
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
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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