A Level >Power Switching< Capacitor Phase Diac MOSFET Phase Relay Thyristor Thyristor AC Transistor Triac |
Power Switching |
|
Device |
Advantages |
Disadvantages |
Cost |
Switch (hand operated) | Simple and cheap. | No automation is possible. | £1 |
Electromagnetic Relay |
High voltages and currents can be switched. AC can be switched. Minimal energy lost in the switch. No waste heat. |
Slow. Moving parts make it unreliable. The coil current has to be present to keep the device on. Quite expensive. |
£3 |
Bipolar Transistor |
Fast. No moving parts. Reliable. Low cost in low power scenarios. Saturation RCE is about 10 Ohm (ballpark figure) |
Low power switching only. High voltages are not possible. AC switching can't be achieved with a single device. There is significant waste heat in the device. The base current has to be present to keep the device on. Saturation Vce = 0.25 to 0.6V |
0.1p |
MOSFET |
Faster than bipolar. No moving parts. Voltages up to about 100 V are possible. Less waste heat than bipolar devices. Better for high power switching. Saturation RDS is about 1 Ohm |
AC switching can't be achieved with a single device. There is significant waste heat in the device. The gate voltage has to be present to keep the device on. |
£1 |
Thyristor |
Very simple circuitry. |
DC only. |
40p |
Triac |
DC and AC. This is effectively two thyristors back to back. The properties are bi-directional. Waste heat less problematic. Low cost. |
30p | |
Diac |
It's turned on by voltages above its break-over voltage. This can be used to "crowbar" power supply over-voltage surges. If the voltage becomes too great, the fuse gets blown. Its speed and performance are similar to Thyristors and Triacs. |
On its own it's not suitable for control system switching. | 20p |
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