Agilent Technologies 90B manual Constant Current Remote Programming, Remote Programming Accuracy

In situations where only low programming voltages are being used, forward conducting silicon diodes (0.7V per junction) can be used in place of zener diodes.

CONSTANT CURRENT REMOTE PROGRAMMING

Most of the general principles discussed under Constant Voltage Programming are also applicable when considering remote programming for constant current supplies. Remote programming of the constant current output of any programmable supply can be accomplished either by:

1.Applying a resistance of voltage to the programming terminals of a CV/CC supply, or

2.Modifying a constant voltage programming supply for constant current operation and then controlling the output current by means of a resistance or voltage applied to the terminals normally used for constant voltage control. A Constant Voltage Supply is modified for Constant Current operation by adding an external current monitoring resistor as described later in this chapter.

Method 1 is used with any Constant Current or Constant Voltage/Constant Current Agilent power supply, while Method 2 is used for any remotely programmable Constant Voltage/Current Limiting supply.

Method 1 has one important disadvantage: the normal current limiting protection, which is dependent upon the constant current setting of a CV/CC power supply, is negated if the constant current programming terminals are accidentally opened. Particular care must be taken in the design of the constant current programming network to insure that no open circuit condition can exist even for a short interval of time, because such an open circuit will program the power supply to an output current in excess of its rating--with almost certain destruction of the series regulating components and other components within the regulator circuitry. Therefore any constant current programming mechanism involving switches must use make-before-break switches.

A good safety precaution is to place directly across the constant current programming terminals of the power supply a control resistance corresponding to the maximum output current. The remote switching mechanism can then be used to shunt this "safety" resistor to the degree necessary to achieve any lower values of output current. The resistor can only be used if non-linear programming of the output current can be tolerated. The speed of response associated with constant current programming is determined by the output voltage change required as a result of change in output current being programmed. The equations given in Figure 61 are applicable in determining the time required for the newly programmed value of constant current to be achieved.

REMOTE PROGRAMMING ACCURACY

Figure 59 shows the relationship between programming resistance and output voltage for a power supply with perfect remote programming. Zero ohms across the programming terminals results in exactly zero volts output, and all other values of programming resistance result in the output voltage predicted by the programming coefficient KP.

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