Hello,

I have limited experience with microcontrollers, all with freescale HC12
and HCS12 systems. I have dealt with 0-5V sensors through ADC before, too.


I currently have a project where I might have to use a 4-20mA sensor. How
would I go about electrically interfacing a 4-20mA sensor to a generic
microcontroller? How would you power it and read it? Is there an IC
available?

Any suggestions or help would be greatly appreciated!

On 22 Dec, 19:33, "eeboarder" <jme...@emittechnologies.com> wrote:
> Hello,
>
> I have limited experience with microcontrollers, all with freescale HC12
> and HCS12 systems. I have dealt with 0-5V sensors through ADC before, too.
>
> I currently have a project where I might have to use a 4-20mA sensor. How
> would I go about electrically interfacing a 4-20mA sensor to a generic
> microcontroller? How would you power it and read it? Is there an IC
> available?
>
> Any suggestions or help would be greatly appreciated!

ADI makes a nice interface chip you could use.

Leon

On Mon, 22 Dec 2008 13:33:39 -0600, "eeboarder"
<j...@emittechnologies.com> wrote:

>Hello,
>
>I have limited experience with microcontrollers, all with freescale HC12
>and HCS12 systems. I have dealt with 0-5V sensors through ADC before, too.
>
>
>I currently have a project where I might have to use a 4-20mA sensor. How
>would I go about electrically interfacing a 4-20mA sensor to a generic
>microcontroller? How would you power it and read it? Is there an IC
>available?
>
>Any suggestions or help would be greatly appreciated!
>
In general, a precision load resistor (E = I *R ) and a bit of signal
conditioning. Usually you'll need something like a 12-24VDC power
supply depending on the type of sensor (eg. 2-wire vs. 4-wire) and
maybe isolation if it's a real-world application with long cables and
serious EMC considerations. Usually you'll want to be able to handle a
certain amount of overrange (somtimes more than 60% of span, eg. 30mA)
under some conditions, so it's usually not best to make full scale
equal to the full scale of your ADC, and you certainly want to be able
to protect against fault conditions and transients. There is at least
one >10 year old "chip" available, but I don't think it's a good
solution in modern systems. 

"eeboarder" <j...@emittechnologies.com> wrote in message 
news:CuKdnQ6h--o...@giganews.com...
> Hello,
>
> I have limited experience with microcontrollers, all with freescale HC12
> and HCS12 systems. I have dealt with 0-5V sensors through ADC before, too.
>
>
> I currently have a project where I might have to use a 4-20mA sensor. How
> would I go about electrically interfacing a 4-20mA sensor to a generic
> microcontroller? How would you power it and read it? Is there an IC
> available?
>
> Any suggestions or help would be greatly appreciated!
>
>


Why not just use an opto coupler?

PhilW 

"PhilW" <p...@microcoin.com> writes:

> "eeboarder" <j...@emittechnologies.com> wrote in message 
> news:CuKdnQ6h--o...@giganews.com...
>> Hello,
>>
>> I have limited experience with microcontrollers, all with freescale HC12
>> and HCS12 systems. I have dealt with 0-5V sensors through ADC before, too.
>>
>>
>> I currently have a project where I might have to use a 4-20mA sensor. How
>> would I go about electrically interfacing a 4-20mA sensor to a generic
>> microcontroller? How would you power it and read it? Is there an IC
>> available?
>>
>> Any suggestions or help would be greatly appreciated!
>>
>>
>
>
> Why not just use an opto coupler?

Because you still have to power the sensor - also the "ADC" in the
question implies an analog quantity, not a simple switched input.

-- 

John Devereux

On Mon, 22 Dec 2008 22:15:30 +0000, John Devereux
<j...@devereux.me.uk> wrote:

>"PhilW" <p...@microcoin.com> writes:
>
>> "eeboarder" <j...@emittechnologies.com> wrote in message 
>> news:CuKdnQ6h--o...@giganews.com...
>>> Hello,
>>>
>>> I have limited experience with microcontrollers, all with freescale HC12
>>> and HCS12 systems. I have dealt with 0-5V sensors through ADC before, too.
>>>
>>>
>>> I currently have a project where I might have to use a 4-20mA sensor. How
>>> would I go about electrically interfacing a 4-20mA sensor to a generic
>>> microcontroller? How would you power it and read it? Is there an IC
>>> available?
>>>
>>> Any suggestions or help would be greatly appreciated!
>>>
>>>
>>
>>
>> Why not just use an opto coupler?
>
>Because you still have to power the sensor - also the "ADC" in the
>question implies an analog quantity, not a simple switched input.

You will need the current source anyway, regardless if galvanic
isolation or not is used to carry the signal. 

If a galvanic signal connection is used, a floating power supply
should be used  with exactly one point of the current loop circuit
connected to any kind of external ground, which in this case must be
connected to the ADC analog ground. This can be problematic,
especially when multiple current loops are used.

When galvanic signal isolation is used, the processor power supply can
be completely separated from the current loop(s) and the loops can be
grounded according to other requirements. If the optoisolator is not
linear enough, various feedback systems can be used involving a dual
optoisolator.

Especially when multiple sensors or long sensor cables are used,
keeping the current loops totally separate from the processor circuit
simplifies things a lot at the system level.

Paul

> Especially when multiple sensors or long sensor cables are used,
> keeping the current loops totally separate from the processor circuit
> simplifies things a lot at the system level.

You could also connect a cheap microprocessor, e.g. ATmega8 with 
integrated ADC to the same power supply as the sensor and transmit the 
signal via serial port e.g. as digital 20mA curent loop signal over a 
optocoupler. So you would not need an analog isolation amplifier which 
might be a bit tricky.

In this case, you would simply connect the output of the sensor to a 150 
Ohm resistor. The ATmega has a resulution of 10 Bits. This might be much 
better as if you use an analog isolation amplifier.

But the best solution depends on your demands, e.g. sampling rate, 
resolution and accuracy.

Stefan

On Tue, 23 Dec 2008 16:15:17 +0100, Stefan Brröring
<s...@broering.de> wrote:

>> Especially when multiple sensors or long sensor cables are used,
>> keeping the current loops totally separate from the processor circuit
>> simplifies things a lot at the system level.
>
>You could also connect a cheap microprocessor, e.g. ATmega8 with 
>integrated ADC to the same power supply as the sensor and transmit the 
>signal via serial port e.g. as digital 20mA curent loop signal over a 
>optocoupler. So you would not need an analog isolation amplifier which 
>might be a bit tricky.

This is OK as long as you can run the ADC, processor and opto
transmitter with less than 4 mA. With suitable loop power supply, you
might have about 5-15 V voltage drop available, the circuit would have
20-60 mW available.

Paul

I may be missing the point but it seems much simpler than all that. If you
can input 0-5v then connect the 4-20 mA output to ground through a 250 ohm
resistor and the voltage at the output will be 1 to 5 volts for 4-20 mA. If
you want some room a 200 ohm resistor gives a maximum of 4 volts. A two
wire sensor is loop powered. I would connect the positive side of the
sensor to a voltage source like 10 or 12 volts. The only problem with this
is that the accuracy of the load resistor will determine the accuracy of
your measurement. This may not be a big issue - many sensors are accurate
to only 1% anyway. An 8-bit ADC has about 0.4% resolution.
>