Above is the schematic of the unit.
As you can see this is simple circuit. The XTR-117 is a current amplifier, plan and simple. By using a the 5 volt reference built into the chip and a resistor you can control the current that will be amplified.
If you have a 125kohm resistor between the 5 volt and the Iin pin you will have current flowing at this pin. This chip will amplify the current to 4 ma on the output when a the circuit is built as in the PDF above.
Every 125kohm resistor you put in parallel with the first 125kohm resistor will increase the input current and increases the current by 4 ma.
So as you decrease the resistance you increase the output current of the chip. Below is the prefect world resistor values to achieve the 4-20ma output in 4 ma steps.
125kohms=4 ma, 62.5kohms=8 ma, 41.67 kohms=12 ma, 31.25 kohms=16 ma, 25kohms=20 ma
Now in my world it was hard to find a 125Kohm resistor so I used a 124kohm and get pretty good results. Each step is a about .03 ma short of the 4 ma step I wanted to achieve. This was good enough for my requirements at this time.
If I was to build one for calibrating or tuning a current loop I would spent the time and get 125kohm .5% resistors. There are out there, just not something you find at the local electronics store.
The input through a bridge rectifier makes it so there is no polarity required. Positive can go on either terminal. Made it this way to handle any transients that may occur.
So far I have 45 units in operation and find them very helpful in testing analog systems with either a 4-20ma or 1-5 volt input.
Oh, for some of you who didn’t know, if you need a 1-5volt source, just put a 250 ohm resistor in the return leg of the unit you will get the desired 1-5 volts across it.
Got to Love that Volts(V)=Current(I) x Resistance(R)
Till Next time, thanks for your time.