GoPiGo-3 Voltage Droop - research and a theory

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One of the problems I’ve been experiencing with my GoPiGo-3 is unexplained “low voltage” alerts when things should be working fine.

Today I finished reassembling Charlie and began my initial research.

What I did:

  1. I added an on-board voltmeter that is currently connected to the top of the reverse polarity protection diode immediately after the power input USB connector.  This was done as a part of the final assembly process so I now have a continuous reading of the Pi’s 5v buss.

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  1. I verified using an external voltmeter that the voltage I was reading was accurate.

  2. I “calibrated” the external 5v boost supply to provide “exactly” 5.00 volts (± 0.02v), with both battery and the adapter connected to a “running” Pi-4 with no operating system present.  This was to provide a "fully powered baseline with all power sources attached.

The tests:

Battery only, fully charged, reading at least 12v.:

  1. During initial boot the voltage started at about 4.90v, but as the system went through the bootup process the voltage drooped to 4.70v and possibly lower.  Since I wasn’t using a 'scope, I could not tell if there were any excursions that were faster than the meter’s approximately 0.5 sec. time constant.

  2. I measured at the 5v pins on the top of the GoPiGo’s red-board connector (pins 2 and 4).

    • There was no measurable voltage drop between there and the Raspberry Pi’s internal 5v buss.

  3. I made the same text between the Raspberry Pi’s 5v pins on the GPIO connector with the same apparent result, regardless of load.

  4. Motion of the servos controlling Charlie’s “head” did not cause any voltage changes outside the ± 0.02 tolerance I measured before.

  5. Testing with a battery more significantly discharged, (open circuit voltage of about 10.7v), showed a proportional increase in voltage instability at the output of the GoPiGo’s 5v supply - at times dipping as low as 4.5 - 4.6 volts.

Testing with the 5v external supply connected through the USB C power port:

  1. Repeating all the tests, using both batteries, the 5v buss on the Raspberry Pi never dipped below 5v (± 0.02)

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Conclusion:

  1. Contact resistance and in-circuit voltage drop was negligible within the tolerances of both the attached voltmeter and my external voltmeter which I used periodically to verify the onboard voltmeter’s readings.

  2. I strongly suspect that the GoPiGo’s onboard +5v supply cannot supply the power requirements of it’s own onboard circuitry and a Raspberry Pi-4 without external supplemental power.

Test notes:

  1. These tests were done with an"older generation" (26 pin) GoPiGo-3 board.

  2. These tests were NOT done with a later generation GoPiGo controller board.  At least not yet. :wink:

  3. These tests were NOT done with an earlier generation Pi, neither were they done with a Pi-5.  I suspect, (but cannot prove), that the GoPiGo’s 5v regulator would be more strongly affected by something with the power requirements of a Pi-5.

  4. Testing was done using a Seagate “Expansion” 500 gig external SSD attached.  Attaching an additional Seagate SSD caused a voltage difference of about 0.10 volts.

More extensive testing remains to be done.

What say ye?

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It is interesting that due to all the power supply / cable drop issues reported by Raspberry Pi users over the years, the official Pi supplies spec 5.1v now.

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That’s at the connector, and accounts for the protection diode’s drop.

I calibrated for 5v as measured at the internal +5v buss, (measured at a point close to the +5v power connector and far away from the GPIO connector).  That way I could guarantee a solid +5 to all the 5 volt components.

There are a lot of procedural defects in this, but I tried to get a quick “damp rag” test to try to narrow down the scope of the problem.

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