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How can I design a system that would allow me to put multiple receivers on a single 4-20mA current loop? Assume that I have design control over only one of the receivers.

Sensor:

  • Loop powered (2-wire)
  • Supply voltage: 12-28VDC
  • Max Load: 250 Ohms @ 12VDC (500 Ohms @ 24VDC)

Current Receiver Design:

  • 250 Ohm resistor to ground
  • Loop power supply ground is connected to ADC ground
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  • \$\begingroup\$ Do you have a data sheet for the sensor? \$\endgroup\$ – tcrosley Nov 18 '10 at 22:20
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Current loops were initially designed to connect multiple teletypewriters in series (originally 60 ma was used instead of 20 ma). This scheme was latter adapted for instrumentation use, with 4-20 ma as the standard.

In your current receiver design, one end of the resistor is connected to ground. Instead it should be passed on to the next receiver, like the old series Christmas tree light strings. The last receiver connects to a lead which goes back to the transmitter ground.

At each receiver, a 250 ohm resistor is placed in the loop. The two sides of the resistor can then be fed into an optical coupler, which will turn a transistor on/off which you can feed to your circuit. (Or if you need to still be able to power your sensor from the loop, you will need to make the sensor run off a floating ground reference tied to one end of the sensor.)

If the sensor requires a minimum of 12v, using a 250 ohm resistor this implies a current of 48 ma, not 20 -- so I'm not sure if this is really a 4-20 ma loop or not.

If you have to use the current receiver design, it should be placed last. All of the others must allow the loop to be passed on. (Which limits you to two receivers, one new and one original design if you are allowed only one new receiver.)

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    \$\begingroup\$ Can you add another 250 ohms resistor in the loop? I thought from the question that the maximum load is only 250 ohms. \$\endgroup\$ – RMAAlmeida Nov 18 '10 at 21:41
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    \$\begingroup\$ That's the whole point of current loops -- you can several receivers in series, because the source (current generator) generates a constant current, not a constant voltage. For a 20 ma loop, each 250 Ω resistor will develop .020A*250Ω = 5 volts across it. The power supply must output a voltage equal to or greater than all the voltage drops in the system combined. \$\endgroup\$ – tcrosley Nov 18 '10 at 21:57
  • \$\begingroup\$ Of course, but the sensor in the question can have a maximum load of 250 ohms. \$\endgroup\$ – RMAAlmeida Nov 19 '10 at 14:31
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As the sensor you are using has a maximum load I think that one (maybe good) alternative is use a hall sensor on the loop. This way you can extract the information on the current without interfering in the main loop. The other receiver can be a regular one.

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Have a look in to the HART protocol - it allows multi-drop communications over existing 4-20mA lines

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