Nuts and Volts - November 2014

■ 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.