Help with 4-20mA signal
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Posts: 46,084
Hey everyone,
I am working on a project to monitor and control temperatures in an oven
up to around 1000 degrees F. I believe the type of temperature sensor I
want to go with is an RTD hooked to an RTD transmitter. The specs on this
transmitter (Model TT246 from Minco) tell me it gives a 4 to 20mA output
over a certain temperature range.
I'm not exactly sure how to read this 4-20mA signal into the stamp. First
thing that came to mind was to hook a 250ohm resistor across the
transmitter output which I 'guess' should give me a 5V output at 20mA and
a 1V output at 4mA. I could then use an ADC like a Maxim MAX187 to sample
that voltage and then read it into the stamp.
Does this sound like it's going to work or am I on the wrong track? I'm a
little confused on the RTD transmitter specs and this 4-20mA signal.
According to the specs the transmitter can be supplied with anything from
10V to 35VDC, and that Rloopmax = (Vsupply - 10) / .020 amps. I'm not
exactly sure what this 10-35V supply and that resistance equation mean in
the big picture.
If anyone could help me out on this 4-20mA standard or could point me to
some resources that may help I'd greatly appreciate it!
Thanks,
Steve
I am working on a project to monitor and control temperatures in an oven
up to around 1000 degrees F. I believe the type of temperature sensor I
want to go with is an RTD hooked to an RTD transmitter. The specs on this
transmitter (Model TT246 from Minco) tell me it gives a 4 to 20mA output
over a certain temperature range.
I'm not exactly sure how to read this 4-20mA signal into the stamp. First
thing that came to mind was to hook a 250ohm resistor across the
transmitter output which I 'guess' should give me a 5V output at 20mA and
a 1V output at 4mA. I could then use an ADC like a Maxim MAX187 to sample
that voltage and then read it into the stamp.
Does this sound like it's going to work or am I on the wrong track? I'm a
little confused on the RTD transmitter specs and this 4-20mA signal.
According to the specs the transmitter can be supplied with anything from
10V to 35VDC, and that Rloopmax = (Vsupply - 10) / .020 amps. I'm not
exactly sure what this 10-35V supply and that resistance equation mean in
the big picture.
If anyone could help me out on this 4-20mA standard or could point me to
some resources that may help I'd greatly appreciate it!
Thanks,
Steve
Comments
>
>I am working on a project to monitor and control temperatures in an oven
>up to around 1000 degrees F. I believe the type of temperature sensor I
>want to go with is an RTD hooked to an RTD transmitter. The specs on this
>transmitter (Model TT246 from Minco) tell me it gives a 4 to 20mA output
>over a certain temperature range.
>
>I'm not exactly sure how to read this 4-20mA signal into the stamp. First
>thing that came to mind was to hook a 250ohm resistor across the
>transmitter output which I 'guess' should give me a 5V output at 20mA and
>a 1V output at 4mA. I could then use an ADC like a Maxim MAX187 to sample
>that voltage and then read it into the stamp.
That is perfect.
>Does this sound like it's going to work or am I on the wrong track? I'm a
>little confused on the RTD transmitter specs and this 4-20mA signal.
>According to the specs the transmitter can be supplied with anything from
>10V to 35VDC, and that Rloopmax = (Vsupply - 10) / .020 amps. I'm not
>exactly sure what this 10-35V supply and that resistance equation mean in
>the big picture.
What that means is that the RTD transmitter itself wants to have at
least 10 volts for its own correct operation. Example:
;
ADC input, 5 volts @ 20 ma
;
; |
+15 volts
| RTD xmtr |---o--/\/\
common
`
' 250 ohms
The resistor is in series with the RTD transmitter. Your ADC is
going to read the voltage across the resistor. When the current is
20 milliamps, there will be 5 volts across the 250 ohm resistor, the
difference, (15volts-5volts)= 10 volts will appear across the RTD
xmtr. That is enough to run it. Whent the current is less, the
voltage across the RTD xmtr is more, but that is okay too. The
transmitter works by regulating the current in the circuit.
