Chapter 4 Theory of Operation
Power Mesh and Control
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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