For the guy like me who would rather buy than build, I'm re-publishing my review of a cheap 4-20mA source that I posted at
http://www.plctalk.net/qanda/showthread.php?t=107307
several weeks ago.
I bought five of the simulators (on ebay, from China for about $10 each) shown in the graphic images below and just finished testing them out-of-the-box. (use 4-20mA source as a search term)
Observations:
a) the digital display reads to one digit to the right of the decimal point, ie, to 0.1. The sales description mentions 0.01, but the hundredths digit is not displayed. Some of the ebay listing photos do not show powered displays so it isn't obvious what the display resolution is.
b) The generated signals are off by a 0.1mA or 0.2mA (measured with a calibrated mA calibrator), so the device is not a calibrator, it's an indicating source. There are no adjustments (pots or otherwise) to correct zero or span.
That doesn't bother me because I am looking for cheap 0-20mA source and if I need an exact mA value I'll put meter in the circuit.
c) I use two 9V batteries in series as a day-to-day power supply. The field connections are on the Phoenix Contact style removable connector which makes testing and disconnecting the battery 'pack' easy. (the higher the voltage the greater loop resistance it can drive.
According to the specs, 12V will drive 350 ohms, 24V will drive 800 ohm; so 18V will drive 600 ohms.
d) I give them a thumbs up for the price at $10-12 each. They are perfect for sourcing a mA signal.
Just be aware of the display resolution and the inaccuracy of the displayed value (it's an approximation) to see if it meets your needs.