Let's say that you don't have 15 volts available, all you have is 12
volts. The circuit above wont work. The maximum resistor value is
then 100 ohms. {100 = (12 - 10) / .020 amps}. If you attempt to use
the 250 ohm resistor there (hoping to get 5 volts output), the
voltage across the RTC xmtr would have to drop below 10 volts, no,
no. In this new circuit, all you can get for the ADC is 2 volts full
scale.
;
ADC input , 2 volts @ 20 ma
;
; |
+12 volts
| RTD xmtr |---o--/\/\
common
`
' 100 ohms
Industrial instrumentation typically operates off of a 24 volt power supply.
>If anyone could help me out on this 4-20mA standard or could point me to
>some resources that may help I'd greatly appreciate it!
>
>Thanks,
>Steve
I hope that helps,
- Tracy
looking for a device that will allow me to read a remote 4-20mA sensor
with no luck. Its a level sensor if you ask.
The Doctor wrote:
>
> Hey everyone,
>
> I am working on a project to monitor and control temperatures in an oven
> up to around 1000 degrees F. I believe the type of temperature sensor I
> want to go with is an RTD hooked to an RTD transmitter. The specs on this
> transmitter (Model TT246 from Minco) tell me it gives a 4 to 20mA output
> over a certain temperature range.
>
> I'm not exactly sure how to read this 4-20mA signal into the stamp. First
> thing that came to mind was to hook a 250ohm resistor across the
> transmitter output which I 'guess' should give me a 5V output at 20mA and
> a 1V output at 4mA. I could then use an ADC like a Maxim MAX187 to sample
> that voltage and then read it into the stamp.
>
> Does this sound like it's going to work or am I on the wrong track? I'm a
> little confused on the RTD transmitter specs and this 4-20mA signal.
> According to the specs the transmitter can be supplied with anything from
> 10V to 35VDC, and that Rloopmax = (Vsupply - 10) / .020 amps. I'm not
> exactly sure what this 10-35V supply and that resistance equation mean in
> the big picture.
>
> If anyone could help me out on this 4-20mA standard or could point me to
> some resources that may help I'd greatly appreciate it!
>
> Thanks,
> Steve
--
"Si desarmasemos a los poderosos y armasemos a los debiles,
alterariamos el orden social que nuestra mision nos obliga a conservar.
La justicia es la sancion de la injusticia establecidas"
Anatole France
"Pero hay que correr el riesgo, de levantarse y seguir cayendo"
Los Enanitos Verdes
The only difference between you and me is that you can see my handicap,
but I can’t see yours. We all have them. When people ask me how I’ve
been able to overcome my physical handicaps, I tell them that I haven’t
overcome anything. I’ve simply learned what I can’t do - such as play
the piano or eat with chopsticks - but more importantly, I’ve learned
what I can do. Then I do what I can with all my heart and soul."
By Jack Canfield
I'm also interested in 4 to 20 ma sensors, though have no direct
experience. This is my application.
I have a 4 to 20 ma pressure transducer(Made by Druck), a 24 vdc supply
powering the transducer and a Bristol Babcock Chart recorder which is
recording water levels from the transducer.
The chart recorder uses a 40 ohm dropping resistor to convert the current
to a voltage.
(0.16 vdc to 0.800 vdc). I would like to measure and record this data
electrically in addition to the chart recorder. I'm thinking that I too
can measure the drop across this resistor with an A to D converter attached
to the stamp.
Not sure how best to do this.
I'm thinking of using a Instrument amplifier to measure across the resistor
and convert the value to a single ended voltage for input to a MAX144 12
bit ADC.
Accuracy is my primary concern. The transducer is accurate to + or - 0.1% fso
Which I believe is = + or - (20 ma - 4 ma = 16 ma) * 0.001 = 0.016 ma
then, 0.016ma * 0.040 ohm resistor = + or - 0.64 millivolts
I thought I could feed the external reference of the A to D with 1.024 vdc,
then the resolution should be 1024/4096= 0.25 millivolts.
