Practical case: Reducing the brightness of a DC motor with

Materials

• 1 × Small DC motor (3–6 V typical from toys or mini fans).
• 1 × Power resistor 10 Ω, 2 W (or a nearby value between 5–22 Ω, minimum 1 W).
• 1 × 3–6 V cell or battery (for example, 2×AA or a 9 V battery if the motor supports it).
• 1 × Battery holder or clip.
• 2 × Alligator clip leads (or several breadboard wires).
• 1 × Digital multimeter (able to measure DC voltage and DC current up to at least 1 A).
• 1 × Simple switch (optional, it can be replaced by connecting and disconnecting a wire).

Wiring guide

• Connect the positive terminal of the battery (+) to one end of the resistor [R1] 10 Ω.
• Connect the other end of the resistor [R1] 10 Ω to the positive terminal of the DC motor [M1].
• Connect the negative terminal of the DC motor [M1] to the negative terminal (–) of the battery (GND).
• If you use a switch:
• Connect the switch [S1] in series between the battery (+) and the resistor, or between the resistor and the motor (choose just one place in series).
• Prepare two measurement points:
• Node VA: junction between battery (+) and resistor [R1] (before the resistor).
• Node VB: junction between resistor [R1] and motor [M1] (after the resistor, before the motor).

Schematic

          +V BAT
          |
          o VA node
          |
        [S1] Interruptor (opcional)
          |
        [R1] 10Ω 2W
          |
          o VB node
          |
        [M1] Motor DC
          |
         GND (batería -)

Measurements and tests

• Verify basic operation:

  • Disconnect the resistor and connect the motor directly between +V BAT and GND.
  • Observe the motor speed (visual/auditory “100%” reference).
  • Go back to the connection with the series resistor as in the schematic.
  • Check that the motor is still spinning, but at a lower speed.

• Measure motor voltage (V_MOTOR):

  • V_MOTOR means “voltage between the motor terminals”.
  • Set the multimeter to DC voltage mode (V⎓).
  • Place the red probe on the positive terminal of the motor [M1] (node VB).
  • Place the black probe on the negative terminal of the motor [M1] (GND).
  • Write down V_MOTOR with resistor and compare it with the no-load battery voltage (V_BAT).

• Measure voltage across the resistor (V_R):

  • V_R means “voltage across the ends of resistor R1”.
  • Keep the multimeter in DC voltage mode.
  • Red probe on VA node (before the resistor).
  • Black probe on VB node (after the resistor).
  • Write down V_R with the motor spinning; it should be V_R = V_BAT − V_MOTOR (approx.).

• Measure motor current (I_MOTOR):

  • I_MOTOR means “current flowing through the motor and the resistor”.
  • Switch the multimeter to DC current mode (A⎓) and use the appropriate input (mA or A, depending on the range).
  • Open the circuit between the battery (+) and resistor [R1].
  • Connect the red probe of the multimeter to the positive terminal of the battery (+).
  • Connect the black probe of the multimeter to the free end of resistor [R1] (VA node now goes through the multimeter).
  • Turn on the circuit (close [S1] if you use it) and read I_MOTOR.
  • Repeat the measurement also without the resistor (motor directly on the battery) to obtain I_MOTOR_SIN_R and compare.

• Check resistance–speed relationship:

  • If you have several resistors (for example, 4.7 Ω, 10 Ω, 22 Ω), repeat the V_MOTOR and I_MOTOR measurements for each value.
  • Observe:
    • The higher the resistance, the lower the I_MOTOR.
    • The lower the I_MOTOR, the lower the V_MOTOR and the lower the motor speed.
  • Write down in a small table: R, V_MOTOR, I_MOTOR, observed speed (fast/medium/slow).

Common mistakes

• Using a resistor with too little power rating:

  • A motor can draw hundreds of mA; the resistor dissipates power P = I²·R.
  • If R = 10 Ω and I = 0.3 A, P ≈ 0.9 W; a 1/4 W resistor will overheat and may burn.
  • Use resistors of at least 1 W, preferably 2 W for this kind of test.

• Connecting the resistor in parallel with the motor:

  • This barely reduces the motor speed and only wastes current.
  • The resistor must go in series with the motor, in the only current path.

• Confusing voltage measurement with current measurement:

  • Voltage: the multimeter is connected in parallel (one probe on each side of the component).
  • Current: the multimeter is connected in series (open the circuit and the multimeter “acts as a wire”).

• Powering a low-voltage motor with a battery that is too high without a resistor:

  • For example, a 3 V motor directly on a 9 V battery: it may heat up, make strange noises, or get damaged.
  • If you notice the motor heating up a lot or producing a smell, stop the experiment and reduce the voltage or increase R.

Safety and good practices

• Do not touch the motor shaft when it is spinning at high speed; it can catch on clothing or hair.
• If the resistor gets so hot that you cannot touch it comfortably, disconnect the power and let it cool down.
• Do not leave the setup powered unattended, especially with 9 V batteries or large packs.
• Always check the battery polarity before turning on the circuit.

Possible improvements

• Replace the fixed resistor with a power potentiometer (for example, 100 Ω, 2 W) to have continuous speed control.
• Add a fixed switch [S1] for on/off without disconnecting wires.
• Use a MOSFET with PWM (pulse-width modulation) instead of a resistor, to improve efficiency and have better speed control.
• Add a small optical tachometer or a Hall sensor to measure RPM and relate them to V_MOTOR and I_MOTOR.

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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