Field device current limiting

A field device for an industrial process comprising:

• a pair of terminals configured to receive power from a 2-wire Advanced Physical Layer (APL) spur;
• a current limiting circuit configured to limit a current through the terminals comprising:
• a current source connected to one of the terminals and configured to source a current conducted through the terminals having an amplitude that is at a current source threshold;
• a first current limiter connected to the current source and configured to limit the amplitude below a first threshold;
• and
• a second current limiter connected to the first current limiter and configured to receive the current from the first current limiter and limit the amplitude below a second threshold;
• and
• field device circuitry connected to the second current limiter and powered using the current,
• wherein:
• the current source includes a pass transistor through which the current travels;
• and
• an emitter of the pass transistor is disconnected from a base of the transistor.

A method of controlling a current in a field device, which includes a pair of terminals, field device circuitry and a current limiting circuit that includes a current source connected to one of the terminals, a first current limiter connected to the current source, and a second current limiter connected to the first current limiter and the field device circuitry, the method comprising:

• receiving power from a 2-wire Advanced Physical Layer (APL) spur at the pair of terminals;
• sourcing a current conducted through the terminals using the current source, which limits an amplitude of the current to a current source threshold;
• receiving the current from the current source at the first current limiter;
• limiting the amplitude of the current below a first threshold using the first current limiter;
• receiving the current from the first current limiter at the second current limiter;
• limiting the amplitude of the current below a second threshold using the second current limiter;
• and
• powering field device circuitry using the current,
• wherein:
• the current source includes a pass transistor through which the current travels;
• the current travels through first and second terminals of the pass transistor;
• and
• the amplitude of the current is based on a voltage at a third terminal of the pass transistor, which is disconnected from the first and second terminals.