The transducer is located in a well 100' deep, along with a very large 220
vac pump.
I'm thinking that noise from the pump will be my biggest enemy, this is why
I'm thinking of using a I-Amplifier, I believe that this will eliminate
alot of the interference?
As you can see, I'm in over my head...
All comments, suggestions, things to watch out for, good A to D converter
for this etc would be very appreciated,
Thanks
Steve
I just did a search here for ( 4 to 20 ma ) came up with lots of answers, I
have one question now, they all say use a 250 ohm resistor and read voltage
across it, how many resistors can you stick on a transmitter loop??
sargent@s... wrote:
>
> Hi Everyone,
>
> I'm also interested in 4 to 20 ma sensors, though have no direct
> experience. This is my application.
>
> I have a 4 to 20 ma pressure transducer(Made by Druck), a 24 vdc supply
> powering the transducer and a Bristol Babcock Chart recorder which is
> recording water levels from the transducer.
>
> The chart recorder uses a 40 ohm dropping resistor to convert the current
> to a voltage.
> (0.16 vdc to 0.800 vdc). I would like to measure and record this data
> electrically in addition to the chart recorder. I'm thinking that I too
> can measure the drop across this resistor with an A to D converter attached
> to the stamp.
>
> Not sure how best to do this.
>
> I'm thinking of using a Instrument amplifier to measure across the resistor
> and convert the value to a single ended voltage for input to a MAX144 12
> bit ADC.
>
> Accuracy is my primary concern. The transducer is accurate to + or - 0.1% fso
>
> Which I believe is = + or - (20 ma - 4 ma = 16 ma) * 0.001 = 0.016 ma
>
> then, 0.016ma * 0.040 ohm resistor = + or - 0.64 millivolts
>
> I thought I could feed the external reference of the A to D with 1.024 vdc,
>
> then the resolution should be 1024/4096= 0.25 millivolts.
>
> The transducer is located in a well 100' deep, along with a very large 220
> vac pump.
>
> I'm thinking that noise from the pump will be my biggest enemy, this is why
> I'm thinking of using a I-Amplifier, I believe that this will eliminate
> alot of the interference?
>
> As you can see, I'm in over my head...
>
> All comments, suggestions, things to watch out for, good A to D converter
> for this etc would be very appreciated,
>
> Thanks
>
> Steve
I'll check it out. I know almost enough to be dangerous. This is how I
understand the "how
many resistors" situation.
The sensor, generally termed the transmitter will require a certain amount
of voltage to operate. In my case, it's a minimum of 9 vdc.
the power supply in my case is 24 vdc, so I have 24 vdc - 9 vdc= 15 volts
left over for my resistor.
As long as my resistor does not try to drop more than 15 vdc, then I'm ok.
With my project, the dropping resistor is 40 ohms. The max current through
it is 20 ma,
so it will drop a max of 0.8vdc, leaving 23.2 vdc to run the transmitter.
I'm thinking of adding another resistor in the loop for my A to D
converter, a 250 ohm resistor would work well as it will generate 1 volt
out at 4 ma, 5 vdc for 20ma.
This resistor will drop 5 vdc at 20 milliamps.
So I add this value (5 vdc) to the 0.8 vdc of the 40 ohm and these two
resistors combined
drop 5.8 volts from 24, leaving me with 18.2 to drive my transmitter. Still
plenty of voltage
I'm Just not sure how to get a "very good" measurement into my stamp
I need a circuit with low temp drift, and a resolution of at least 12 bits.
a clearer explanation can be found here
Measuring Current Output Transducers with Campbell Scientific Dataloggers
http://www.campbellsci.com/apnotes.html#sensors
At 03:15 PM 02/15/2001 -0800, you wrote:
>http://www.jamesrusso.com/stamp/search.html
>I just did a search here for ( 4 to 20 ma ) came up with lots of answers, I
>have one question now, they all say use a 250 ohm resistor and read voltage
>across it, how many resistors can you stick on a transmitter loop??
>
>sargent@s... wrote:
>>
>> Hi Everyone,
>>
>> I'm also interested in 4 to 20 ma sensors, though have no direct
>> experience. This is my application.
