Possible to use USB Power AND External Power on PCB?

T

Thundercat

Well-known member
Hi, apologies if this is obvious or stupid, but I've got a large project that is going to need more than the 500mA that a USB cable can supply. I'll be using a Teensy 4.1, as well as an ESP32-S3.

Yes I know USB-C can deliver much more power, but I also hear it's tricky and may not work reliably in all cases, and needs a special IC etc etc.

I want something really simple and reliable.

My thought is this: use the USB power for both the MCUs, and then add an additional wall wart that will supply 5V DC with more amperage.

I ran across a post where someone said you can do this, just connect the grounds of the Teensy and ESP32 and the external wall wart, and then use the external wall wart power for the higher wattage items (lots of LEDs and a BLDC motor that itself will take 1.2A).

Is this viable? Is this safe?

If so, what do I need to think about when introducing the wall wart positive rail onto the PCB? Should I add some bulk capacitance or EMF filtering, like a TVS, or an inductor?

Again, sorry if this is really obvious, but this is new territory for me. I really, really wanted to keep the project powered only by USB, but I'm afraid it will require a lot more power than the 500mA USB can deliver.

Thanks for any insights. And if anyone has an suggested circuits or specific ideas, I'd love that.

Mike
 
Yes you can power up your power hungry devices such as LEDs from separate power supply and connect ONLY GND between devices powered with different supplies. Make sure you do NOT connect +5V from external supply to USB 5V line because you are likely to damage USB port
On Teensy you can cut the trace to disconnect VUSB power from 5V pin.
 
Thank you Tomas.

I do not want to cut the trace on the Teensy. And it's not practical on the ESP32 either. I want to run the Teensy and ESP32 directly from the USB bus, and have everything else run off the external PSU supply.

Is this a bad idea? It seems like some things do not work as well on USB bus when the device isn't using the actual USB VBUS.
 
Thank you Chris. I don't see how this would be useful in the project I'm doing, but it is interesting for sure. I think it would add a lot of unnecessary complication and make it really confusing to implement in my situation. Also, I'm going to need north of 6A, and that part only goes to 2A.

I'm leaning towards using a buck converter to take input from a wall wart and step it down to the right voltage, and then as previously mentioned, use the USB power to power the MCUs.

I haven't heard anyone say this is a bad idea yet...

Thanks again.
 
As long as you connect ONLY GROUND and don't connect +5V coming from USB and external power source together, you would be fine. You just don't want different power supplies to "fight" with each other.
 
Another option, and the route that I took, is to hook up your device to the battery via Vin, then connect to the computer via a Hub with individual switched ports. Something like this: https://www.amazon.co.uk/dp/B00JX1ZS5O. Each port has a push button that toggles power whilst maintaining D+, D- and Gnd.
This allows one to connect your self powered Teensy to the computer without having to break the Vin to USB power track.
 
MichaelMC said:
Another option, and the route that I took, is to hook up your device to the battery via Vin, then connect to the computer via a Hub with individual switched ports. Something like this: https://www.amazon.co.uk/dp/B00JX1ZS5O. Each port has a push button that toggles power whilst maintaining D+, D- and Gnd.
This allows one to connect your self powered Teensy to the computer without having to break the Vin to USB power track.
Click to expand...
Clever! Thanks Michael.
 
Chris O. said:
TPS2113A Power Multiplexer Carrier with USB Micro-B Connector https://www.pololu.com/product/2596
Good for Teensy 4.1 with pogo pins under for usb data lines D+, D-.
Click to expand...
Hey again @Chris O. , I've looked at this more deeply and I think you actually gave me a great suggestion! Apologies for dismissing it so quickly. I couldn't see it at first. Also, I don't want to have another board on top of the custom PCB I'm making.

But there's no reason I can't use the actual chip involved, the Fairchild’s FPF1320/1321 onboard my own PCB to do the switching. And I think it's brilliant, because I can use the "PGOOD" circuit from the onboard regulator to send the high signal to do the switching - only for the USB line. This is so simple, and seamless, for the user, and for me! And safe too, since no chance to connect two power sources to sensitive IC.

So thank you again, and I really appreciate the suggestion.
 
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