I'm checking current of kitchen timers to estimate operation time.
The obstacle here is that in a "passive" mode (no blinking/beeping/key pushes, only working counters + LCD) devices consume 8-12 uA but my multimeter breaks line if current out of 20 uA range and this happens during power on & menu navigation by pushing buttons.
So I started with 200mA range, altered device state into desired mode and started to lower range via mechanical switch on the multimeter (VC830L): 200mA => 20mA => 2mA => 200uA => 20uA.
I saw some "blinks" on device LCD, fortunately I am able to lower range for all tested devices without ruining their operation.
I'm an amateur and the question how such kind of measurements are performed "professionally":
- is there special auto auto-adjustble measuring devices (that jumps through the ranges)?
- is there a special design for current measurement that doesn't break a line when ranges are switched?
I saw:
- Current measurement over a fairly large range
- Dynamic Range Current Measurement
- Wide range and high precision current measurement
- How to measure supply currents ranging from 1μA to 10mA?
and they are about schematic design. My question is about "industry practice" to measure in lower current ranges with market available multimenters.
Best Answer
Use a shunt resistor, but use a large enough shunt resistor that you can see a meaningful voltage at the currents of interest.
As in your original experiment, this shunt resistor may present too much of a burden voltage to get through some more power-hungry tasks you need to pass through to get to the phase of operation you are interested in measuring, so temporarily bridge the resistor with a clip lead until you get to the situation of interest.
In some cases it's also useful to add a capacitor to the circuit, possibly by putting it across the resistor.
These techniques can be extended to some types of time-varying behavior by using a scope instead of a simple voltmeter. Sometimes it's useful to use a fairly high resistance shunt to determine the actual sleep current, then a lower one and a scope (or MCU datalogger) to determine the percentage of active vs. sleep time - even if the sleep current is misread in such case, if you combine the duty cycle from one study with the carefully measured sleep current from another, you can form a good picture with only cheap tools.