>>
>> I have a 4 to 20 ma pressure transducer(Made by Druck), a 24 vdc supply
>> powering the transducer and a Bristol Babcock Chart recorder which is
>> recording water levels from the transducer.
>>
>> The chart recorder uses a 40 ohm dropping resistor to convert the current
>> to a voltage.
>> (0.16 vdc to 0.800 vdc). I would like to measure and record this data
>> electrically in addition to the chart recorder. I'm thinking that I too
>> can measure the drop across this resistor with an A to D converter attached
>> to the stamp.
>>
>> Not sure how best to do this.
>>
>> I'm thinking of using a Instrument amplifier to measure across the resistor
>> and convert the value to a single ended voltage for input to a MAX144 12
>> bit ADC.
>>
>> Accuracy is my primary concern. The transducer is accurate to + or -
0.1% fso
>>
>> Which I believe is = + or - (20 ma - 4 ma = 16 ma) * 0.001 = 0.016 ma
>>
>> then, 0.016ma * 0.040 ohm resistor = + or - 0.64 millivolts
>>
>> I thought I could feed the external reference of the A to D with 1.024 vdc,
>>
>> then the resolution should be 1024/4096= 0.25 millivolts.
>>
>> The transducer is located in a well 100' deep, along with a very large 220
>> vac pump.
>>
>> I'm thinking that noise from the pump will be my biggest enemy, this is why
>> I'm thinking of using a I-Amplifier, I believe that this will eliminate
>> alot of the interference?
>>
>> As you can see, I'm in over my head...
>>
>> All comments, suggestions, things to watch out for, good A to D converter
>> for this etc would be very appreciated,
>>
>> Thanks
>>
>> Steve
>
>
>
Have you checked your AC out put?· Sometimes that can kick you in the butt when you are not even looking for it.· What does you specs tell you for your ac output?· Your 4-20 mA is your AC output, and your DC is what you are putting in.· Can you send me a scematic so I can look at it?· That will help me to help you better.
Thanks
Alicia······
······················ p.s.i hope that will help
>From: The Doctor
>Reply-To: basicstamps@yahoogroups.com
>To: basicstamps@yahoogroups.com
>Subject: [noparse][[/noparse]basicstamps] Help with 4-20mA signal
>Date: Mon, 12 Feb 2001 00:51:10 -0500 (EST)
>
>Hey everyone,
>
>I am working on a project to monitor and control temperatures in an oven
>up to around 1000 degrees F. I believe the type of temperature sensor I
>want to go with is an RTD hooked to an RTD transmitter. The specs on this
>transmitter (Model TT246 from Minco) tell me it gives a 4 to 20mA output
>over a certain temperature range.
>
>I'm not exactly sure how to read this 4-20mA signal into the stamp. First
>thing that came to mind was to hook a 250ohm resistor across the
>transmitter output which I 'guess' should give me a 5V output at 20mA and
>a 1V output at 4mA. I could then use an ADC like a Maxim MAX187 to sample
>that voltage and then read it into the stamp.
>
>Does this sound like it's going to work or am I on the wrong track? I'm a
>little confused on the RTD transmitter specs and this 4-20mA signal.
>According to the specs the transmitter can be supplied with anything from
>10V to 35VDC, and that Rloopmax = (Vsupply - 10) / .020 amps. I'm not
>exactly sure what this 10-35V supply and that resistance equation mean in
>the big picture.
>
>If anyone could help me out on this 4-20mA standard or could point me to
>some resources that may help I'd greatly appreciate it!
>
>Thanks,
>Steve
>
>
>
>
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Get your FREE download of MSN Explorer at http://explorer.msn.com
reason why you can't low-pass filter the data to remove the A.C. stuff from
the pump? Seems like the water pressure would change much more slowly than
60 Hz.
Regards,
Arlen Fletcher
Original Message
From: <sargent@s...>
To: <basicstamps@yahoogroups.com>
Sent: Thursday, February 15, 2001 4:36 PM
Subject: Re: [noparse][[/noparse]basicstamps] Help with 4-20mA signal
> Thanks Larry,
>
> I'll check it out. I know almost enough to be dangerous. This is how I
> understand the "how
> many resistors" situation.
