At 01:47 PM 7/4/2003 +0200, you wrote:
>I am looking for an introduction/tutorial into 4-20mA 'transmission'. I
>am no fool, but have no idea about which party provides the power,
>whether it is to ground or from a positive voltage etc.

I've seen just about every combination possible. Two-wire transmitters
draw their power from their voltage drop in the loop and grounding isn't
an issue (except, perhaps with inputs, where isolation may be required).

If you're supplying multiple outputs in a "4-wire" situation, best to
make the common (-), even though it complicates things a bit.

If you're allowing for inputs that could come from 2-wire transmitters,
a 24VDC supply is typical.

If you're designing a 2-wire transmitter, keep the drop to a minimum,
there may be other parties in series with their own drop. More than
about 8-10V would be substandard. Keep the worst-case maximum consumption
of your circuit to about 3.6mA total.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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