View Full Version : 4-20mA output from Propeller...

02-13-2007, 05:34 AM
Hi All,
Sorry if this is more hardware related, but I am looking at using a Propeller to control some valves which have a 4-20mA signal to set their position.

Can anyone point me to some links which would show the hardware required to produce a 4-20mA output.

Does having a 3.3v supply complicate things ?

West Oz.

02-13-2007, 07:00 AM
Hi Chris

There is a chip called an AD421·· this does the trick.
Not sure if anybody carries this in stock in OZ but try Farnell RS and possibly Arrow Electronics.
PDF file attached

Ronald Nollet··

02-13-2007, 07:39 AM
Thanks Ron,

Will look into that...

Phil Pilgrim (PhiPi)
02-13-2007, 10:55 AM
Here's a circuit I remember from a Design Idea in one of the trades several years ago. It uses a triple opto isolator, like the Sharp PC817 (http://rocky.digikey.com/WebLib/Sharp/Web%20Data/PC817%20Series.pdf) series, available from DigiKey. It has the advantage not only of optoisolation, but of being able to either source or sink current, depending on the app. The receiver, of course, will have to provide the excitation voltage.

The input is a single Propeller pin, driven by one of the counters' DUTY outputs. This is filtered to a voltage that ranges from 0 to 3.3V. The op amp drives the optoisolator LEDs in series, so each receives the same amount of current. The op amp's output voltage will have to be at least three LED diode drops above ground at max drive current. This will probably mean a 5V or greater supply voltage for the op amp. For 4-20mA output, the op amp's input common mode range will need to include 0.66V and 3.3V. For 0 to 20mA, the input common mode range will have to include ground. Depending on the op amp, it may also be necessary to add a transistor follower to boost its drive current to at least 10mA. This will also raise the op amp's Vdd requirements by another 0.6V to account for the transistor's VBE.

The feedback for the op amp comes from one of the phototransistors. It will range from 0 to 10mA which, across the 3.3K resistor, results in a feedback voltage of 0 to 3.3V. The other two phototransistors are paralleled, which yields double the feedback current, or 0 to 20mA. These may be wired to the receiver in any fashion, consistent with the device limitations of the optical isolators.

This circuit relies on the fact that three isolators in the same package will be farily well matched and will be subjected to identical temperatures. So the feedback current should accurately track the output current.

I've never tried this circuit. But if I were going to produce a 4-20mA output device, this is where I'd start.


Update: Added a link to the opto datasheet and additional requirements for the op amp.

Post Edited (Phil Pilgrim (PhiPi)) : 2/13/2007 4:39:13 PM GMT

02-13-2007, 11:43 AM
Many many thanks Phil

02-13-2007, 09:30 PM

Is the Sharp PC817 the op-amp of you drawing or is your drawing a breakdown of the Sharp PC817.

I too could benifit from a homegrown 4-20ma sensor conditioner. If I could beef it up to 11V output I could use it.

Graham Stabler
02-13-2007, 09:39 PM
The sharp device is a quad opto-coupler, the op-amp is an additional device.


02-13-2007, 09:46 PM
Any link to a datasheet for the coupler, all I get is salesmen.

Phil Pilgrim (PhiPi)
02-13-2007, 11:40 PM

I added a link to the datasheet.


02-14-2007, 12:24 AM
Graham, Phil,

Thanks, That definitly gave me something to chew on.
The sensor conditioner sold with my accelorometers goes for $150 a hit.
In the end we have always said we where going to build our own, And this gives me yet another way to do it. My other circuit relied heavily on precision resistors, transitors not readily available.