A Precision Voltage Reference
Don Wilhelm W3FPR
I found myself with the confusion of the ham with too many meters. The quip about the man who had 2 watches and didn't know what time it was certainly was hitting close to home. In the past I had access to calibrated lab equipment and could believe what the meters said, but I left that world behind, and found myself wondering just how accurate my voltmeters really were.
As I was looking at the fine print in a DigiKey catalog, I found a group of voltage reference devices and ordered one. The LM4040 device comes in several voltage flavors and have a tolerance of 0.1% -- not good enough for a calibration lab, but certainly good enough for the majority of ham use. With this device, I could calibrate my meters to an accuracy of 1% (based on the rule of thumb that the standard must be 10 times better than the instrument being calibrated.)
The LM4040-2.5 which I chose requires a minimum operating current of 60 microamps and a maximum operating current of 15 milliamps. I sized the series resistor (620 ohms) for a current of approximately 10 ma. so I could draw a small current without upsetting the operating parameters. I added a string of 100 ohm resistors to give me voltage steps at 0.5 volts increments. I hand selected these resistors from a pile and managed to find 5 resistors that measured right on 100 ohms with an ohmmeter calibrated at 0.1%. A reasonable alternative would be to use 1% resistors which would still yield a unit having an accuracy of 0.1% for the 2.5 volt output and a bit over 1% at the other voltage levels -- still good enough for most ham work.
As you can see in the photo above, I used manhatten style construction, and I chose not to create a case for this project. It is not something that is used all the time, so I felt no need for a case -- besides, bare electronics is beautiful in my opinion. Layout is not critical since we are dealing with DC -- as long as there are no unintentional voltage drops from bad connections or cheap connectors, this circuit can be built most any way one would chose.
The Schematic is shown below.