Fluke 5725A User Manual
Page 107
Theory of Operation
Detailed Circuit Description
4
4-37
4-60. AC Line Voltage Selection Section
In addition to the primary power switch, S401, this section contains three line-voltage
selection switches, S402 through S404. These three switches allow the 5725A to be set
to eight nominal ac line voltage levels, each with a 10% tolerance.
Thermistors RT401 and RT402 have negative temperature coefficients, and limit inrush
current at power-up. Relay K401 provides a pair of shunt switches to bypass thermistors
after about a half second. This allows greater efficiency than with thermistors alone, and
the use of thermistors allows operation if the relay fails. CR401 through CR404 and
C401 form an unregulated power supply that is not isolated from ac line power for
controlling K401. R401, R402, and C402 provide a time delay that prevents Q401, and
hence K401 from switching before the effective power supply capacitance is sufficiently
charged.
Metal Oxide Varistor RV401 prevents high voltage transients on the ac line from
reaching the transformer by clamping voltages above 275V ac.