# 4-20mA scaling ???????

Discussion in 'Electronic Design' started by Fred Bloggs, Aug 17, 2004.

1. ### Fred BloggsGuest

You can make an active current divider with R2 a variable resistor like
so, it can be linear enough for small scale changes:

View in a fixed-width font such as Courier.

R2
Iin x -------
R1 + R2
Iin ---> vfd input
---> +-----------+
R1 | |
| | | | |
4-20mA | | | | |
loop | | | | |
| | | | |
<------|-------------------|----+---+ |
| | | |
| | +-----------+
| |
| |
| / | |
| R2 / +|--+
+----/\/\--+-< |
| \ -|--+
| \ | |
| |
| |
+--------+

2. ### Tim WescottGuest

(a) Use a panel-mount PLC. Overkill, but you'll have lots of useful
features.

(b) Post your question on sci.engr.control. There's folks there who
know _way_ more about industrial control than I do, and will be able to
help you out with finding a simple, rugged & maintainable adjustment
device to work on current loops.

3. ### PabloGuest

I am working on a project where I have 1 DC drive which outputs a 4-20mA
signal to a VFD, I want the VFD to follow the DC drive which it does,
however I want to be able to scale the VFD. That is if the DC drive is at
say 50% then the VFD would be at 50%, but, in some cases I want the VFD to
be say 45%. I have gone through the programming and found that the DC drive
has no means of scaling the 4-20mA signal and the VFD would be too
complicated for my production people to program each time they wanted to
change scaling. Is it possible to adjust the 4-20mA signal in some way, say
with a potentiometer or some device. I would like to do it this way so as to
make it simple for my operators. This setup is basically a machine which is
driven by a DC drive and a colour feeder which sends a pigment into the
machine, when we push the output up or down on the machine the colour must
follow, but as you may have guessed the colours are fed at different
percentages. Thanks to all whom reply. Paul

4. ### nospamGuest

Trouble is that will also divide the 4mA offset. Dividing by 2 would give
you 2..10mA not the 4..12mA you want.

If the VFD has a voltage input then a commercial 4..20mA -> 0..10v
converter and a pot is probably as simple as it gets?

Also even cheap VFDs are getting quite sophisticated, I'm sure some will
have the facility to scale one analog input with another.

5. ### BanGuest

Pablo, the best would be to convert the signal back to a voltage, scale this
with a pot and create again the 4-20mA current. There are ICs dedicated to
this task. AD694 comes to mind. This is a very precise and reliable method
with well defined output current, that works with every transducer and
connected data logger under all circumstances.
You could of course add or subtract a certain current with a bipolar current
source as well, which would require more components.

6. ### Jim ThompsonGuest

Fred, You also moved the zero point.

...Jim Thompson

7. ### John PopelishGuest

There are analog building blocks that can be put together to do this
sort of thing. They are industrial parts that accept and output 4-20
ma signals.

Take a look at the catalog from an example such as Action Instruments.
http://www.actionio.com/
http://www.actionio.com/products/actionpak/signalcond/index.html

8. ### Roger HamlettGuest

So much depends on how the existing input behaves, and the output is used.
At the 'simplest', the input may well have a fixed DC resistor, across
which the voltage is measured to sense the current. In this case, the
current can be effectively scaled, by simply paralleling a second resistor
with the existing one.
However there is then a significant 'caveat', in that this moves the
entire 'range', so (for instance), if the input had a 100ohm resistor, and
a 1400ohm resistor was paralleled with this, the measurement at 12mA,
would reduce by 5%, but the new 'zero' point, would become 4.28mA, rather
than 4mA. Also some sensors use an overcurrent (normally at about 22mA),
as an 'alarm' indication, with the current reduced like this, the
overcurrent value would no longer be properly sensed.
To do the job 'properly', requires a lot more care, with probably a
circuit to subtract the 4mA offset, scale the value, and duplicate any
'special' values like the overrange signal, then add back the offset...

Best Wishes

9. ### PabloGuest

Most of the time we will be mid scale(like 12 mA) , thus if the bottom end
is affected this does not matter, I just want to add a pot or something so
that we can add the pot setting to a setup manual for a particular product.
Thus the operator can do a setup and simply dial in the pot setting from the
manual and know that it is the same as the last time and the percentage of
color being added is the same. I was thinking of a three digit pushbutton
type pot whereas the setting is very easily recorded and set.

10. ### PabloGuest

Fred

Not sure I understand the device beside R2, can you exagerate. I am thinking
that this would work but I am a little confused. Also what value would one
use for R1. Thanks Paul

11. ### Spehro PefhanyGuest

It's pretty easy to do this sort of thing electronically, but I
suggest you just buy a module that will be protected against reversed
inputs, have a power supply (if necessary), have a reasonable
frequency response (and rolloff) and all that practical crap that
makes it into work.

Best regards,
Spehro Pefhany

Good Luck!
Rich

13. ### Fred BloggsGuest

This impractical thing will take care of that offset:
View in a fixed-width font such as Courier.

..
..
..
..
..
..
.. CS R2
.. +-----+ (Iin-4mA)x------- + 4mA
.. +--|4mA->|--+ R1 + R2
.. Iin | +-----+ | vfd input
.. ---> | | +-----------+
.. | R1 | ---> | |
.. >------+---+---/\/\----+---+--------------------+---+ |
.. | | | | |
..4-20mA | | | | |
..loop | | | | |
.. | | | | |
.. <------|-------------------|--------------------+---+ |
.. | | | |
.. | | +-----------+
.. | |
.. | |
.. | / | |
.. | R2 / +|--+
.. +----/\/\--+-< |
.. | \ -|--+
.. | \ | |
.. | |
.. | |
.. +--------+
..

14. ### Spehro PefhanyGuest

It sounds like you really need an operator interface panel that
provides this minor fiddling of the process signals- something that's
repeatable and easy/error-free to set up for each job. In retrospect,
the modules that I and others have suggested might not be the best for
that purpose.

Are you doing compounding of polymer resins? Is the environment
extremely dusty?

I might be able to help with this little project, feel free to contact
me if you'd like.

Best regards,
Spehro Pefhany

15. ### Wouter van OoijenGuest

It's pretty easy to do this sort of thing electronically, but I
I make such modules for a customer. They include a PIC18F252, have two
current inputs and one output, and can perform any computation
inbetween. Also a character LCD. But these are not cheap

Wouter van Ooijen

-- ------------------------------------
http://www.voti.nl
PICmicro chips, programmers, consulting

16. ### Rich GriseGuest

Spehro Pefhany wrote:
....
....

Geez! Is _everybody_ out of work?

17. ### Roger GtGuest

: Spehro Pefhany wrote:
: ...
: > I might be able to help with this little project, feel free to
contact
: > me if you'd like.
: ...
:
: Geez! Is _everybody_ out of work?
:
:
:

Pretty much.... I know 37 unemployed Engineers myself!

18. ### Rich GriseGuest

I guess it's a good thing they needed that new drywall taped today then. ;-)

Cheers!
Rich

19. ### PabloGuest

I have now two options (that is bought type options) and I am waiting for
salesman on the one. First is a KB product which does exactly what I need it
to, however at a cost of 175.00 canadian, add that with a good ten turn or
pushbutton pot which cost about \$50-75, thus overall cost would be about
250.00 per machine. We are looking at 8 machines. The other option which is
getting me firm is to use a operator interface which Control Techniques
sells and use it to scale the current output.

20. ### Spehro PefhanyGuest

What model number is that?

Best regards,
Spehro Pefhany