Idle Curiosity -- 4-20mA Signalling

That's really a system spec. The limit is the power supply voltage. 40V is about the most you'll find, and 24V most likely. The drop across other devices that may be in seried tells you how much you have left. At

20ma. At 4 ma, you have 1/5th of that. Google will turn up a lot of stuff like Jerry

I'm curious about a "standard" as well. I _think_ the device electronics must simply consume less than 4mA, so that "zero signal input" can cause a draw of exactly 4mA??

This one...

dates back 8 years, in answer to an original post by Spehro. ...Jim Thompson

Don't know either - I think I would make it less then the SELV voltage limit in any case?

Loop powered systems usually have a worst-case low end of about 3.6mA

+/-. They have a minimum and maximum voltage so you can determine the maximum supply voltage and calculate the maximum load resistance. They can go over the 20mA, usually by 10% or more, sometimes as bad as 30mA total (for example, as a result of sensor failure) so you should allow for that. The electronics can use more than 4mA but you'd have to use a SMPS and eat more minimum voltage. Most will work to well under 10V, so maybe you could get 6 or 8 mA at 3.3V. Often you need to run at least one power supply with galvanic isolation plus a microcontroller and perhaps a display off of that power, plus whatever signal conditioning, sensor excitation, ADC converters and other stuff is required to get a signal into the micro, meaning the power supply can't be wasteful (nor are commercial modules usable).

The maximum load resistance limitation usually comes into play when several loads are connected in series (and perhaps a lot of wire in the middle). For example, a field-mounted loop-powered indicator connected in series with the SCADA system input.

Four wire (non-loop-powered) systems don't have the lower end limitation, but if you want to tell the difference between railed at the low end and a broken loop you need to avoid going right to zero. There's a maximum load resistance specification. If you're going

4-wire it's best to galvanically isolate things as much as possible as the number of ways to f*** things up increases exponentially.

All the systems I've seen don't mind zero resistance load.

I don't know of any standards, but the above are examples of common usage. You might be able to find some more stringent 4-20mA specs as the analog part of the HART (hybrid analog/digital) standard.

all the 4-20mA pressure transmitter and such I use have a voltage spec of 11-32Volts written on the them

you'll ofcourse have to stay under 4mA if you want to use the full range.

-Lasse

You'd have to go by the compliance range given in the datasheets:

The standard would be ANSI/ISA S50.1 which I don't have but parts are outlined here:

As Lasse hinted you'd have to obtain the compliance ranges of all the devices in the system. After all, it won't do you any good if the whole thing complies to the standard but doesn't work reliably.

But, as the scouts say, be prepared :-)

Likewise I have never come across a specific standard for the 4-20mA loop systems but I have seen a range of excitation voltages used in circuits that feature them. Most I dealt with were the teleprinters running on 60VDC supplied 4-20mA loops.

Wikipedia has some useful info.

Tim, I may be corrected but I'm not aware of anything. I've always consulted the system specs, which generally specify in terms of maximum allowable loop resistance. It's pretty common for equipment these days to be powered from

24V, so that gives one an indication, there's some voltage loss across the current source electronics, plus the loop resistance voltage, plus whatever you use for current sensing. AFAIAA, 250 ohms, or 1-5 volts, for input circuits is still typical (but in practice it can be far lower).

I would say an excellent question; another way is to ask what is the voltage compliance. If one wanted to be really bizarre and have a large voltage compliance, then the open circuit voltage could be up to 100KV which would allow a small break in the circuit to be "ignored".

Yes, the 24V (or 32V or 40V or whatever) gives you an idea of worst-case power dissipation, but there can be long wires on these things and they need protection against relatively high transient voltages (think lightning strike at the old refinery), which usually means discrete components, not ICs, will take the brunt it.

Best regards, Spehro Pefhany

This would require a quite large loop current also.

While the high open loop voltage may allow sparks to occur across air gaps, the created ionization does not last long, unless sufficient power is supplied to compensate for the radiation losses.

Of course with long wires and large stray capacitances that would create a resonant circuit, so this would be a nice spark gap transmitter and as such, could be used for wireless communication :-).

Paul

Some of the transmitters that are 4 to 20 ma loop powered require about 2 ma to operate. Typically, the receiver device has a 250 resistor to provide a 1 to 5 volt signal to it. The power supply is typically 24 vdc. So if you allow 2 ma for operation, then for a zero signal input to the transmitter you get 4 ma output and likewise an extra 18 ma is added to get a 20 ma output at the 100 percent input to the tramsmitter. The Motorola Veritrak Diff. Press and Press transmitters did this. You could adjust the transmitters to use any 25 percent of the range for the 0 percent to 100 percent outputs. Foxboro used a 10 to 50 ma system but used a 100 ohm resistor to get the 1 to

5 volt signal to the receiver device or controller.

Any way, you have to live with between 0 ma up to 4 4 ma electrical output zero for your device. The transmitter amps are just fancy electronic rheostats that vary from an effect 23K ohms to get 4 ma at a 0 percent signal to the transmitter down to 950 ohms to get 20 ma at the 100 percent signal to the trasmitter. That could be some thing like 0 to 400 PSIG or 0 to 100 inches of mercury differential pressure. Dave Foreman

The abouve effective resistance values are with just the 250 ohm resistor at the receiver device. Dave Foreman

Thanks for reminding me, if your loop is category Ex-I (intrinsically safe) then the loop has to support the voltage drop of the safety barrier if one is used. Barriers can have significant voltage drop.