Constant Current Sources

An iron-hydrogen resistor (also known as a barretter) is a unique component that utilizes hydrogen gas and an iron filament to act as a constant current source. It achieves this by exploiting the positive temperature coefficient of resistance (PTC) property of iron.

Operating Principle:

1. Current flow: When current passes through the iron filament, it heats up.

2. Increased resistance: As the temperature rises, the resistance of the iron filament also increases due to its PTC characteristic.

3. Stabilized current: This increase in resistance counteracts any increase in voltage, ultimately stabilizing the current flowing through the circuit.

4. Hydrogen's role: The hydrogen gas amplifies this effect by increasing its solubility in iron with rising temperature, further enhancing the resistance rise and current stabilization.

Applications:

Iron-hydrogen resistors were once widely used in electronics, particularly for:

• Voltage stabilization: Stabilizing voltage supply for sensitive circuits.

• Current limiting: Protecting components from excessive current flow.

• Soft-start circuits: Limiting the inrush current when powering up circuits.

Limitations and Replacement:

Despite their advantages, iron-hydrogen resistors have been largely replaced by modern technologies due to:

• Size and bulkiness: Their glass bulb construction makes them less space-efficient.

• Limited current range: They are typically limited to handling currents in the milliampere range.

• Fragility: The glass bulb can be easily broken due to mishandling.

A Field Effect Transistor (FET) is a voltage to current device. Connecting the gate to the source (gate voltage =0) gives a 2 terminal constant current source. You can make the gate source voltage negative by including a source resistor, to reduce the current. This can be adjustable to deal with FET parameter variability.