underclocking raspberry pi 3 B+ to conserve power

pi lightning

While loading all the packages and software I need for my battery capacity tester / analyzer on my new pi 3 B+,  I would get the undervoltage lightning bolt, and the pi would reboot.  Half the time, it would crash again before getting to the desktop.  If I was lucky enough to get that far,  it would crash again while trying to install packages like apache2 and php7.

I have been using a simple power supply circuit for a long time based on an LM7805  voltage regulator chip in a TO-220 package plus a good heat sink, and with a couple of reservoir capacitors to power my pi projects.

LM7805 5VDC power supply circuit
LM7805 5VDC power supply circuit

It has worked great for me with no issues using a pi 2B and the pi 3B.  On the new pi 3B+, however it simply is not enough.  The new pi 3B+ has an aluminum heat sink to improve  thermal management issues due to the 1400MHz clock rate of the CPU and requires a 2.5 amp power supply.  The particular LM7805 chip I use can supply 2 amps;  enough to power on, but cannot supply power under a significant load.

I am looking into using an LM2576, however it requires several external components that I did not have on hand like a 100uH inductor,  and I wanted to get this dude up and running right now.

I decided to try and see if underclocking the CPU would get me by.   I added the following to /boot/config.txt to throttle the clock speed to 900MHz:


arm_freq=900

After rebooting,  I still got the occasional lightning bolt, but it never shutdown.  I then put the pi 3 B+ under a prolonged stress test that maxed out one core of the CPU for over 10 minutes and browsed heavily on chromium during the test.  I would reach 100% capacity on the CPU, but it never shut down.  I will pursue a better LM2576-based  power supply circuit, but this is a good temporary fix.

raspberry pi 3 B+ based battery capacity test analyzer
raspberry pi 3 B+ based battery capacity test analyzer
raspberry pi 3 B+ based battery capacity test analyzer
battery load test analyzer internals. Uses 3ea MCP3208 ADC’s, pi 3B+, adafruit pi cobbler, BPS POW-3U prototype board.

2 Replies to “underclocking raspberry pi 3 B+ to conserve power”

  1. Hello,

    very interesting article. I have the same problem with pi3B+.

    I’ve tried LT1085CT-5 which is said able to deliver until 3 amps, but even if I cooled it, it did not solved the problem.
    RPI3B+ did not stop rebooting.

    Did you finally succeeded with LM2576 replacement ?

    thanx.

    M.

    1. No, I never did get around to trying the LM25756, however I did pick up a couple of IRM-15-5 power supplies that convert 120-240VAC directly to 5VDC @ 3.0A. These are cheap and much easier to use.

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