On Wed, 21 Jan 2009 11:02:12 -0800, "David Duley"
wrote:
> You should use a metal film 1% resistor. This will
minimize the effect of
> temperature on your measurement. If this is a loop powered 4-20 ma
device
> you wont burn it up. It will never draw more than 20ma. Just check the
> specs
Correct, It is quite difficult to "burn up" a 4-20ma loop. I
consistently
short the outputs
across my ma Fluke meter to verify proper 4-20 ma output.
The resistor inserted into the loop must have one leg grounded.
You lose 20% resolution when inserting this resistor since your range is
now 1-5vdc
instead of 0-5vdc as previously pointed out.
Some systems may well draw more than 20 ma under fault conditions.
The owner would have the opportunity upon initial setup for the loop to
default
with a 0 ma or a 22 ma output. Most opt for the 22 ma since this failure
mode isolates the
failure to the transmitting device and not the loop. More of these options
are available as
the complexity of the devices increase. More below!
> The reason I said "Using just a resistor
seems too easy." is because
> all the current loop equipment I have found is expensive. I doubt
> they use JUST A RESISTOR. The pressure sensor cost about $500 and
> it's not mine. I don't want to burn up this pricy sensor! Wish I
had
> something cheap to experiment on.
>
> Max
Many loop transmitters are connected to more than one receiver. If you tap
into the loop
somewhere in the middle, then ground one leg, you may inadvertently cause
ground loop
problems. This is more of a concern if you are using switch mode power
supplies than with
an isolated transformer or battery based power for your PIC.
See this Maxim App Note that takes its power from the loop and gives you a
0-5vdc output.
It only needs a 10 ohm resistor.
http://www.maxim-ic.com/appnotes.cfm/an_pk/823
Also Burr-Brown has a RCV420 that also converts 4-20 ma into 0-5 vdc.
Notice how one leg
of the resistor must be grounded.
focus.ti.com/lit/ds/symlink/rcv420.pdf
The best place to connect these devices is at the control panel where the
24Vdc loop power
first leaves before picking up any receivers.
If you want to possibly avoid setting off alarms, solder the resistor into
the uncut loop wire than cut the wire
between the resistor leads. That way you never lose your loop.
It's all a question of accuracy and convenience of installation.
Let us know what you decided!
Dennis
4-20ma Analog Current Loop to Pic A/D
Started by ●January 20, 2009
Reply by ●February 6, 20092009-02-06
Reply by ●February 6, 20092009-02-06
On Jan 21, 2009, at 10:56 AM, kg4pid wrote:
> The reason I said "Using just a resistor seems too easy." is because
> all the current loop equipment I have found is expensive. I doubt
> they use JUST A RESISTOR. The pressure sensor cost about $500 and
> it's not mine. I don't want to burn up this pricy sensor! Wish I had
> something cheap to experiment on.
>
> Max
That pressure transmitter was designed to be installed in a harsh
environment by "Bubba." You are not going to burn it up. The worst
thing that you can expect in what you are doing, is frying the
microcontroller.
Your right the current loop measuring stuff is expensive but they use
resistors. They may have some buffers and an optical isolator or two
but these are to protect the module, backplane, PLC processor etc.
Not the pressure transmitter.
We've used 250 Ohm 1% Metal Film resistors around here for years to
convert 4-20mA to 1-5VDC anytime we want to send the input to multiple
controllers. It works great.
Phil Birkelbach
p...@petrasoft.net
http://www.madsciencenotebook.com/
> The reason I said "Using just a resistor seems too easy." is because
> all the current loop equipment I have found is expensive. I doubt
> they use JUST A RESISTOR. The pressure sensor cost about $500 and
> it's not mine. I don't want to burn up this pricy sensor! Wish I had
> something cheap to experiment on.
>
> Max
That pressure transmitter was designed to be installed in a harsh
environment by "Bubba." You are not going to burn it up. The worst
thing that you can expect in what you are doing, is frying the
microcontroller.
Your right the current loop measuring stuff is expensive but they use
resistors. They may have some buffers and an optical isolator or two
but these are to protect the module, backplane, PLC processor etc.
Not the pressure transmitter.
We've used 250 Ohm 1% Metal Film resistors around here for years to
convert 4-20mA to 1-5VDC anytime we want to send the input to multiple
controllers. It works great.
Phil Birkelbach
p...@petrasoft.net
http://www.madsciencenotebook.com/
Reply by ●February 6, 20092009-02-06
On Wed, 21 Jan 2009 11:02:12 -0800, "David Duley"
wrote:
> You should use a metal film 1% resistor. This will minimize the effect of
> temperature on your measurement. If this is a loop powered 4-20 ma device
> you wont burn it up. It will never draw more than 20ma. Just check the
> specs
Correct, It is quite difficult to "burn up" a 4-20ma loop. I
consistently short the outputs
across my ma Fluke meter to verify proper 4-20 ma output.
