Spark protection circuit for measuring current in high-voltage circuits

A circuit has been designed to protect electrometers and other sensitive instrumentation from damage by electrical sparks. Measuring current is necessary in a variety of high-voltage applications, including the evaluation of ionizers designed to eliminate static charge. The currents are often small, perhaps only several microamperes (μA) or less. Instruments designed to accurately measure such small currents are expensive and sensitive. Consequently, the instruments are susceptible to damage from unexpected electrical sparks that are common in high-voltage experiments. The protection circuit has two layers of protection. The first layer is a Zener Transient Voltage Suppressor that limits the input voltage by shunting the spark energy to ground potential. The second layer of protection is a passive, low-pass filter that blocks the high-frequency energy of the spark while passing the low-frequency current to be measured. The filter design is based on a resistively terminated, LC Cauer network. Three inductors are required to obtain high-frequency attenuation exceeding 10+6, resulting in a sixth-order low-pass filter. Small resistances to ground through connectors and cables, that are normally negligible, limit the high-frequency attenuation to about 10+5, which provides sufficient protection.