Convert Voltage Signal to 4-20 mA

Hi all! I wonder if anyone knows of a simple way for me to convert the signal from one of our instruments (0-2 V, resolution of 1 mV) to a 4-20 mA signal so that we can use our current PLC configuration. (Modifying the PLC would be about $770 - cheap by some standards, but cheaper is better!) Input impedance must be 1 MegOhm at least.

I contacted a company who said they would be happy to make me a custom card for $1200 that would convert 0-2V to 0-20 mA but then I lose the bottom 20% of my data since the card wouldn't register anything below 4 mA!

Thanks for your help!

Contact Action Instruments, API, and M-Systems. They make all kinds of signal conditioners, and may have a T/C or other unit that you can scale to suit your needs.

Tom Jenkins said:

Contact Action Instruments, API, and M-Systems. They make all kinds of signal conditioners, and may have a T/C or other unit that you can scale to suit your needs.

Click to expand...

Add to the above list:

http://www.miinet.com/products/sg_signal.shtml

http://web4.automationdirect.com/adc/Shopping/Catalog/Process_Controls/Signal_Conditioners

Diy

If you are really intent on doing this on the cheap and dont mind a bit of experimenting for about a dollar you could build a voltage amplifier followed by a voltage to current stage from a single operational amplifier. +V can be connected to the 24V supply


Sorry I can't be specific I'm a digital guy and a bit rusty on my analog theory but you basically multiply the voltage by 5 in the first stage (plus provide a bias voltage otherwise we would do 0-20mA) then we need to drive 4-20mA through a R1 by selecting a suitable value for R2.

I dont use any 4-20mA stuff these days as I mainly work on micro-controllers not PLC's but from memory there were loads of cricuits on the internet to do what your asking as a lot of micro-controller based control systems need them as they normally work at 0-5V
.

Try 500 ohms (10v/0.02A) or there bouts for R2 and (looking at the above again) you may also want to look at a better biasing arangement than my 1M pot to improve stability.

Also if you just want to get a 0-10V ouptut which all PLC's I know will use then simply remove the second op-amp stage and the bias pot on the first stage.

The input resistance of all opamps is well over 1meg so no probs there.

weidmuller has bunch of them and silver bullet is Wave DC Pro.
it is very compact and does coverts anything to anything...

http://www.weidmuller.com/downloads/pdfs/prodinfo/Waveanalog_PRO_DC.pdf

and here is WaveTool setup utility (highly recommended):
http://www.weidmuller.ca/portal/page?_pageid=320,960715&_dad=portal&_schema=PORTAL

use it and specify range and type of input and output and it generates wiring diagram, dipsw. configuration and setup procedure.