I would like to measure the power consumed (or current drawn) by an RFM69HCW 915 MHz module (picture below) connected to an Arduino Pro Mini (on a customized board). I put the radio to sleep for 750 ms, then it wakes up for 250 ms and then goes back to sleep, so on and so on. How do I use an oscilloscope to see the current drawn by the radio module?

RFM69HCW module

Here is what I tried: The RFM69HCW module is supplied with 5 V, which is brought down to 3.3 V using resistors. A voltmeter across the VIN and GND pins show 5 V. In the picture above I circled a pin in red on the RFM69. A voltmeter across that port and the GND pin shows that the voltage toggles between 3.3 V to 0 V, and I guess that is due to the radio going to sleep and waking up.

To reiterate, I know that to measure the current I have to put the probes of the oscilloscopes in series in the circuit. But which pins do I probe?

FYI, I can use the radio to receive data from a transmitter. I can use the data to perform tasks on the Arduino. So the hardware and software work fine.

  • 1
    \$\begingroup\$ "supplied with 5 V, which is brought down to 3.3 V using resistors" no, it isn't. The board clearly shows a linear regulator. \$\endgroup\$ – Chris Stratton Oct 11 '17 at 3:46
  • \$\begingroup\$ Dave from the EEVBlog made a video explaining how to use the integration function on a scope to determine the current consumption of a product with non-constant current consumption. Find this video here: youtube.com/watch?v=Dh0xYu8YvaE Dave's general way of working also applies to your situation. \$\endgroup\$ – Bimpelrekkie Oct 11 '17 at 8:47

An oscilloscope is not designed to measure current, but rather voltage.

Therefore, to measure current, you need something which transforms current to voltage, such as:

  • A low value sense resistor in series with the circuit. However, this may disturb the operation of the circuit. Sometimes you can get this to work by adding a capacitor across the circuit being tested.

  • A current sense transformer or hall based current sensor

  • A current probe, which often clamps around one of the power leads and has its own amplifier with range selection which then feeds into the scope.

For something where the power consumption is relatively stable over time, you wouldn't use a scope at all - you'd only use it for something where the demand is pulsed and you want to see the actual waveform of current, and not just a simple time average.


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