Capacitors, diodes, transistors

These three small parts appear in nearly every project. One stores charge, one is a one-way valve, and one is an electronic switch.

Capacitor: a tiny tank

A capacitor stores charge and releases it quickly. When a motor starts and the supply dips, the capacitor fills the gap.

• 100 nF beside a chip removes fast noise.
• 470 µF across a servo supply handles sudden current.

Round electrolytic capacitors have a minus stripe, and they can burst if fitted backwards. Pick a voltage rating well above your supply.

Diode: a one-way valve

Current flows only from the anode to the cathode, which is the end with the stripe. An LED is a diode that gives light.

Motors, pumps, relays and valves contain coils. When a coil switches off it throws back a sharp voltage spike. A diode across the coil, with the stripe towards plus, gives that spike a safe path. This is called a flyback diode, and you will see it in our pump and valve wiring.

Transistor: a small signal controls a big load

A board pin can give only a small current, which is not enough for a motor. A MOSFET solves this. The pin controls the gate, and the MOSFET switches the heavy current through the load.

1. Connect the load between the supply and the MOSFET.
2. Connect the MOSFET source to ground.
3. Connect a 220 Ω resistor from the pin to the gate.
4. Connect a 10 kΩ resistor from the gate to ground, so the load stays off during start-up.

Choose a logic-level MOSFET. It turns fully on at 3.3 V or 5 V.