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Aug 05, 2015, 01:02 AM
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The amazing uCurrent

A nice bit of kit arrived at the day job. Modern gadgets live & die by their power management firmware. It's not like the old days when a power switch was the final word in whether the batteries drained or lived. Now, the typical gadget has a full power mode, low power mode, off mode, charging mode, all controlled by software with no direct user control. If the software doesn't work, the gadget bricks with no way out for the user.

Many a gadget has been instantly wiped off the face of the Earth because it didn't turn off properly, killing its batteries, or didn't turn on. History is littered with Gopro's draining their batteries in standby mode, iPad's going dark & never turning on again, Lipo's drained past their self destruct point or overcharged.

Keeping the day job gadget from becoming a statistic consumed a lot of testing. The mane problem was measuring current through the gadget's full range of hundreds of mA to tens of uA. The traditional "mowti"meter couldn't do it without shutting down to change ranges & faking the battery voltage with a bench supply set 2V higher than the battery voltage. Burden voltage was a big issue. There was no way to test transitions into charging mode with the bench supply.

As simple as it is, the uCurrent was the easiest way to do the job. It was a lot more reasonable than picking out the right resistors, op-amps, & switches, building up a circuit to test current ourselves. The mane advantage is being able to change ranges without shutting down the circuit.

Not sure if it's an intended feature or a lucky side effect of being new, but by some miracle, the switches don't interrupt the current when they're changed. The OFF & SHORT positions both bypass the shunt resistors. The ON position goes through the shunt resistors.

The shunt resistor is chosen with the range switch, which doesn't interrupt the circuit either. The mane problem is accidentally switching into a shunt resistor which is too high & killing the circuit with burden voltage. Avoiding this requires switching into OFF or SHORT before changing shunt resistors.

The uCurrent has a 100khz bandwidth, allowing intriguing plots of high frequency transitions in the current usage. A mowimeter could only show the average current. The uCurrent revealed the square wave current usage of an LED driven by PWM. The quiescent current usage of a voltage regulator is revealed to be a similar PWM signal. The periodic blips of an interrupt handler can be seen.

The waveforms from the day job are trade secrets, but the waveforms were very useful in showing if the mane user of current was an oscillating or constant signal, thus showing if it was a voltage regulator, interrupt handler, or resistor & knowing exactly how much current was used by a circuit which periodically spiked to service a realtime clock.

The mane limitation with the uCurrent is limited range. The shunt resistors are in powers of 1000. The output voltage can go up to +/- 2.2V by using more batteries. The day job oscilloscope can resolve 10mV, which lets it see 0-2mA waveforms in 10uA precision or 0-2A in 10mA precision. Resolving the key 2mA-10mA range is impossible without building another circuit to boost the uCurrent's output, once again facing all the issues of op-amp dropout voltage & gain bandwidth product. There are a lot of circuits in the key 2mA-10mA range.

It's surprising there aren't more gadgets like the uCurrent & no reviews of it, but it's aimed at the $parkfun market & the $parkfun market is driven by hobbyists. Optimizing the power management for indefinite fault tolerant transitions between different modes isn't on the mind of the typical wearable pinball machine tinkerer or quad copter maker who drains the battery in 10 minutes.
Last edited by Jack Crossfire; Aug 05, 2015 at 01:19 AM.
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