Reactive Loads; Capacitive Loads - Keithley 3706A User Manual

Section 4: Operation considerations

Reactive loads

The 3762 is specified for resistive loads only. Since reactive loads can cause excessive currents and
voltages, current surge limiting (for capacitive loads) and voltage clamping (for inductive loads) are
required to prevent damaging the relays and external circuitry.

Capacitive loads

The surge current from a capacitive load must be less than 0.5 A to protect the relays. The next figure
shows typical circuits to limit current surges. Also, consider the maximum load of 10 VA when
determining the current limit. For example, when switching 500 V, the current must be limited to 20
mA (A = 10 VA/500 V) and the limiting resistor calculation of the next figure would be:
R = V/I = 500 V/20 mA = 25 kΩ
Use an appropriately rated resistor in the test.
The Thermistor must have high resistance when cold, low resistance when hot, and have fast
thermal recovery.
Figure 6: Model 3762 Limiting capacitive reaction current (thermistor limited)
4-2
Model 3762 10-Channel High Voltage Multiplexer Card User's Manual
Figure 5: Model 3762 Limiting capacitive reaction (resistor limited)
3762-900-01 Rev. A / July 2017