4-20mA generator

134

02:07:12

Very simple and cheap but necessary tool in the work of automation.

published 6 years ago

hurz

6 years ago

Looks like a improvisation. Is the sensor (transmitter) and where is the receiver. I guess receiver is 251Ohm resistor, while 930 or 47Ohm are for TX? Anyway, there is no precise current source in your circuit.

hurz

6 years ago

here a alternative more precise solution for a 20mA current source http://everycircuit.com/circuit/5199127741464576

BillyT

6 years ago

In 4 to 20 circuits the source generates the current regardless of the reciever resistance, to be able to do this the source uses the first 0-4 mA a the working energy. The output is not necessarily a set value, if a devices input is is required to be 1-5V a 250 Ohm resistance might be used if the input is 2-10V a 511 Ohm resistance might be used. I have even had systems with a 1000 Ohm input a few times. Also there could up 3 to 4 loads in series, this can present problems for system earthing.

hurz

6 years ago

four receiver with single resistor as current to voltage "transformer" does cause the following problem: 250Ohm x 20mA x 4 = 20V! So if your transmitter works with an overvoltage of 24V, there is only 4V left for the transmitter current source circuit and the rest does drop on long wires. Test or increase the overvoltage and thicker wires might help.

BillyT

6 years ago

The specs for 4 to 20 systems is for a nominal voltge of 24Vdc. Most systems will accept at least 32Vdc. Some 4 to 20 systems will happily run off 12Vdc. Some systems will bitch if the voltage is only 24Vdc (a HP device is one I've run into). Some systems are a mixture of current and voltage devices, some of the current devices having little load effect on the system. And finally what is the voltage of a 24V lead acid battery.

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