Using USB 5V directly to power product during development - Page 1
Forum section: electronics / projects

author: BurnedResistor
author_rank: Regular Contributor
timestamp: July 15, 2016, 05:24:36 pm
content: Dear all,I am currently working on a battery powered project (11-13V) which includes both a 3v3 processor and a few peripherals which run off 5v. I am currently using a linear regulator to take the battery voltage down to 5v, and powerin the relevant peripherals with that. Connected to the 5V line is a 3v3 LDO which generates the supply needed for the processor and other various functions on the board.It is quite a hassle during debugging and programming because it requires me to constantly have my lap supply plugged into the board. I would much rather be able to power the board off the USB port which I use only during prototyping. I intended to simply include a jumper that would switch the 5V rail between the battery voltage regulator and the USB power, making it much easier to prototype.However, I have concerns about powering some peripherals off USB, namely an nrf24+ wireless module, seeing that the USB spec allows for the voltage to drop down to 4.4V.Is the indicated power scheme feasible? If not, who could I make sure my 5v line is nice and stable?Thank you,

author: Ravenghost
author_rank: Contributor
timestamp: July 15, 2016, 05:48:45 pm
content: I don't see why not a TTL input signal is defined high when between 2.2V and Vcc(5V). Also i don't see why USB output voltage would drop to 4.4V.From nRF24L01 datasheet

author: Ice-Tea
author_rank: Super Contributor
timestamp: July 15, 2016, 05:54:35 pm
content: If you look at the chipset for your RF, you'll see it will have its own LDO for power needs and the IO is 5V tolerant but does not need it to work

author: MarkF
author_rank: Super Contributor
timestamp: July 15, 2016, 11:50:14 pm
content: I have been using a DLP Design USB interface with a pic16f877a powered from the USB for a long time to do development and debugging. You just need to be aware of the current loading.If you're not using the USB, you could try one of the USB charger bricks.I too have powered the nRF24L01 plus a pic16f877, a pic16f876, a mcp4802 and an OLED display with no problems. To power the nRF24L01, I just put 3 1N4002 diodes in series and a parallel capactor for it. The PIC did have trouble communicating with the nRF24L01 when the supply reached 3.0V.

author: TheDirty
author_rank: Frequent Contributor
timestamp: July 16, 2016, 01:20:34 am
content: nrf24l01+ is a 3.3v device. Not certain why you are worried about 4.4v.

author: Ice-Tea
author_rank: Super Contributor
timestamp: July 16, 2016, 08:51:51 am
content: Because the chip is mounted on a module that wants +5V. So there's a dropper on that module. And depending on the dropout, that could be an issue even though I doubt it has a worse dropout then 1V so there should be no issue.

author: Jeroen3
author_rank: Super Contributor
timestamp: July 16, 2016, 10:41:59 am
content: Get a buck/boost converter. Dropping 12 to 5 is a loss anyway.I've used the TPS63060DSCT in a product with 3xAAA's and 5-12 external power.http://nl.farnell.com/2078528It works from about 3 to 13 volts Vin. excl orring diode loss.An extra small low quiescent ldo powers the on/off latch. If you use lipi battery you might need additional undervoltage lockout.

author: MarkF
author_rank: Super Contributor
timestamp: July 16, 2016, 11:11:57 am
content: Quote from: Ice-Tea on July 16, 2016, 08:51:51 amThe nf24L01 moduledoes NOThave a voltage regulator to 3.3V. You will destroy it if you apply 5V.

author: Brutte
author_rank: Frequent Contributor
timestamp: July 16, 2016, 12:58:11 pm
content: Quote from: BurnedResistor on July 15, 2016, 05:24:36 pmOnly bus powered devices that were granted >100mA load have to operate down to 4.4V , that is a necessity for certification. Your nRF won't pull more than 20mA so I would not be concerned with that. Supplying a development board from USB is risky - cannot you get a cheap, isolated 5V power supply from somewhere? Cell phone charger maybe?And please be smarter than one dude here that was powering a 4mA uC through a power supply set to 1.3A limit - use fuses or resistors to limit the current before smoke escapes.

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 16, 2016, 02:26:40 pm
content: with tons of 5V SP charger wallwart nowadays, i dont quite get it since, why the risk of short circuit damaging a PC/Laptop USB outlet which is mainly purpose for powering/communicating with proven PC peripherals, not ee experimentation.

author: Ravenghost
author_rank: Contributor
timestamp: July 16, 2016, 03:31:53 pm
content: Quote from: Mechatrommer on July 16, 2016, 02:26:40 pmIntresting point about ee experimentation. But what about arduino? Most people play with arduino powered from PC usb port, shorting pins left and right, summoning magic smoke from connected IC's and i haven't seen threads pop up saying that they damaged their USB ports.

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 16, 2016, 05:07:19 pm
content: Quote from: Ravenghost on July 16, 2016, 03:31:53 pminstead of shorting and damaging IC pins, next time do it again by shorting USB power supply rail and see what happen (been there).

author: Jeroen3
author_rank: Super Contributor
timestamp: July 16, 2016, 06:20:56 pm
content: Or you just buy an indestructible USB hub.https://www.eevblog.com/forum/reviews/bb-electronics-uhr304-isolated-usb-hub-teardown/msg837577/#msg837577I've shorted it accidentally a few times already, it just turns the port off for a while.

author: Ravenghost
author_rank: Contributor
timestamp: July 16, 2016, 06:50:06 pm
content: Quote from: Mechatrommer on July 16, 2016, 05:07:19 pmIf product(motherboard, usb hub) manufacturer is half decent it should provide over-current protection as stated in USB 2.0 specificationhttp://www.usb.org/developers/docs/usb20_docs/.QuoteI don't have much experience in shorting USB power rails so it would be interesting to hear from someone who shorted USB power rails and what was the effect of that.As someone stated above safest bet is to use external power supply or USB hub.

author: Ice-Tea
author_rank: Super Contributor
timestamp: July 16, 2016, 07:43:45 pm
content: Quote from: MarkF on July 16, 2016, 11:11:57 amYou're right. Was misled by the OP. He may want to look into that

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 17, 2016, 01:23:59 am
content: Quote from: Ravenghost on July 16, 2016, 06:50:06 pmagreed, and my USB that is shorted while ago is still ok. still i dont want to "risk" screwing my $1000 machine, but thats just me.