DC power supply choice


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I am using ten 4-20MA loop powered transmitters. I want to run all of the loops off of one power supply. The spec on each transmitter is 750 ohms at 20MA insertion loss. The PLC input card states a resistance per channel of 250 ohms. I calculate that each loop has a resistance of 750+250 ohms(1000ohms). How do I figure the amount of current that the DC power supply at 24VDC has to put out? Thanks

Gerald Moore

The max. current is 10 x 20mA = .2 A The min. Voltage is 1000 x .02 = 20 V So a 24V power supply with at least .2A is sufficient. Good Luck, Gerry
I would be lot more generous in the amperage of the power supply. Two (2) amps capability. What difference in price ? Probably, 2 amps is the smallest size available from reliable suppliers.
May I add something which I feel is an important point which you shouldn't overlook. The loop power supply when it powers a loop, the maximum loop current should be limited. Your PLC burden resistor may have a maximum value for the input current as well as for input voltage, usually in the range of 30-40maDC/ 40 VDC respectively.Also burden resistor of the input channel is a high stability metal film resistor with less than 250mW ratiing.If the the transmitter terminals get short circuited for some reasons it may damage the PLC input circuit as well.Also , when many loops are fed in parallel from a single power supply , then the short circuit across one transmitter will effect the readings of the other loops as well.(PLC logic may trigger some trip / alarm logic attached to these loops). If the loop power supply has an electronic current limit feature the maximum current will be limited to the short circuit current pemissible.If a fuse is used for overcurrent protection then , it may blow leaving all the loops dead until short circuit is removed and the fuse is replaced. The best way is to select a PLC input module which provides an internal loop power supply for each channel, if one is available from the PLC manufacturer.Diagnostic functions are also available. You can also have loop currents below upto 2 maDC and above upto 22maDC a requirment for some cases. Otherwise power the transmitters from specially designed loop power supplies.( For example M-System Japan has a 8 channel loop current supply which can be DIN rail mounted.It is excellent for such purpose.If they are expensive; try designing a circuit with a 723 regulator.It shouldn't be difficult.) Please keep me posted if there are any more comments. Bye Kamath RL Indonesia