Looking for 4-20ma circuits

I have a request for a 4-20ma interface. (input and output)

googleing has found TI / Analog parts.

These parts seem a bit pricey for the function.

Does anyone have any links for in-expensive 4-20ma circuits.

Thanks

hamilton

 

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

John Larkin posted one

<ftp://jjlarkin.lmi.net/Ipump.JPG>

 

Thanks, I take it the input is PWM ?

 

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

 

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

 

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

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

hamilton

 

I always assumed they were related somehow though?

 

Digital was 20mA, (or 60mA), 4-20 is analogue.

 

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 <a for 300ms> <b for
300ms> <c for 300ms> 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?

?-)

 

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
4-20 mA it was a current loop.

?-)

 

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

?-)

 

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.

?-)

 

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

?-)

 

Found some cool stuff.

http://www.laurels.com/linearizing_pricing.htm


And their interfaces:

http://www.laurels.com/interfaces_serial.htm

 

Now tell us the science as to why it begins at 4mA.

 

Current is current. It is VERY precise, and properly linearized, very
accurate at any point in the loop.