Agilent Technologies 90B manual Converting a CV Supply to CC Output

As Figure 69 indicates, it is only necessary to add a single external current monitoring resistor to a remote programming constant voltage power supply in order to convert it to constant current operation. (Also any remote sensing protection resistor or diode connected inside the supply from –S to - OUT must be removed.) Because the proper operation of the regulator circuitry requires that the positive output and positive sensing terminals be at nearly the same potential, the external current monitoring resistor RM must be connected to the positive output terminal, while the constant current load must be connected to the negative output terminal. The front panel control (or remote programming control) is used to determine the voltage E across the current monitoring resistor RM. Since this voltage E will be held equal to the voltage EP across the control resistance by feedback action, a constant current IT = E/RM will be caused to flow through the current monitoring resistor RM. The load current IL consists of the current flowing through the monitoring resistor plus the programming current IP (normally negligibly small compared to IT). Both the current through the monitoring resistor and the programming current are held constant by regulator action; thus the net load current is also constant.

Figure 69. Converting a CV Supply to CC Output

Since any change in the value of the resistance RM will result in a change in the load current, the current monitoring resistor should have a low temperature coefficient and should be operated at less than 1/10 (or even 1/100) of its power rating . This, plus the restriction that the total IR drop across RM and RL in series cannot exceed the voltage rating of the power supply, means that RM will be selected so that its IR drop will be of the order of 1 volt, depending upon the constant current value required.

Generally speaking, the constant current performance of a supply connected in the method shown in Figure 69 can be predicted by dividing the constant voltage specification by the value of RM, and then adding on a percentage basis any change in the value of RM due to temperature effects. The lowest constant current output level is limited to the programming current IP, typically 5 milliamps.

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