Looking for 4-20ma circuits

I refer you to my (other) thread of last year. Well I would if I could find it.

John Larkin posted one

Thanks, I take it the input is PWM ?

hamilton expressed precisely :

In the OLDEN DAYS that was called Current Loop, a comunication protocol like RS232 (Not directly but used for communication between devices)

No, that's something else.

This is an analog instrumentation standard.. 4mA = "Zero", 20mA = "Full Scale", similar to the old pneumatic 3-15 PSI standard.

HART stacks a digital protocol on top using a 1200 baud modem.

Best regards, Spehro Pefhany

There was this thing I did for you 11 years ago...

Seems there was something wrong there, something about I attached ground at the wrong node, but it was OK once that was moved... I don't remember now ?:-) ...Jim Thompson

I missed the request for an input - but can't you just use a 250 ohm resistor say?

Hi, No, it's an analog voltage.

But, It has a cap in there for a integrator process..

so, it may still work as a PWM input..

Jamie

It's in the wrong place to integrate the +input.

No, the caps just stabilise the local voltage:current loops. An input voltage change notionally results in an instantaneous output current change. Of course a real circuit will have bandwidth and slew rate limits, but it is not intended to convert PWM.

John

No idea what the target application is, you may not have to concern yourself with this at all. Just be aware that for signal transmission within areas where there is a possibility of ignition or explosion, such as process plants or storage areas for flammable materials, regulatory requirements apply. You can't just build a box then provide it for installation.

Thank you all for your insights into 4-20ma circuits.

The application is for machine motor control, not dangerous. (I hope)

hamilton

Spehro Pefhany expressed precisely :

Yes there are both analog and digital Current Loop protocols. But what did the OP want? he was not very specific.

I always assumed they were related somehow though?

Perhaps arbitrarily picked as the same 20mA as the current loop, and with the same zero supression as the pneumatic standard. It sort of feels in the right order of magnitude for long runs, immunity from EMI and immunity to corrosion in connections. There were other 'standards' too, 10-50mA, for example. I doubt anything could run off the 4mA (better to count on 3~3.6mA) in the olden days, but nowadays it's quite common to have computers and displays as well as sensor excitation and signal conditioning that work off of '2-wires'.

Best regards, Spehro Pefhany

I had an ASR33 teletype once, I think that used the 20mA loop.

Hey, does anyone know... We have an application where the customers want "4-20mA" but there are several variables to communicate. A competitor uses a "time multiplexed" arrangement where the values are output in a repeating sequence like etc. There is some kind of sync value using an out-of-range current to indicate the start of the sequence.

Do you know if any of this is standardized or directly supported in PLCs? Or is it all ad-hoc?

Thanks,

John

So, are you looking for transmitter or receiver?

?-)

=20

I suspect you may have misunderstood the analogy. Nor was it presented well. Digital 20 mA and 60 mA were bit serial "teletype" standards were king of the hill long before what is now TIA-232. Like the control = system

?-)

protocol=20

Nonsense. For those familiar with the art 4-20 mA is clearly the analog loop. Right in the subject line.

?-)

protocol=20

The standard did not exclude locally powered (transmitter) devices. They did have to be isolated though. Ground loop thing. Worked way back in the tube days.

?-)

protocol=20

I think it is ad hoc, but i will bet that it uses 24 mA (error value) as the sync.

?-)