GCSE    Switches    Questions 0 to 23   -->  View All  

Compare Switch Types

• BJT switches are fast and the transistors are not expensive. A minus point is the current needed to operate the switch.
• MOSFETS are even faster but they cost more. A plus point is the minute current needed to operate the switch - nearly zero.
• OP-Amp comparators are slow, more expensive, but much more sensitive to input voltage changes.
• Logic GATE switches are fast and cheap but not very sensitive to small input voltage changes.
• Mechanical switches are less reliable due to the moving parts.
• Electromagnetic relays are less reliable due to the moving parts.
• Mechanical switches and relays can switch high voltages and currents and also alternating current.

In this circuit a high logic level on the input switches on the N Channel MOSFET and the motor will run.

The diode prevents MOSFET damage due to back emf by allowing the motor current to die away gradually if the MOSFET is suddenly switched off.

A power MOSFET with a heat sink can switch nine amps. Multiple MOSFETS can switch much more current.

Calculations

• The drain (d) current,   I_D   = g_M ( V_GS - 3 )
• The electron flow is always from source to drain and never goes the other way.
• g_M is called the transconductance ...
  • It's measured in Amps per Volt. For example increasing V_GS by one volt might increase I_D by 10 Amps and the transconductance would be 10.
  • The unit of transconductance (often ignored) is the Mho (Ohm backwards) and the symbol is ℧.