Robots often use motors, solenoids, and relay coils, and all of these are inductive loads. An inductor stores energy in its magnetic field while current flows. When a transistor or MOSFET suddenly switches the current off, the collapsing magnetic field can create a large voltage spike.
A flyback diode protects the driver circuit by giving that current a safe path.
Key Facts
- Inductor voltage is v = L di/dt, so faster current changes create larger voltage spikes.
- Energy stored in a coil is E = 1/2 L I^2.
- A flyback diode is connected in parallel with the coil or motor, reverse biased during normal operation.
- When the switch opens, the diode becomes forward biased and current circulates through the load and diode.
- For a silicon diode, the clamp voltage is about V_supply + 0.7 V in a low-side switch circuit.
- The diode current initially equals approximately the load current just before turnoff, then decays over time.
Vocabulary
- Flyback diode
- A diode placed across an inductive load to safely conduct current when the load is switched off.
- Freewheeling diode
- Another name for a flyback diode because it lets current continue circulating after the switch opens.
- Inductive kick
- A high voltage spike produced when current through an inductor is interrupted quickly.
- MOSFET
- A transistor commonly used as an electronic switch to control motors, relays, and other loads.
- Reverse bias
- A diode condition in which the applied voltage blocks current flow except for tiny leakage current.
Common Mistakes to Avoid
- Placing the diode in series with the motor is wrong because it will not provide a path for the inductor current when the switch opens.
- Reversing the diode polarity is wrong because it may short the supply during normal operation instead of staying reverse biased.
- Choosing a diode with too low a current rating is wrong because the diode must carry nearly the full load current right after turnoff.
- Ignoring switching speed is wrong because a simple flyback diode protects well but can make relays and solenoids release more slowly.
Practice Questions
- 1 A relay coil has L = 80 mH and carries I = 0.50 A before turnoff. How much energy is stored in the coil?
- 2 A motor winding has L = 20 mH, and its current drops from 2.0 A to 0 A in 10 microseconds without a diode. What average voltage magnitude is produced by the winding?
- 3 In a low-side MOSFET motor driver, explain why the flyback diode is reverse biased while the motor is on but becomes forward biased when the MOSFET turns off.