4-20mA Analouge input

4-20 dc ma is a standard industrial process control interface signal between field sensors (pressure,temp,flow,etc) and process control electronics in the control house. This 'loop' is generally powered from a 24vdc power supply. The advantage, of course, is that there is no error induced because the length (resistance) of the signal wire pair that can be hundreds to a few thousands feet long if needed. The loop voltage also 'powers' the field sensor device. Most input circuits just wire the 4-20 in series with a 250 ohm resistor to measure the 1-5vdc voltage drop using a diff input voltage amplifier to further process or convert it with a A/D convertor. The reason the 4ma = 0% level (live zero) is to have a means of detecting an open loop condition and of course to have a small amount of power for the field device to consume independent of the measurement value. Works very well and has been in use for decades.

Lefty

Yes, you supply a 24 volt current source from your input to the transmitting device. Normally, you do that via a 24 volt supply through a resistor where at which point, you measure the return voltage and do the math./. just subtract what ever is coming back and use that to do the basic ohms law of the source resistor you're using. just put this into a op-amp as a voltage follower.

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and remember, that in cases where you have multiple devices connected in series, each device will attempt to maintain 20 ma on the TX in the loop as an off condition so that the other devices in the loop can lift the loop to 4 ma so that a change in condition can be seen at the receiver. This is mostly for SERIAL communications. Just thought i would through that in!

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
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quoted text -

I'm not sure at all what you are describing but it's not the industrial standard instrumentation 4-20ma interface. This is strickly a DC analog measurement standard and not a digital communication interface. There can be digital communications superimposed by injecting a small ac fsk signal onto the DC measurement (HART protocol) but the basic 4-20ma standard is to support one field device hardwired to a central control point and was in use well before digital electronics came onto the industrial control world.

Lefty

You need to know a little more about what you are connecting to.

4-20mA, 'does what it says on the tin', delivering a varying current, dependant on some signal that wants to be transmitted. Now it is common on some stuff, for the source to provide isolation, and to power the loop. However on other areas using the same interface, the receiver powers the loop. Even worse, there are some sensors, which require receiver isolation... So you need to know, which end of the loop is going to supply power, and if the other end is isolated or not (if not, then you should provide isolation). Some applications will require both ends to provide isolation. AN54, from Siemens, shows how to use their IL300 analog opto-coupler, to provide a line powered isolated interface. TI (Burr Brown), do an op-amp, which provides an isolation barrier in the amplifier, as an alternative way of doing this.

Best Wishes

Thanks heaps Lefty, Jamie and Roger, Been a great help.

Best regards

AJ