Switching high currents

Relay

• Easy to understand
• If coil energized, arm is drawn down, making connection
• Control current: ~100mA
• Control voltage: 12V
• Switched current: tens of amps
• Switched voltage: hundreds of volts, AC/DC
• Loss: very low (metal-to-metal connection, microohms)
• Speed: low (tens of milliseconds to switch)
• Contact bounce, mechanical, coil is inductive
• Cost: ~$5

Field-effect transistor

• Voltage-controlled
• Gate voltage = source voltage + 10, current flows from drain to source
• Control current: zero (but gate capacitance)
• Control voltage: 10V (logic level FETs with 5V)
• Switched current: tens of amps
• Switched voltage: tens of volts, DC
• Loss: low (milliohms)
• Speed: high (with sufficient gate drive)
• Static sensitive
• Cost: $2

Bipolar transistor

• Current-controlled
• Current flowing from base to emitter allows current to flow from collector to emitter
• Control current: 1% - 5% of switched current
• Control voltage: ~0.6V
• Switched current: hundreds of milliamps
• Switched voltage: tens of volts
• Loss: low (0.1V drop between collector and emitter)
• Speed: high
• Easy to find, huge variety
• Cost: $0.10

Previous: H-bridge

Next: PIC intro