Many electric locomotives used them in conjunction with transformers to convert high voltage AC from the catenary to relatively low voltage DC for the traction motors.

Low voltage DC from the battery is chopped by a trembler circuit and this pulsed square wave used to drive a transformer, providing the 90V or so required by valves.

However, the solution to be applied to overhead power lines, short of conversion to underground cable or high voltage DC transmission, is not so easy considering the very substantial voltages and currents involved and the heavy capital investment in existing structures.

The power supply then rectified this high voltage to make the high voltage DC required for the vacuum tubes, (called valves in British English).

Some equipment in fixed stations may run off low voltage DC instead of AC, and require a separate power supply.

A proposed high voltage DC transmission line would transmit wind generated electrical power to the Chicago area from northwest Iowa.

Dynamos still have some uses in low power applications, particularly where low voltage DC is required, since an alternator with a semiconductor rectifier can be inefficient in these applications.

Miniature indicator LEDs are normally driven from low voltage DC via a current limiting resistor.

High voltage DC (380 V) is finding use in some data center applications, and allows for small power conductors, but is subject to the more complex electrical code rules for safe containment of high voltages.

They then use high voltage DC and a magnetic lens to focus a modulated high energy electron beam through a small drift tube like a klystron.