Practical case: Soft start of direct current motor

Materials

• 1 × DC motor (6–12 V), M1
• 1 × Logic‑level N‑MOSFET, Q1 (e.g., IRLZ44N, AOI514)
• 1 × Flyback diode, D1 (e.g., 1N5819 Schottky or 1N4007)
• 1 × Resistor R1 = 100 kΩ (gate charge, +V to gate)
• 1 × Capacitor C1 = 10 µF, ≥16 V electrolytic (gate to GND)
• 1 × Sense resistor R_S = 0.22 Ω, ≥2 W (current sense, source to GND)
• 1 × DC power supply +V (9–12 V) capable of driving the motor
• 1 × Multimeter (DMM)
• Breadboard and jumper wires

Wiring guide

• Connect M1 (DC motor) between +V and Q1 drain. Add D1 across the motor: cathode to +V, anode to the motor’s lower node (Q1 drain).
• Connect Q1 source to the top of R_S; connect the bottom of R_S to GND.
• Build the soft‑start RC:
• R1 from +V to the gate node.
• C1 from the gate node to GND (observe capacitor polarity: negative to GND).
• Connect the gate node to Q1 gate.
• Probing abbreviations (used in schematic):
• V_G: Gate voltage at the black dot labeled V_G, measured to GND.
• V_M: Motor voltage measured between black dots V_M+ and V_M−.
• V_S: Sense voltage across R_S measured between black dots V_S+ and V_S−. Motor current I_M = V_S / R_S.
• Double‑check: No component leads left unconnected. Diode orientation correct. MOSFET pins: D (to motor), S (to R_S), G (to RC node).

Schematic

                 PS1 Fuente DC 12 V
                       +12 V
                         │
                         ├───────────────● I_M
                         │               │
                         │              ┌┴┐
                         │              │ │   M1 Motor DC 6–12 V (M1)
                         │              │ │
                         │              └┬┘
                         │               ● V_M+
                         │               │
                         │              ┌┴┐
                         │              │ │   D1 1N5819 (flyback)
                         │              │ │   (cátodo arriba)
                         │              └┬┘
                         │               ● V_M−
                         │               │
                         │             ┌─┴─┐
                         │             │   │   Q1 TIP122 (NPN Darlington)
                         │─────────────┤   │   (entrada izquierda = Base)
                         │             │   │
                         │             └─┬─┘
                         │               │
                        GND             GND


        +12 V
          │
        ┌─┴─┐
        │   │  R1 22 kΩ (base)
        │   │
        └─┬─┘
          │
          ● V_C ──────────────────────────────── (conecta a la Base de Q1, línea horizontal hasta el lado izq. del rectángulo)
          │
        ┌─┴─┐
        │   │  C1 100 µF 25 V  (+ arriba)
        │   │
        └─┬─┘
          │
         GND

Measurements and tests

• Power‑up ramp:

  • With the DMM on DC volts, measure V_G (black dot V_G to GND). You should see an exponential rise from 0 V toward +V with time constant τ ≈ R1 × C1 ≈ 1 s.
  • Observe that the motor starts smoothly as V_G increases.

• Motor voltage (soft start evidence):

  • Measure V_M between V_M+ and V_M−. V_M should ramp up as the MOSFET turns on (it will start near 0 V and approach the supply as Q1 saturates).

• Motor current (from shunt):

  • Measure V_S between V_S+ and V_S−. Compute I_M = V_S / R_S.
  • Expect I_M to rise gradually (no abrupt inrush). Example: with R_S = 0.22 Ω, 220 mV corresponds to 1 A.

• Time constant exploration:

  • Increase C1 to 22 µF for a slower start (τ ≈ 2.2 s). Decrease to 4.7 µF for a faster start (τ ≈ 0.47 s).

• Polarity and reset check:

  • If repeating tests quickly, ensure C1 has time to discharge. If V_G remains high after power‑off, wait a few seconds or briefly short the gate node to GND (with power off) to reset.

Common mistakes

• Using a non‑logic‑level MOSFET: it may not turn on fully at 9–12 V gate drive losses, causing heating and poor torque.
• Missing or reversed flyback diode: leads to large voltage spikes and potential MOSFET failure.
• Electrolytic capacitor reversed: C1 must have its negative lead to GND.
• Wrong R_S value or low power rating: excessive voltage drop or overheating will distort measurements.

Safety notes

• Motors can draw several times their rated current at startup; verify your supply current limit.
• R_S dissipates P ≈ I_M² × R_S; for 1.5 A, that is ~0.5 W—use ≥2 W for margin.
• MOSFETs can get hot if not fully enhanced; consider a small heatsink if the motor current is high.

Improvements

• Add a small gate stopper (e.g., 100 Ω in series with the gate) to reduce ringing in long wiring.
• Add a supply bulk capacitor (e.g., 100–470 µF across +V to GND near the motor) to reduce supply dips.
• Replace the passive RC with a dedicated soft‑start or PWM controller for adjustable ramp profiles.

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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