>
> The sensor, generally termed the transmitter will require a certain amount
> of voltage to operate. In my case, it's a minimum of 9 vdc.
>
> the power supply in my case is 24 vdc, so I have 24 vdc - 9 vdc= 15 volts
> left over for my resistor.
>
> As long as my resistor does not try to drop more than 15 vdc, then I'm ok.
>
> With my project, the dropping resistor is 40 ohms. The max current through
> it is 20 ma,
> so it will drop a max of 0.8vdc, leaving 23.2 vdc to run the transmitter.
>
> I'm thinking of adding another resistor in the loop for my A to D
> converter, a 250 ohm resistor would work well as it will generate 1 volt
> out at 4 ma, 5 vdc for 20ma.
>
> This resistor will drop 5 vdc at 20 milliamps.
>
> So I add this value (5 vdc) to the 0.8 vdc of the 40 ohm and these two
> resistors combined
> drop 5.8 volts from 24, leaving me with 18.2 to drive my transmitter.
Still
> plenty of voltage
>
>
> I'm Just not sure how to get a "very good" measurement into my stamp
>
> I need a circuit with low temp drift, and a resolution of at least 12
bits.
>
>
>
> a clearer explanation can be found here
>
> Measuring Current Output Transducers with Campbell Scientific Dataloggers
> http://www.campbellsci.com/apnotes.html#sensors
>
>
>
>
>
>
>
> At 03:15 PM 02/15/2001 -0800, you wrote:
> >http://www.jamesrusso.com/stamp/search.html
> >I just did a search here for ( 4 to 20 ma ) came up with lots of answers,
I
> >have one question now, they all say use a 250 ohm resistor and read
voltage
> >across it, how many resistors can you stick on a transmitter loop??
> >
> >sargent@s... wrote:
> >>
> >> Hi Everyone,
> >>
> >> I'm also interested in 4 to 20 ma sensors, though have no direct
> >> experience. This is my application.
> >>
> >> I have a 4 to 20 ma pressure transducer(Made by Druck), a 24 vdc supply
> >> powering the transducer and a Bristol Babcock Chart recorder which is
> >> recording water levels from the transducer.
> >>
> >> The chart recorder uses a 40 ohm dropping resistor to convert the
current
> >> to a voltage.
> >> (0.16 vdc to 0.800 vdc). I would like to measure and record this data
> >> electrically in addition to the chart recorder. I'm thinking that I
too
> >> can measure the drop across this resistor with an A to D converter
attached
> >> to the stamp.
> >>
> >> Not sure how best to do this.
> >>
> >> I'm thinking of using a Instrument amplifier to measure across the
resistor
> >> and convert the value to a single ended voltage for input to a MAX144
12
> >> bit ADC.
> >>
> >> Accuracy is my primary concern. The transducer is accurate to + or -
> 0.1% fso
> >>
> >> Which I believe is = + or - (20 ma - 4 ma = 16 ma) * 0.001 = 0.016 ma
> >>
> >> then, 0.016ma * 0.040 ohm resistor = + or - 0.64 millivolts
> >>
> >> I thought I could feed the external reference of the A to D with 1.024
vdc,
> >>
> >> then the resolution should be 1024/4096= 0.25 millivolts.
> >>
> >> The transducer is located in a well 100' deep, along with a very large
220
> >> vac pump.
> >>
> >> I'm thinking that noise from the pump will be my biggest enemy, this is
why
> >> I'm thinking of using a I-Amplifier, I believe that this will eliminate
> >> alot of the interference?
> >>
> >> As you can see, I'm in over my head...
> >>
> >> All comments, suggestions, things to watch out for, good A to D
converter
> >> for this etc would be very appreciated,
> >>
> >> Thanks
> >>
> >> Steve
> >
> >
> >
>
>
>