■ FIGURE 4. ■ FIGURE 5.
Measuring Microamps
and Nanoamps
In the course of working on this project, I discovered
that measuring current in the micro-amp and nano-amp
(nA) range is no easy task. Most digital voltmeters can
measure milliamps. However, measuring anything below
that with accuracy is difficult. Even my multi-hundred
dollar professional multimeter was not up to the task.
To verify that my circuit was truly drawing a microamp or
less, I needed a reliable measuring device.
In researching for a solution to this, I discovered a
device made by David Jones in Australia. If you have
been learning about electronics for a while, you may have
come across one of David Jones' enthusiastic videos on
YouTube or his video blog. He has over 600 videos
covering basic electronics, how to use your oscilloscope,
product reviews, and more. I've learned a lot from him
over the years.
In 2009, David Jones presented the open source
µCurrent, and described both the need for it and how
to make it in an article in the April 2009 issue of Silicon
Chip Magazine. The µCurrent is a low cost (less than
$100) adapter that lets you accurately measure current
down to the nano-amp with any digital multimeter
capable of measuring millivolts. In 2013, Jones
announced the version 5 µCurrent Gold and initiated a
Kickstarter project to fund production, exceeding his
funding goal more than ten-fold. His video on the
Kickstarter page does a good job of explaining why you
need something like the µCurrent to measure current
without impacting your circuit.
He is currently in production, but it may still be a
while before units are available in the US. I wasn't
prepared to wait, so I downloaded the publicly available
PCB manufacturing files and parts list. I made some
changes to the PCB files so they would be accepted by
OSHPark, and I checked with David Jones about
substituting for a part that was only available in full reel
quantities. All of the parts were available from Mouser
Electronics. Once the boards came back, I assembled my
own µCurrent and sent David Jones a donation. You can
see the bare boards and my assembled µCurrent in
Figures A and B. The µCurrent uses several precision
op-amps and precision resistors to generate a voltage in
the millivolt range that is proportional to the current
being measured. The µCurrent can be seen in action back
in Figure 2. The 16. 8 millivolts displayed on the meter
translates to a measurement of 16. 8 nA for this circuit in
sleep mode. I'm using a
PIC12LF1840
microprocessor, and
the spec sheet says the
sleep mode current is
20 nA. For anyone
working with low
currents, a tool like
the µCurrent is
indispensable.
See the References
section for more
information about
David Jones, the
µCurrent, and the
vendors I use.
■ FIGURE A. ■ FIGURE B.
44 November 2014