Current measurement

[jupiter8]

Just a little electronic noobie here, how do you guys measure the current draw of the your various devices? Scopes? DDM? else?

And what is the cheapest solution for in the box and diy?

Thanks.

[jupiter8]

Thanks, i,m getting at it right now.

[robert-hh]

@jupiter8 I use different kind of methods. The simple and convenient one is a USB current meter, which displays the current. There a various models, up to a resolution of 100 µA. For the current drawn by the battery I use a DMM. If you want to measure extremely low currents, like that during deepsleep, then you have the challenge that the range of lower cost DMM does not cover ~100mA mach current at a resolution of ~1µA. That would be 8 digits. Switching the range very often ínterrupts the connection. So you might need, as indicated, a setup with a resistors as measuring shunt, which can be bypassed e.g. with a jumper for high currents. That way you can change the range without interruption. Then you would use the DMM in voltage mode.
I modified also a INA219 breakout using a Schottky Diode/resistor set-up as poor man's logarithmic shunt, giving an range of 1µA to 1 A, although with onls 4096 discrete steps and a large temperature drift. But for a rough estimation that is sufficient. And I can sample up to about 4000 values per second. A better choice would be a temperature compensated precise log amplifier. That is on task on my to-do choices.

[jcaron]

@jupiter8 as you probably want to measure things even when the device is sleeping or waking up, you usually want something external.

If you want to measure instant draw, a simple resistor in series between your battery and the device and a multimeter are enough. Pick a resistor with a “round” value to simplify conversions, low enough that it doesn’t drop voltage too much, high enough to be in the measuring range of your meter.

If you want to measure draw over time (e.g. graph what happens during boot or a complete sleep-wake up-transmit-sleep cycle), you need a device that can capture the info over time and show it to you.

Cheap options include this cheap digital oscilloscope which requires a bit of assembly and is extremely crude, but does what you need and only costs €31.

There are also logic analysers you can connect to your computer, like this cheap one for less than €20 but I haven’t used it yet.

[andyh13]

You could put a resistor in series with the device, then measure the voltage drop over the resistor with the adc. Divide the voltage drop by the known resistance, and that's the current going through the resistor.