HW b8: Battery protection FET removed


I was browsing through Ruuvi schematics and noticed that battery reverse polarity protection FET (Q1) was removed from HW b8 (was there in HW 71).

Was it removed because there was some issue with that?

How does 47uF cap (C20) which is now added help Ruuvi tag functionality?



The Q1 was unnecessary for practically all use-cases. The coin cell cannot be inserted with wrong polarity, center pad will remain unconnected. Even in case where wires are soldered to battery terminals and power profiler kit is connected with reverse polarity, ESD protection diodes of components on tag can save the tag against ~80 mA or so of current that the profiler sources into tag.

It helps to smooth radio current spikes, especially in cold and when battery is old.

How about CR2477N and other reverse polarity (negative on the sides) batteries though? Although “obviously” wrong type of battery, but it’s easy to mistake the N as a “meaningless letter” since there are all sorts of H, HT, HR etc which don’t affect the physical geometry of the battery

The peak current capacity of the coin cells is small enough that the ESD diodes should be able to protect the tag. The battery itself might get damaged or drained, especially if left in for a longer period of time.

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Thanks Otso, makes sense!

Can you tell what is manufacturers part number for Q1 in HW 71? I think I need to replace it on my PCB. BOM .csv of KiCAD doesn’t give the part number.


I don’t have exact part number for Q1, it might be a generic one where it is substituted by whatever happens to be available. As long as the gate threshold voltage is low and pads are in right order any part should do.