Power for Devices

Hello, this is probably a general question, but my current use case is an ultrasonic sensor. I bought the sensor from Adafruit that is rated for 3.3v. It works great when my Microbit is plugged into the computer. It does not work on battery pack ;-(

When the MB is plugged into the computer, I read 3.2v between the 3v and GND. When I only plug into the battery pack (with fresh batteries), I only read 2.8…hence the reason the ultrasonic no longer works.

I am a trained programmer and school of hard knocks electrician. I assume this convo has to do with current and amps as well. Can anyone shed light on this behavior between the two power sources?

Thanks!

Hi

What type of battery pack are you connecting?
How are you connecting it to the Micro:Bit?

Standard 2xAAA battery pack to power the MB. When adding extra power, I use 4xAA like this:
image

The question is still why the difference between USB power from the computer vs. the standard 2xAAA?

Thanks!

With lots more research, looks like USB provides 5V and the on board regulator turns it into 3.3. So with more input voltage, I assume it can maintain higher currents, hence the reason USB acts more powerfully than a battery pack. Am I understanding this correctly?

I’ve been playing around with the micro:bit, to better be able to teach my son how to use it, and I’ve noticed that when powering from a battery directly, that is via the 3V/GND rails, there is no onboard regulation (which makes sense, when you think about it.)

The block schematic[1] shows that the JST (JST S2B-PH-SM4-TB) and USB connections pass through the regulator but as you can see, the rails don’t. (How could they, they can’t have two voltages at once?)

Fig A

So if you power it directly with 3.6V for example (3 x 1.2V NiMH) the 3V rail will be 3.6V, and as in your case, a supply of 2.8V (assuming two alkaline batteries at 1.4V), the 3V rail will be 2.8V, thus under voltage for your peripheral. (Note, the micro:bit still runs fine as the nRF52832[2] chip on the v2 allows for 1.7–3.6V supply voltage range.)

Looking into this more, the nRF52832 specs[3] state that it has a “fully automatic LDO and DC/DC regulator system”. My understanding is that it will step-down an input voltage, but won’t step low input voltage up.

But let’s test it. I’ll connect a number of power supplies and test the voltage with all LEDs active, and with them all inactive.

1 x 5V USB (nominally 5V) via USB.
Rail reads: 3.328V (idle), 3.325V (active)
Regulator active (5V → 3.33V)

3 x 1.2V NiMH (nominally 3.6V, actual 3.85V) via JST.
Rail reads: 3.328V (idle), 3.325V (active)
Regulator active (3.85V → 3.33V)

3 x 1.2V NiMH (nominally 3.6V, actual 3.85V) via rails.
Rail reads: 3.36V (idle), 3.13V (active)
Regulator inactive, some voltage drop.

2 x 1.5V Zinc Carbon (nominally 3V, actual 2.59V) via JST.
Rail reads: 2.34V (idle), 2.21V (active)
Unknown, some voltage drop, but less than direct.

2 x 1.5V Zinc Carbon (nominally 3V, actual 2.59V) via rails.
Rail reads: 1.99V (idle), 1.88V (active)
Regulator inactive, some voltage drop.

It looks like there is some voltage drop for the direction connections but that could be the way I’m reading it, or some consumption along the way, but I think it’s fair to make the follow speculative statement:

  • Regulator only active when powered via USB or JST ports
  • Regulator can step down voltage, but cannot step up voltage
  • Even under supplied, regulator appears to keep voltage more stable

Hope this little foray into the power supply helps you, or others who land here.

[1] https://tech.microbit.org/hardware/
[2] https://www.nordicsemi.com/products/nrf52832
[3] https://infocenter.nordicsemi.com/index.jsp?topic=%2Fstruct_nrf52%2Fstruct%2Fnrf52832.html