4 Theory of Operation
108 Keysight E3633A and E3634A Service Guide
source - it cannot be both; transfer between these two modes is accomplished at
a value of load resistance equal to the ratio of the output voltage control setting
to the output current control setting.
Full protection against any overload condition is inherent in the Constant Voltage/
Constant Current design principle since there is not any load condition that can
cause an output which lies outside the operating region. For either constant
voltage or constant current operation, the proper choice of front panel voltage
and current limit settings insures optimum protection for the load device as well
as full protection for the power supply.
The diodes connected across the output terminals in reverse polarity protect the
output electrolytic capacitor and the series pass transistors from the effects of a
reverse voltage applied across the output terminals.
Earth-Referenced Logic
Microprocessor U1 handles GPIB (IEEE-488) control through bus interface chip U6
and bus receiver/driver chips U3 and U11. The RS-232 interface is also controlled
through microprocessor U1. RS-232 transceiver chip U19 provides the required
level shifting to approximate ±9 volt logic levels through on-chip charge-pump
power supplies using C3 and C9. Communication between the earth referenced
logic interface circuits and the floating logic is accomplished through an
optically-isolated bi-directional serial interface. Isolator U5 couples data from U1
to processor U19. Isolator U2 couples data from U19 to microprocessor U1.
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