Isolated corona current monitoring using a compensated light-emitting diode as an unpowered sensor

Measurement of the emission current at a high voltage is necessary in monitoring ion production from a corona source, to provide independent confirmation of operation. The wide common mode range required is usually obtained through an isolated system, which requires isolated power to operate, adding complexity and volume. Passing the current through a light-emitting diode (LED) provides an alternative measurement method as the LED's brightness can be used to signal the current's magnitude. The forward voltage loss across the LED is negligible compared with the emitter voltage. Selection of a discrete LED for this task rather than using one within a standard integrated optocoupler package improves the low current sensitivity by two orders of magnitude. A high efficiency discrete infrared LED-photodiode pair is demonstrated to provide measurements of corona currents between 0.2 and 20 µA using a second LED-photodiode pair for analog linearity compensation. The inherent simplicity is well suited to new applications of ion emission in propulsion and weather modification.