Electronic Parts

Discrete Components

Electronic and Digital Switching

Mechanical contacts are only one way to control current. Semiconductor devices can perform the same general switching function with no moving parts. They are usually faster and can be controlled directly by electrical signals, but they behave differently from an ideal metal contact.

Three electronic ways to switch

Transistors as switches

A bipolar transistor can be driven between a low-current "off" condition and a conducting "on" condition. In switching circuits the transistor is normally driven well into the required state rather than used in the middle of its analog operating range. A small base current can therefore control a larger collector current.

MOSFET switches

A MOSFET is controlled primarily by voltage at its gate. When selected correctly and driven properly, a power MOSFET can have very low on-resistance and can switch loads quickly. MOSFETs are widely used for motors, lamps, solenoids, power conversion, and logic-level switching.

Unlike a mechanical contact, a MOSFET has polarity, voltage limits, capacitance, and an internal body diode that may matter in the circuit. "Off" and "on" are useful switching descriptions, but a semiconductor switch is not literally an open piece of metal or a perfect short circuit.

Low-side and high-side switching

A transistor or MOSFET placed between a load and ground is commonly called a low-side switch. A device placed between the supply and the load is a high-side switch. High-side switching may require a different transistor polarity, a special driver, or a gate voltage that can move above the supply, depending on the device and circuit.

Analog switch ICs

Integrated-circuit analog switches use MOS devices to connect and disconnect signal paths. Some are bilateral, allowing signals within their permitted voltage range to travel in either direction. Multiplexer and demultiplexer ICs extend the idea by selecting one of several signal paths using digital control inputs.

Analog signals need special care

When switching audio or other analog signals, the signal voltage must remain inside the permitted range of the electronic switch. On-resistance, distortion, leakage, capacitance, and charge injection can matter even when the digital control signal is perfectly valid. This is one reason an analog-switch IC chosen for audio routing may be quite different from a power MOSFET chosen to run a motor.

Digital logic as switching

Digital circuits represent information with defined voltage ranges, often discussed as logical 0 and logical 1. Internally, logic gates are built from transistors that switch conduction paths in carefully arranged networks. A mechanical switch can also provide a digital input, but its contact bounce may need to be removed before a logic circuit interprets the signal.

Open-collector and open-drain outputs

Some digital devices provide an output transistor that can pull a line in one direction but cannot actively drive it in the other. These are called open-collector (BJT) or open-drain (MOSFET) outputs and normally use an external pull-up resistor. They are useful for combining control signals, level interfacing, and driving loads within the transistor's rating.

Electronic switch versus relay

Electronic switches can be extremely fast, silent, and long-lived because there are no mechanical contacts to wear. Relays, on the other hand, can provide a very clear physical separation between the control coil and isolated contacts and can switch signals that do not share the control circuit's reference. Solid-state relays occupy a middle ground: they are controlled like relays but switch with semiconductor devices.

Continue to Relays.

Return to the Switches overview.