Computing stuff tied to the physical world

RFM12B startup power consumption

In Hardware on Dec 29, 2012 at 00:01

For quite some time, I’ve wanted to know just how much current the RFM12B module draws on power-up. Well, time for a test using the power booster described recently:

JC's Grid, page 51

So the idea is to apply a sawtooth signal to the RFM12B, rising from 0 to 3V at the rate of say 10 Hz, and to measure the voltage drop across a 100 Ω resistor at the same time. This will have a slight effect on measurement accuracy – but no more than 2%, so I’m ok with it.

Here is the outcome:

SCR51

The yellow trace is VCC, the supply voltage – from 0..3V. The magenta trace is the current consumption, which turns out to be 0..650 µA. As you can see, the current draw quickly rises between 1 and 2V, and then continues to increase sort of linearly.

Note that this power consumption can’t be reduced: we don’t have the ability to send any commands to the RFM12B until it has started up!

This type of analysis can also be done using the X-Y mode on most oscilloscopes:

SCR48

It’s essentially the same picture as before, because the sawtooth is a straight line, and so voltage rise is the same thing as time in this case. Here’s what happens when the input signal is switched to a sine wave:

SCR49

As expected, the essence of the curve hasn’t changed one bit. Because it really doesn’t matter how we vary VCC over time. But there’s an intriguing split in the curve – this is most likely caused by a different current consumption when VCC is rising vs when it is dropping. Keep in mind that the changes are occurring at 10 Hz, so there’s bound to be some residual charge in the on-board capacitors of the RFM12B module.

Anyway. It’s a bit of a silly distraction to do things this way, but now I do have a better idea of how current consumption increases on startup. This relatively high 0.65 mA current draw was the main reason for including a MOSFET in the new JeeNode Micro v2, BTW.