The idea
A robot’s energy almost never arrives in the form its motors need. Power electronics converts it: AC to DC, one DC voltage to another, DC to AC for motors. This page is the map; the Week 3 guides fill in each piece.
The four conversions
| Conversion | Circuit | Example in a robot |
|---|---|---|
| AC → DC | Rectifier | The charger plugged into the wall (Bridge rectifiers) |
| DC → DC | Buck (down) or boost (up) converter | 48 V battery to 5 V logic (DC-DC converters) |
| DC → AC | Inverter | Driving a brushless motor (Motor drives) |
| AC → AC | Frequency converter | Rare in robotics |
The components
| Component | What it does |
|---|---|
| Resistor | Limits current, turns energy into heat |
| Capacitor | Stores energy in an electric field; smooths voltage |
| Inductor | Stores energy in a magnetic field; smooths current |
| Transformer | Steps AC voltage up or down, and isolates |
| Diode | One-way valve for current |
| BJT, MOSFET, IGBT | Electronic switches (Power switches) |
A MOSFET as a low-side switch
The most common way a microcontroller switches a motor on and off:
+V ──── motor ──── Drain
MOSFET Gate ──R_G── microcontroller pin
0 V ───────────── Source └─R_PD── 0 V
- Main loop: battery + → motor → drain → source → ground. The switch is on the ground side, so it’s a low-side switch.
- Gate resistor : limits the current spike that charges the gate.
- Pull-down : holds the gate low so the motor stays off if the control pin floats, at power-up or if a wire comes loose.
- Common ground: the controller’s ground must connect to the power ground, because the MOSFET responds to gate voltage relative to its source.
- Flyback diode across the motor: when the switch turns off, the motor’s inductance tries to keep current flowing. The diode gives it a path, instead of a voltage spike that would kill the MOSFET.
Check yourself
1. Why does a motor need a flyback diode but a heater doesn't?
A motor winding is an inductor, which resists sudden changes in current and creates a voltage spike when switched off. A heater is (almost) pure resistance.