Outcome 2.7 Power Supplies: Isolation Choices and Overcurrent Protection

Transformer-powered supply

Uses a mains-frequency transformer to change mains voltage and provide galvanic isolation.

Galvanic isolation

Electrical separation between the mains and output, often provided by a transformer.

Line-connected supply

Powered directly from the mains without a mains-frequency isolation transformer.

Resistive dropper

Uses resistors to drop voltage, inefficient and generates heat.

Capacitive dropper

Uses a series capacitor’s reactance to limit current, common in low-power applications.

Non-isolated rectifier

Rectifies mains to high-voltage DC without providing electrical isolation.

Safety barrier

A feature of transformer-powered supplies that prevents direct contact with live mains.

Voltage regulation

The process of maintaining a constant output voltage despite variations in input or load.

Current limiting

A method used in line-connected supplies where output depends on load current.

Thermal trip

A circuit protection mechanism that opens based on heat generated by overload current.

Magnetic trip

A circuit protection mechanism that responds quickly to very high short-circuit currents.

Fuses

Sacrificial devices that open the circuit when current exceeds a threshold.

Time-delay fuse

A fuse designed to tolerate brief overloads but opens on sustained overload.

Interrupting capacity

The maximum fault current that a fuse or breaker can safely clear.

Rated fuse holder

A proper housing for a fuse that secures it and ensures safe operation.

Nuisance blow

An unwanted opening of a fuse or breaker due to brief overloads like motor inrush.

DC rating in fuses

Specific ratings needed for fuses used on DC circuits due to difficulty interrupting current.

Circuit breaker

A resettable switch that opens automatically under overcurrent conditions.

Selectivity in protection devices

The desired behavior where protective devices coordinate to trip in the correct order.

Short circuit condition

A fault condition where current through a low-resistance path becomes extremely high.

Electrical noise

Unwanted disturbances in electrical signals, often more prevalent in non-isolated supplies.

Load current

The current drawn by a device during normal operation.

Overcurrent protection

Devices designed to interrupt current that exceeds intended operating values.

Current rating of a fuse

The nominal current a fuse can carry without opening.

Voltage rating of a fuse

The maximum voltage at which a fuse can safely operate.

Inrush current

The initial surge of current that occurs when an electrical device is powered on.

PCB trace ampacity

The maximum current that PCB traces can safely carry without overheating.