User manual

Take the above example again, the Meterman 37XR would drop a whopping 2V (10mV*200) on its
mA range for 200mA. Not much good when your supply voltage is only 3.3V, or 5V, or even 12V.
And the 37XR is a relatively expensive 10000 count meter that is supposed to be capable of
measuring 999.9mA on its 1A range – which it will try to do. But that would be a gigantic 10V drop
which the meter itself cannot even handle, so it’s limited to a nominal 400mA with a 4V drop on that
range. Crazy huh?
I’m sure by now you are getting the idea that burden voltage can be a real hidden problem lurking in
your meter. What is your meter rated at?
Accuracy
And the second problem mentioned? That would be one of accuracy. Most multimeters will have a
much wider accuracy specification for current than they will for the DC voltage ranges, or the “Basic
DC accuracy” as it’s called.
The Meterman 37XR for example is quite an accurate meter at 0.1%(+5 counts) on DC volts, and is
sold and marketed as such. But its current accuracy?, a not so impressive 0.5%(+10 counts) on DC
current, jumping to 1.5% for the 10A range.
An even better example is the Fluke 27, with 0.1%(+1 counts) DC volts accuracy and 0.75%(+2
counts) mA/uA DC current accuracy.
Most other meters are very similar, with a factor of 5 or more between the DC volts and DC current
accuracy being quite typical.
This issue is more prevalent with DC than AC, but can also apply to the AC voltage vs AC current
ranges as well. Some meters can actually have very poor AC current accuracy and/or reduced AC
frequency response compared to their AC millivolt range. Take the Fluke 27 again as an example.
The ACV accuracy is 0.5%+3 to 2KHz, but the AC current ranges are only 1.5%+2 to 1KHz.
The Solution
You guessed it, the project to be presented here presents a neat solution to these issues. The
µCurrent (pronounced “micro current”) is a simple yet accurate professional grade precision amplified
current adapter for multimeters. It provides in many cases up to a 100 fold reduction in burden
voltage for a given current range!
An additional feature is a nA current range not found on most multimeters at any price. This gives any
cheap 3.5 digit multimeter the ability to resolve 0.1nA (100pA), and 0.01nA (10pA) on a 4.5 digit
multimeter. All with an excellent accuracy of <0.2%.
The µCurrent is in many cases also able to improve upon your meters current range accuracy by
using your meters more accurate mVDC or mVAC voltage ranges instead to display the DC or AC
current. With AC the frequency response extends flat up to 10KHz, but THD increases substantially
above 2KHz. Still a very respectable AC response range, surpassing that of many meters on current
and voltage ranges.
Typical accuracy of the µCurrent itself is better than 0.2% on the µA and nA ranges, and 0.5% or
better on the mA range. Unfortunately it is not easy to obtain a 0.1% precision shunt resistor for the
mA range, as the 10 milliohm value is too low.