2019-12-06 at 9:12 PM #13445Steve OchsParticipant
This will probably be a question for Shannon since it is hardware related but I have a Mayfly 0.5b I purchased earlier this year and it started sending data showing a low LiPo battery voltage (3.4v instead of 3.9v). This board has only been used for software development but I was getting ready to deploy it.
I verified the battery is indeed 3.9v.
Thinking it was an A/D issue, I checked the voltages at the voltage divider circuit and it was indeed only 3.4v.
Tracing it back in the circuit I have found the input V_BATT (drain) terminal on FDN340P MOSFET to be 3.9v but the output (source) terminal of the MOSFET to the voltage regulator is only 3.4v. The gate is 3.5v
I then checked another Mayfly and the difference between the drain and source are within 0.06v (negligible)
Gate 3.5v 2.7v
Drain 3.9v 3.9v
Source 3.4v 3.9v
See attached markup of Mayfly drawing for voltages measured.
Is it possible the MOSFET is bad or possibly D3?
I hate to throw a fairly new board away just for this if I can fix it.
2019-12-07 at 12:48 AM #13447Shannon HicksParticipant
Yes, this is a known issue that I’ve seen with only a very few boards recently where we see those exact same incorrect voltage readings around the Q1 mosfet. I haven’t tried replacing parts to see whether it’s the mosfet or the D3 diode, but it’s definitely either one of those, or possibly both of them. I’ll see if I can experiment with some bad board this weekend and find the culprit. I’ve never been able to purposely cause a failure even though I’ve tried really hard to repeat the conditions and stress out the components with various hardware and power configurations. Were you powering the board from the microUSB port, the FTDI header, a LiPo battery, or a combination of those? And did you have any telemetry (radio, wifi, cellular) xbee modules installed at the time?
2019-12-07 at 9:24 AM #13449Steve OchsParticipant
I had the board powered with the 2500 mah LiPo battery and a 2w Voltaic solar panel and I was monitoring battery life when I saw the sudden drop in voltage.
I am using a XBEE S6B wifi module for telemetery.
Current draw on the battery is between 100ma and 120ma when reading sensors and transmitting
Current draw is not measurable when sleeping
Sampling interval is 15 minutes
I can’t quite make out the serial number but it appears to be 19-0025
2019-12-07 at 12:32 PM #13450Shannon HicksParticipant
Thanks for the additional information, I appreciate you helping us out with this. I’ll send you a PM about what to do next.
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