Possible to create 4-20 mA signal using a PIC?

[jseidmann]

I'm sure some of you are familiar with 4-20 mA signals. Essentially, I was wondering if there was any way to create a 4-20 mA signal using some pins on the PIC? I'm not tied to a particular PIC, so I'm really open to suggestions. I would really like to use only the PIC and possibly a resistor or so, but no external components (no IC's anyway if possible!). Let me know if you know any means of doing this. I know the pins can sink/supply up to 25 mA, is it possible to use that?

[MikeValencia]

Can't you just put a series 250ohm resistor onto the PIC's pin? This way, you are guaranteed that the current sourced will not exceed 20mA.

[jseidmann]

What I would like to do is to be able to produce a current from 4-20 mA that I could change in software. I'd like to be able to supply a current anywhere in between 4 and 20 mA.

[Humberto]

[SherpaDoug]

What do you want to use this 4-20mA signal for? You can easily generate a 4-20mA signal into a low impedance output, either with PWM or with a ladder of resistors, but it won't be able to drive any overhead voltage for the load. Generally a 4-20mA source will supply the prescribed curent into a load that has an overhead of from 0V to at least 12V. That is going to require an op amp at a minimum.
_________________
The search for better is endless. Instead simply find very good and get the job done.

[jseidmann]

[SherpaDoug]

If you are using this 4-20mA transmitter on the bench for calibrating 4-20mA displays you may be able to measure the voltage overhead of the display and compensate for it.
But if you are making a product to ship to a customer in god-only-knows-where and they will expect it to work like an industry standard 4-20mA transmitter, you are going to need some analog circuitry to make a current source that can accomodate an unknown voltage overhead. There is only so much than can be done in the 5V digital domain.
_________________
The search for better is endless. Instead simply find very good and get the job done.

[Graham2]

[jseidmann]

[Ttelmah]

The key (which has been touched upon, but not really emphasised), is that a 4-20mA driver should normally be able to handle a wide range of possible loads. Remember that wiring itself will impose variable loads, and given that targets themselves may have different resistors used as a sensor, it is normal for a 4-20mA source to be able to handle between perhap 50R, and 500R. This ability, really implies an op-amp is needed, and a supply of 12v.
You can form a very simple 4-20mA source, by using one of the PWM outputs of the PIC to feed an integrator, to generate a controlable voltage. A simple op-amp, like a TL072, with a transistor, can then form a current source. All you do is compare the voltage being developed across a sense resistor, feeding into the loop connections, with the voltage being developed by the PWM, and adjust the base drive to the transistor to keep the two matched. Choose a suitable value of sense resistor, and you have a controllable current source, for the cost of a few cents. The downside here, is that the source will not be a floating supply (it is common for industrial 4-20mA drives to float the source, so that the sense resistor at the other end can be referenced to ground). Using PWM like this, it is relatively easy to make a floating source, by using an opto-coupler in the PWM signal (obviously requires a floating supply for the op-amp).

Best Wishes

[MikeValencia]

Don't know if this helps, but look at the article "Single Transistor Constant-Current Generator" from Electronics Design magazine...

http://www.elecdesign.com/Articles/ArticleID/6268/6268.html#

[EY]

This Circuit can be your wants...

[EY]

or http://img382.imageshack.us/img382/3020/pccs9rb.jpg

[TimMoody]

I wonder if you are just looking for the old 20mA current loop interface. It was used back in the "day" for a lot of communications equipment like teletypes. Basically 20mA represents a 1 and no current represents a 0 and is a very simple dollar ciruit. If you want an analog 4-20mA control loop(0 mA being a alarm, 4mA being a true 0 and up to 20mA indicating control), you're best bet is to buy an off the self part like a RCV420JP(receiver), about 3.00 and a XTR116U(transmitter) about 1.50.