected result

  • When opening the switch, the voltage spike across the load (V_CARGA) is smoothed: you see a less abrupt tooth-shaped waveform on the oscilloscope.
  • The intensity of the visible spark at the switch contacts is clearly reduced (for example, it goes from an intense arc to just a small flash).
  • The maximum current drawn from the supply remains below the nominal limit, for example < 500 mA on a 12 V / 1 A supply.
  • The supply voltage V_CC = 12 V remains stable, with variations smaller than 0.5 V during switching.
  • The fall time of the voltage on the load when turning off follows a measurable RC time constant (for example, a fall to 37% in a few milliseconds depending on the value of the capacitor).

Target audience: Electronics hobbyists and students who build small 12 V projects; Level: Beginner–intermediate.

Architecture/flow: 12 V DC supply → mechanical switch → resistive load (lamp or resistor) in series → capacitor in parallel with the switch to smooth the transient → measurement of V_CARGA and V_CC before and after adding the capacitor.

Materials

  • 1 × 12 V DC power supply (minimum 500 mA).
  • 1 × SPST switch (simple, toggle or slide).
  • 1 × 100 Ω, 5 W power resistor (resistive load)
  • (Alternative: 1 × 12 V / 5–10 W lamp).
  • 1 × 100 µF electrolytic capacitor, minimum 25 V (C1).
  • 1 × 100 nF ceramic capacitor, minimum 50 V (C2) (optional, to see combined effect).
  • 1 × Breadboard or terminal strip.
  • 6–8 × Male–male jumper wires.

Wiring guide

  • Connect the positive terminal of the 12 V supply to the fixed contact 1 of the switch (we will call this node VCC).
  • Connect the fixed contact 2 of the switch to the first terminal of the 100 Ω, 5 W resistor (node VCARGA).
  • Connect the second terminal of the 100 Ω resistor to GND (negative of the supply).
  • Connect the positive terminal of electrolytic capacitor C1 (100 µF) to node VCARGA.
  • Connect the negative terminal of C1 to GND.
  • Connect one of the terminals of the ceramic capacitor C2 (100 nF) to node VCARGA.
  • Connect the other terminal of C2 to GND.
  • Make sure the negative of the supply is tied to the common GND rail of the setup.

Schematic

                    +12V (fuente)
                    |
                    |
                 o VCC node
                    |
                 [S1] Interruptor SPST
                    |
                 o VCARGA node
                    |
                 [R1] 100Ω 5W
                    |
                   GND

          [C1] 100µF
   VCARGA o----+| |-+----o GND
               |   |
          [C2] 100nF
               |   |
   VCARGA o----+   +----o GND
Schematic (ASCII)

Measurements and tests

  • Basic functional check:

    • Measure the voltage V_CC (supply voltage between +12 V and GND) with switch S1 open and closed; it should remain close to 12 V in both cases.
    • Measure the voltage V_CARGA (between node VCARGA and GND) with the switch closed; it should be very close to 12 V (ideally 12 V, it may be slightly less depending on the supply).
    • Check that resistor R1 gets moderately warm but not excessively hot after several minutes of continuous operation.

  • Measuring the spark and the effect of the capacitor (qualitative observation):

    • First disconnect capacitor C1 (100 µF) from the circuit and toggle S1 several times, watching the spark between the contacts (in an environment that is not very bright, always safely: do not bring your face close to the contacts).
    • Reconnect C1 and repeat the test; you should notice a visible reduction in the intensity of the spark when opening/closing S1.
    • Also connect C2 (100 nF) and observe whether there is any additional change in the spark (it usually helps mainly with very fast transients).

  • Measuring V_CARGA over time (if you have a multimeter with “Hold” function or, better, an oscilloscope):

    • V_CARGA means “voltage on the load”: measure between node VCARGA and GND.
    • Close switch S1 and check that V_CARGA ≈ 12 V in steady state.
    • With the oscilloscope, connect the probe tip to node VCARGA and the ground clip to GND; set the time base in the range of 5–20 ms/div.
    • Open S1 and observe how long it takes V_CARGA to fall from 12 V to 0 V; with the capacitor connected there should be a somewhat smoother fall compared to the case without capacitor (exponential curve instead of an almost instantaneous drop).