The resistor inserted into the loop must have one leg grounded.
You lose 20% resolution when inserting this resistor since your range
is now 1-5vdc
instead of 0-5vdc as previously pointed out.
Some systems may well draw more than 20 ma under fault conditions.
The owner would have the opportunity upon initial setup for the loop
to default
with a 0 ma or a 22 ma output. Most opt for the 22 ma since this
failure mode isolates the
failure to the transmitting device and not the loop. More of these
options are available as
the complexity of the devices increase. More below!
> The reason I said "Using just a resistor seems too easy." is because
> all the current loop equipment I have found is expensive. I doubt
> they use JUST A RESISTOR. The pressure sensor cost about $500 and
> it's not mine. I don't want to burn up this pricy sensor! Wish I had
> something cheap to experiment on.
>
> Max
Many loop transmitters are connected to more than one receiver. If you
tap into the loop somewhere in the middle, then ground one leg, you may inadvertently
cause ground loop problems. This is more of a concern if you are using
switch mode power supplies than with an isolated transformer or
battery based power for your PIC.
See this Maxim App Note that takes its power from the loop and gives
you a 0-5vdc output.
It only needs a 10 ohm resistor.
http://www.maxim-ic.com/appnotes.cfm/an_pk/823
Also Burr-Brown has a RCV420 that also converts 4-20 ma into 0-5 vdc.
Notice how one leg
of the resistor must be grounded.
focus.ti.com/lit/ds/symlink/rcv420.pdf
The best place to connect these devices is at the control panel where
the 24Vdc loop power
first leaves before picking up any receivers.
Let us know what you decided!
Dennis
wrote:
> You should use a metal film 1% resistor. This will minimize the effect of
> temperature on your measurement. If this is a loop powered 4-20 ma device
> you wont burn it up. It will never draw more than 20ma. Just check the
> specs
Correct, It is quite difficult to "burn up" a 4-20ma loop. I
consistently short the outputs
across my ma Fluke meter to verify proper 4-20 ma output.
The resistor inserted into the loop must have one leg grounded.
You lose 20% resolution when inserting this resistor since your range
is now 1-5vdc
instead of 0-5vdc as previously pointed out.
Some systems may well draw more than 20 ma under fault conditions.
The owner would have the opportunity upon initial setup for the loop
to default
with a 0 ma or a 22 ma output. Most opt for the 22 ma since this
failure mode isolates the
failure to the transmitting device and not the loop. More of these
options are available as
the complexity of the devices increase. More below!
> The reason I said "Using just a resistor seems too easy." is because
> all the current loop equipment I have found is expensive. I doubt
> they use JUST A RESISTOR. The pressure sensor cost about $500 and
> it's not mine. I don't want to burn up this pricy sensor! Wish I had
> something cheap to experiment on.
>
> Max
Many loop transmitters are connected to more than one receiver. If you
tap into the loop somewhere in the middle, then ground one leg, you may inadvertently
cause ground loop problems. This is more of a concern if you are using
switch mode power supplies than with an isolated transformer or
battery based power for your PIC.
See this Maxim App Note that takes its power from the loop and gives
you a 0-5vdc output.
It only needs a 10 ohm resistor.
http://www.maxim-ic.com/appnotes.cfm/an_pk/823
Also Burr-Brown has a RCV420 that also converts 4-20 ma into 0-5 vdc.
Notice how one leg
of the resistor must be grounded.
focus.ti.com/lit/ds/symlink/rcv420.pdf
The best place to connect these devices is at the control panel where
the 24Vdc loop power
first leaves before picking up any receivers.
Let us know what you decided!
Dennis
Reply by ●February 6, 20092009-02-06
Hello group,
I am just getting started with PICs and this is my first posting to
the list.
An industry standard at my workplace is to have the 4-20ma current
loop terminated with a 250 ohm resistor.
This yields a signal voltage of 1-5 volts DC at the indicating device,
meter, controller, PLC, recorder, etc.
>From Ohms law, E=IR;
.004 amps x 250 ohms = 1 volt
.020 amps x 250 ohms = 5 volts
Hope this is of some help.
I am just getting started with PICs and this is my first posting to
the list.
An industry standard at my workplace is to have the 4-20ma current
loop terminated with a 250 ohm resistor.
This yields a signal voltage of 1-5 volts DC at the indicating device,
meter, controller, PLC, recorder, etc.
>From Ohms law, E=IR;
.004 amps x 250 ohms = 1 volt
.020 amps x 250 ohms = 5 volts
Hope this is of some help.
