Electronic – Lowering current measurement range

current measurement

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:

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.