  • Measuring current in the load (if your multimeter can measure DC current):

    • Disconnect one of the wires between S1 and R1 and put the multimeter in ammeter mode in series, measuring between the output node of S1 (VCARGA) and resistor R1.
    • I_CARGA will be the current flowing through R1; for 12 V and 100 Ω you expect around 120 mA (I_CARGA ≈ 0.12 A).
    • Check that when you close S1 the current rises to that value and when you open it the current falls to 0 A; with the capacitor connected, the drop may not be absolutely instantaneous (depending on the sensitivity of the meter).

Educational explanation (what is happening)

  • Switch S1 opens and closes the current path to the load (R1).
  • When you open the switch without a capacitor, the current is cut off very abruptly and the energy stored in the load and wiring can produce a small voltage spike and spark.
  • Capacitor C1, connected between the load node (VCARGA) and GND, acts as a small energy “storage” that:
  • Charges when the switch is closed.
  • Discharges when the switch is opened, delivering current for a short time.
  • Because of this discharge, the voltage does not drop to 0 V suddenly; the transition is smoother and the energy available to form a spark at the contacts is reduced.
  • The ceramic capacitor C2 (100 nF) has a much smaller capacitance value but responds very quickly to abrupt changes, helping to filter very high-frequency spikes.

Common mistakes

  • Incorrect polarity of the electrolytic capacitor:
  • Connecting it reversed (+ to GND and – to node VCARGA) can damage it; always check the “+” marks or the “–” stripe.
  • Underestimating the working voltage of the capacitor:
  • Do not use 10 V capacitors in a 12 V circuit. Always choose a safety margin (at least 25 V in this case).
  • Forgetting the power dissipation of the load resistor:
  • With 12 V and 100 Ω, the power dissipated is P ≈ V²/R = 144/100 = 1.44 W; a 5 W resistor is suitable, but a 1/4 W resistor will burn out.
  • Creating a direct short circuit:
  • Do not connect the capacitor directly between +12 V and GND without the load, if it is not intended; although it is usually not dangerous for small values, it can produce high peak currents.

Safety and good practices

  • Always work with the power supply disconnected while you are assembling or modifying connections first.
  • Do not touch exposed terminals directly while you operate the switch, even at low voltage.
  • Leave space around the 5 W resistor so it can dissipate heat; do not rest it on flammable materials.
  • If the capacitor gets hot, swells, or emits a strange smell, disconnect the supply immediately and check polarity and rated voltage.

Possible improvements and extensions

  • Replace resistor R1 with a small 12 V relay to see how the capacitor helps reduce sparks when disconnecting coils.
  • Try different values for C1 (10 µF, 47 µF, 220 µF) and compare how much the duration of the V_CARGA decay changes.
  • Add a diode in parallel with an inductive load (if you use a relay instead of R1) and observe how diode + capacitor further reduce spikes and sparks.
  • Measure the RC time constant in more detail and experimentally verify the theoretical formula τ = R × C.

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Quick quiz

Question 1: What is the main objective of the circuit described in the article?




Question 2: Where is the capacitor placed in the small spark suppression circuit?




Question 3: What type of load is explicitly mentioned as an example in the article?




Question 4: One of the purposes of the circuit is to increase the service life of switches. How does it achieve this?




Question 5: According to the expected result, what is observed on the oscilloscope when opening the switch with the capacitor installed?




Question 6: What visible effect is expected on the spark at the switch contacts when using the capacitor?




Question 7: Besides reducing sparks in the switch, what is this experiment for?




Question 8: What benefit does the circuit provide in terms of electrical interference?




Question 9: What kind of switching is sought with the use of the capacitor in the circuit?




Question 10: To what type of people is the described experiment mainly addressed?




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Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

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

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