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Topic: Input output interfacing (Read 1 time) previous topic - next topic

mmoscz

Nice, your sensor will work fine with 12V suppy, but connect the wire + to 12V and wire - to resistor, in your figure is invert..
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MarkT




The polarity symbols you have drawn for the round temp and RH are backwards.



Ah, that's not actually true, since the sensor is drawn as a current-source, and the signs represent current
measurement direction, not voltage.  Usually the signs aren't written as the arrow serves to indicate
polarity.
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retrolefty

#7
Feb 14, 2013, 02:07 am Last Edit: Feb 14, 2013, 04:38 pm by retrolefty Reason: 1




The polarity symbols you have drawn for the round temp and RH are backwards.



Ah, that's not actually true, since the sensor is drawn as a current-source, and the signs represent current
measurement direction, not voltage.  Usually the signs aren't written as the arrow serves to indicate
polarity.



I'm speaking from a real word situation where all such '2-wire 4-20ma transmitters' have just two terminals labeled + and -. The + must wire to the positive loop voltage source and the - must wire to the series 150 250 ohm resistor, which then wires to the negative loop voltage terminal.

Lefty

Veco

I've no idea what, if anything, is the symbol for a 4-20mA sensor. A current source makes sense in my head at least.

I guess if it was drawn correctly the + and - wouldn't actually be around it at all, the diagram only needs to show the 12V source and ground.

The documentation however only gives the electrical connections in terms of a + and - so it's good to have for reference.

In other news, 250ohm resisters ordered, I was shocked at how they start at £5 for such a tiny component. 0.1% tolerance and a specific use market do that i guess.

MarkT

The +/- on a current source symbol are unusual (it is potentially confusing, pun intended!).

250 isn't a standard value, so fewer are sold, 0.1% is a rare requirement, so fewer are sold, price
is strongly inversely proportional to volume sold.  Often an unusual value can be synthesised from
standard values in series or parallel, which can be cheaper.   270 0.1% in parallel with 3k3 1% would
give 250 at about 0.2% accuracy.
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