Searching \ for '[PIC]:4-20mA to PIC' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/microchip/ios.htm?key=4%2D20ma
Search entire site for: '4-20mA to PIC'.

Exact match. Not showing close matches.
PICList Thread
'[PIC]:4-20mA to PIC'
2005\07\13@230125 by alan smith

picon face
Is it as simple as it seems....4-20mA current loops....put across a precision resistor and then feed to the A/D ? Or better....buffered thru a unity gain amp...?

Or is it a little more tricky than that?



               
---------------------------------
Start your day with Yahoo! - make it your home page

2005\07\13@233952 by Chen Xiao Fan

face
flavicon
face
The complexity depends on the 4-20mA transmitter and the
accuracy you want to achieve. Is it loop powered or is it
self-powered (most of the cases)? What is the output impedance
of the transmitter? How accurate do you want to measure the
signal: 1%, 0.1% or 5%? Do you need temperature compensation?

I have seen circuits so complicated that I can never understand
just to transmit the 4-20mA signal (repeater or transmitter
power supply). However in the simplest form, I think a buffer
OPAMP will do.

Regards,
Xiaofan

-----Original Message-----
From: alan smith [spam_OUTmicro_eng2TakeThisOuTspamyahoo.com]
Sent: Thursday, July 14, 2005 11:01 AM

Is it as simple as it seems....4-20mA current loops....put
across a precision resistor and then feed to the A/D ? Or
better....buffered thru a unity gain amp...?

Or is it a little more tricky than that?


2005\07\14@003546 by Jinx

face picon face

> Is it as simple as it seems....4-20mA current loops....put across
> a precision resistor and then feed to the A/D ? Or better....buffered
> thru a unity gain amp...?

I'm getting good results with an F88, a 4-20mA depth sensor
and this

http://www.maxim-ic.com/appnotes.cfm/appnote_number/823

Basically doing what you said. No adjustment or fiddling necessary
to get 1mV / mm over 5m (apart from grounding pin 1 of the 472,
which isn't shown above)

2005\07\14@015900 by Brent Brown

picon face
> Is it as simple as it seems....4-20mA current loops....put across a
> precision resistor and then feed to the A/D ? Or better....buffered
> thru a unity gain amp...?
>
> Or is it a little more tricky than that?

Yes, it is (or, can be) that simple. For example, a 250 Ohm resistor converts
4-20mA to 1-5V, ideal for input into your PIC's ADC. The current loop
topology is important, but if your sensor sources the 4-20mA signal and if it's
OK to put your resistor to 0V then this is indeed the simplest way to go.

Why buffer it with a unity gain Op-Amp? If it's for "protection" then it's better
to do it properly with an appropriate network of voltage clamping and series
protection devices. You could however use the Op-Amp with some gain to
allow you to reduce the current shunt resistor and thereby the voltage drop in
the loop. Or go to the next level and use a differential Amp to allow your
resistor to "float" between the supply/common mode limits making it's
placement in the current loop more flexible. But that just complicates
things...just use a resistor and be amazed how simply it converts current to
voltage!

--
Brent Brown, Electronic Design Solutions
16 English Street, Hamilton 2001, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell/txt: 027 433 4069
eMail:  .....brent.brownKILLspamspam@spam@clear.net.nz


2005\07\14@023623 by Wouter van Ooijen

face picon face
> Is it as simple as it seems....4-20mA current loops....put
> across a precision resistor and then feed to the A/D ? Or
> better....buffered thru a unity gain amp...?
>  
> Or is it a little more tricky than that?

Basically it is just that. You'll probably need a good reference (better
than a 7805-suppied Vdd), of course the precision resistor, some
protection and filtering against strange things on the input (but the
not-too-high precision resistor will help a lot). For realy fancy
applications: opto-isolation. 4-20 is often used in a rough environment
where power is from 24 or 48 V DC, so you might want to make your input
robuust against that, also when applied backwards.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2005\07\14@035106 by Vasile Surducan

face picon face
On 7/14/05, Jinx <joecolquittspamKILLspamclear.net.nz> wrote:
>
> > Is it as simple as it seems....4-20mA current loops....put across
> > a precision resistor and then feed to the A/D ? Or better....buffered
> > thru a unity gain amp...?
>
> I'm getting good results with an F88, a 4-20mA depth sensor
> and this
>
> http://www.maxim-ic.com/appnotes.cfm/appnote_number/823

There is s implest way if you check for a chip containing a voltage
reference and an operational amplifier. Maxim have a few like those.

best regards,
Vasile


>
> Basically doing what you said. No adjustment or fiddling necessary
> to get 1mV / mm over 5m (apart from grounding pin 1 of the 472,
> which isn't shown above)
>
> -

2005\07\14@043032 by Wouter van Ooijen

face picon face
> There is s implest way if you check for a chip containing a voltage
> reference and an operational amplifier. Maxim have a few like those.

like http://www.voti.nl/shop/p/IC-LM10-DIP.html

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2005\07\14@090900 by alan smith

picon face
Just wanted to say thanks to all that replied.  I don't have the details on the sensor yet, but this is the information I need in order to decide exactly how to go about it.

Again, thanks very much.

Al

__________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around
http://mail.yahoo.com

More... (looser matching)
- Last day of these posts
- In 2005 , 2006 only
- Today
- New search...