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
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