24VDC or 32VDC?

[Dale Hayes]

24VDC or 32VDC?

What determines the use of a 32VDC power supply rather than a 24VDC supply for powering several 4-20ma devices?

 

[GoldDigger]

24VDC or 32VDC?

What determines the use of a 32VDC power supply rather than a 24VDC supply for powering several 4-20ma devices?

The main determiner will be the loop resistance of the circuit. If it is a long loop of thin wire, it may take a higher supply voltage to be able to deliver the 20ma full range current. In addition to the resistive drop in the wire there will typically be a drop on the order of 10 volts at the transmitter so that it can draw operating power from the loop.
Four wire transmitters will be slightly different. Above describes two-wire.

 

[weressl]

The main determiner will be the loop resistance of the circuit. If it is a long loop of thin wire, it may take a higher supply voltage to be able to deliver the 20ma full range current. In addition to the resistive drop in the wire there will typically be a drop on the order of 10 volts at the transmitter so that it can draw operating power from the loop.
Four wire transmitters will be slightly different. Above describes two-wire.

Never seen 36VDC on 4-20mA, and I've been dealing with circuits over 2000'. Not with today technology. When 4-20mA was in it's infancy and the devices were much more of a power hogs and when it was used over telephone wire with miles of wire and up to 125VDC power supply. (The need for those have long vanished with digital technology.)

 

[Simplex]

Voltage drop

Voltage drop

Ok... What's the minimum voltage your gear will work on? 24VDC? 16VDC? Read the spec sheet.

Then figure the voltage drop, based on the current draw at that minimum supply voltage. Your wire type, gauge, and distance is a factor here. Use the Vdrop formula (google it).

If you find your device needs 20mA at 16VDC minimum, and your wire will drop 10 volts, you must supply 26 VDC minimum at the source, with enough amps to power the device (20mA... i.e.. hamster)

so play it safe and get a 28VDC or 32VDC PSU rated for an amp.