Chapter 4 Theory of Operation
Power Mesh and Control
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4
Two error amplifiers are included in a CV/CC supply, one for controlling output
voltage, the other for controlling output current. Since the constant voltage amplifier
tends to achieve zero output impedance and alters the output current whenever the
load resistance changes, while the constant current amplifier causes the output
impedance to be infinite and changes the output voltage in response to any load
resistance change, it is obvious that the two amplifiers can not operate simultaneously.
For any given value of load resistance, the power supply must act either as a constant
voltage source or as a constant current source - it can not 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.
The +6V power mesh and control circuits are similar to those of the ±25V power
circuits except that the the TRIAC preregulator is not used. The ac input, after passing
through power transformer, is rectified and filtered. By feedback action, the series
pass transistor alters its conduction to keep the regulated dc output voltage constant.
The +6V and the ±25V power mesh and control circuits are isolated from each other
and the two outputs of the ±25V power mesh and control circuit share a common
output terminal.
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