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Adafruit Feather 32u4 Bluefruit LE - Power Management
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Adafruit Feather 32u4 Bluefruit LE - Power Management

by lonpfrb on Wed Dec 30, 2015 10:29 pm

According to https://learn.adafruit.com/adafruit-fea ... management
the SPX3819 is used to regulate USB or LiPo power from 5 or 3.7v down to 3.3v. The SPX3819 documentation specifies 16v as vMax so my intended 12v power supply (Automotive) should be acceptable. However it's not clear if either the USB or BAT pin inputs or Mini-USB or JST-PH sockets will route power to the SPX3819 inputs and so produce a safe 3.3v supply to the board & pin 3V. Obviously that's the implication of this documentation section but it's not clear.

Please can Adafruit technical experts confirm this likely pin function? Thank you.

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Re: Adafruit Feather 32u4 Bluefruit LE - Power Management

by adafruit_support_mike on Thu Dec 31, 2015 4:05 am

You'll need an external 5v regulator.

The SPX3918 3.3v regulator can handle 12 input, but the MCP73831 LiPo charger and the ATmega3u4's VBUS pin (which monitors the USB 5v line to see if there's a connection) can't.

All three of those devices are connected to the VBUS trace, which also connects to the USB jack's 5v pin and the BUS pin at the side of the board. The BAT pin connects to VBUS through a Schottky diode, and the one we use is specifically designed for a low voltage drop. Connecting 12v to either BUS or BAT will end up sending at least 11.75v to the MCP73871 and the ATmega32u4, killing both of them.

The nRF51822 BLE module would probably survive, but wouldn't be useful.

The Feather doesn't use much current, so an L4931 5v regulator and a couple of 10uF capacitors should provide more than enough buffering from the 12v auto power: https://www.adafruit.com/products/2236

You can connect the output from that to either BAT or BUS safely.

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Re: Adafruit Feather 32u4 Bluefruit LE - Power Management

by lonpfrb on Thu Dec 31, 2015 7:42 am

Mike,

Thank you for an excellent explanation. I'm glad that I asked since my Feather deserves more than to be fried.. At least I suspected that there was rather more to it than just the SPX3918 to consider.

So now I can go to my local Adafruit distributor for the additional regulator and a couple of 10uF capacitors. I found this diagram as an example of using capacitors
Image
and would be grateful if you will confirm that it's the right circuit topology for me to follow? Thanks.

Regards,
Paul.

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Re: Adafruit Feather 32u4 Bluefruit LE - Power Management

by adafruit_support_mike on Fri Jan 01, 2016 3:21 am

Yep, that looks good.

Linear regulators are nice that way.. they're so common that the connections have become simple.

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Re: Adafruit Feather 32u4 Bluefruit LE - Power Management

by lonpfrb on Fri Jan 01, 2016 9:39 pm

Mike,

Thanks for that. I hope this LM2940 regulator lives up to its' datasheet as being robust in an Automotive environment (hostile). Yes, I'm learning from datasheets too, hopefully valid on the Adafruit education journey...

Since we have discussed the power management, and I've completed the prototype breadboard design, I may as well be open with the whole prototype:
https://drive.google.com/file/d/0B6X3QirzV3nvOVgtaHVsRVNHMnM/view?usp=sharing

It's not much, I know, but is my first project with much to do on the software so hopefully the hardware will be robust. That's been my main objective.

As ever, I'm grateful for your observations, and insights since I have no formal hardware training...

Regards, Paul.

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Re: Adafruit Feather 32u4 Bluefruit LE - Power Management

by adafruit_support_mike on Sat Jan 02, 2016 2:12 am

Looks good, thanks for posting!

Don't worry about formal training too much. It has a lot of value, but doesn't replace working with actual hardware. Keep building, keep learning, and you'll be good. ;-)

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Re: Adafruit Feather 32u4 Bluefruit LE - Power Management

by lonpfrb on Tue Feb 09, 2016 6:47 pm

Mike,
Thanks for your advice. I'm having a doubtful response from the opto-couplers that I'm using to isolate my Feather from the harsh outside signals. I'm using the SFH618 here http://www.vishay.com/docs/83673/sfh618a.pdf but not sure that I've got the pins right: 1:Anode, 2: Cathode, 3: Collector, 4: Emitter.
It looks like the printed dot symbol identifies pin 1. I was expecting to see similar resistance/polarity across the LED pins 1 & 2 as a Diode IN4004 but I don't.

Any other way to confirm the pins with a multimeter, please?

Regards,
Paul.

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Re: Adafruit Feather 32u4 Bluefruit LE - Power Management

by adafruit_support_mike on Thu Feb 11, 2016 3:27 am

The forward voltage for the LED will be about a volt higher than the forward voltage of a 1n4148 diode.

The best way to verify the hardware is to connect GND to pin 2 and a 100 ohm resistor between pin 1 and, say, a 5v supply rail. Under those conditions, the voltage between pins 1 and 2 should be somewhere around 1.7v, and the resistance between pins 3 and 4 should be a few ohms.

When you disconnect the 5v supply, the resistance between pins 3 and 4 should go up to a few megohms.

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Re: Adafruit Feather 32u4 Bluefruit LE - Power Management

by roseen on Thu May 09, 2019 1:22 pm

Hi!

I'm sorry for re-opening an old thread, but this is the only hit I get when I try to google., and I thought the answer was very well written.

My question is: does your answer above go the same for an nrf52 (nrf52832) Feather BLE?

And if so, what's the easiest solution today (4 years later...) for a dc-dc converter from 12 to something that the feather likes?

Sincerely,
Göran Roseen

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Re: Adafruit Feather 32u4 Bluefruit LE - Power Management

by adafruit_support_mike on Thu May 09, 2019 11:30 pm

What, specifically, do you want to do? The thread above hit multiple issues.

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Re: Adafruit Feather 32u4 Bluefruit LE - Power Management

by roseen on Fri May 10, 2019 1:26 am

Ah, sorry!
I meant the starting discussion of how to feed the feather from 12V.

I suppose the nrf52832 also needs a step down converter, but to what voltage, USB's 5V, or the lipo's 3.7?

Another thing is that I want a very stable connection (since it is mounted in a boat), so I'd prefer to not use the micro-usb connector.
If I want to solder it to the pins of the feather, should I use VBAT (c:a 4V) or or VBUS (c:a 5V)?

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Re: Adafruit Feather 32u4 Bluefruit LE - Power Management

by adafruit_support_mike on Sat May 11, 2019 2:12 am

All Feathers are limted to 5V maximum input power.

An external 5V regulator connected to the Feather's USB and GND pins, with a diode to USB, is the most common external power connection. The diode keeps the external 5V supply from fighting with the USB host if you plug a USB cable in to program the Feather.

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Re: Adafruit Feather 32u4 Bluefruit LE - Power Management

by roseen on Sat May 11, 2019 2:29 pm

Thanks!

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