I'm totally new to circuits and programming, I need your help with powering my device.

Thread Starter

cap_arcona

Joined Feb 10, 2021
3
I have a Stemma QT Py with a tiny LED that I'm trying to make into a portable device. Currently it only works with a USB C connection. Can I solder one of my 1S RC batteries to the back on the USB's pads?
 

dl324

Joined Mar 30, 2015
13,303
Welcome to AAC!
I have a Stemma QT Py with a tiny LED that I'm trying to make into a portable device. Currently it only works with a USB C connection. Can I solder one of my 1S RC batteries to the back on the USB's pads?
What voltage are the batteries?

It looks like you can apply 5V to the pins under the USB C connector:
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The diode mentioned is to prevent power from the USB port from charging your battery. If you use a diode, a Schottky would be best as it has a lower voltage drop and you'll still be within the spec for 5V tolerance (4.5V is allowed).

EDIT: The voltage regulator used has a worst case dropout voltage of 0.4V with a load current of 600mA. That means the lowest voltage at the 5V pad needs to be 3.7V. The typical dropout voltage is 0.25V.
 

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Last edited:

wayneh

Joined Sep 9, 2010
17,153
I’m afraid you may have trouble putting 3.7V, minus a diode drop, onto a pin expected to be at 5V. Might work, might not.
 

Thread Starter

cap_arcona

Joined Feb 10, 2021
3
You guys are the best! Thanks!


Which is exactly what the power bank is. And he can just plug it right in, no wiring required.

Bob
Unfortunately the device needs to be small. Really small, like roughly the size of a quarter. I'm prototyping an idea I had, but I'm not directly telling you what it is ;)

What voltage are the batteries?
All my batteries are for flying RC planes. I have little 220mAh 1S (single cell) at 3.7v, as well as 2S 380mAh (7.4v). I'm pretty sure the 3300mAh 3S and 5000mAh 4S batteries are way too much, and I'm not donating my $60 6S batteries to this project lol.

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Basically, I want a way to put a battery on my QT. I think soldering it right on isn't going to work, especially since it'll need charging eventually. I am totally clueless and am trying to learn as I go along, so if I am being ignorant then please let me know. How do I get portable power onto my device? Can I just put on a little USB recharging thing like this? If yes, how would I make it smaller? Please bear with me, I'm trying to learn how to write Python for this project and I already have a busy job so if I didn't research something I should have, it's because I have very little time and too many questions.

Thanks so much for your replies
 

dl324

Joined Mar 30, 2015
13,303
All my batteries are for flying RC planes. I have little 220mAh 1S (single cell) at 3.7v, as well as 2S 380mAh (7.4v). I'm pretty sure the 3300mAh 3S and 5000mAh 4S batteries are way too much, and I'm not donating my $60 6S batteries to this project lol.
If you never plan to power through the USB connector while a battery is connected, you can power with a 3.7V battery connected to the 5V pad.

Otherwise, you need the recommended diode and need a battery voltage that's appropriately higher.
 

Thread Starter

cap_arcona

Joined Feb 10, 2021
3
If you never plan to power through the USB connector while a battery is connected, you can power with a 3.7V battery connected to the 5V pad.

Otherwise, you need the recommended diode and need a battery voltage that's appropriately higher.
Oh, okay, I didn't know it was that easy. That's correct, I don't plan on using the USB with a battery connected, but I still need the USB to connect to the computer to make changes. So just to clarify: I can simply solder a couple leads onto the 5v and ground pads under the USB for my battery connector and plug in? The voltage range for these batteries is 4.2v down to 3.8v (maybe 3.7v since the voltage sag won't be nearly as bad as a plane).
 

dl324

Joined Mar 30, 2015
13,303
If you disconnect the battery before connecting power via the USB connector; yes. Otherwise you need the recommended diode and the voltage at the 5V pad needs to be at least 3.7V.

A lower voltage might work if you have a typical LDO regulator and/or load current is less that 600mA.
 
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