barrychapman
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Power demands

Mon Sep 30, 2019 5:34 pm

I have a power-hungry pi device, and while most of the time it is okay, a surge in usage may result in insufficient power to the device causing it to reboot.

I have a RavPower USB power supply that I use in the car, which boasts 40W total output between two ports. My question is - and I know questions have been asked similar to this before, but I wasn't able to find the answers I needed - is can I use some sort of 'merge' cable (opposite of a Y-cable), to essentially merge the power from these two ports?

I would need to make sure it is wired in such a way that it provides 40W @ 5V, and not 40W at 10V of course, which either scenario would be possible depending on how it is wired (series/parallel). Would this be a viable option for supplying continuous power to my pi assuming that the voltage stays at 5VDC?

I have a few buck inverters lying around that I could adjust to deliver 5V/5A - but they do not have the comfort of circuitry to regulate voltage variance. I am not sure WHAT the variance would be, but I was unwilling to test it.

What are my options here?

itimpi
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Re: Power demands

Mon Sep 30, 2019 8:12 pm

Why do you think you do not want a Y-Cable? The standard use of a Y-Cable is to take the power from 2 USB ports into a single device that requires more power than a single USB3 port can provide. In the past this was often used for 2.5” hard drives although modern ones are less power hungry and thus do not need Y-cables.

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Imperf3kt
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Re: Power demands

Mon Sep 30, 2019 9:42 pm

It should work as long as the ravpower device has one regulator between two ports.
If it has one regulator per port then there may be conflicts between the two supplies while they "fight" each other if the voltages are slightly different
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Paul Hutch
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Re: Power demands

Tue Oct 01, 2019 4:38 pm

Even the most power hungry Raspberry Pi setup should use less than 20 Watts. So if the RavPower USB power supply gives 20W per port you should only need one port.
(Pi4 power adapter is 15.3W https://www.raspberrypi.org/products/ty ... er-supply/).

What length and gauge of wire are you using for power? The Pi4 power adapter uses a 1.5m 18AWG cord to ensure adequate power.
Imperf3kt wrote: It should work as long as the ravpower device has one regulator between two ports.
If there is only one regulator circuit for both ports then either port can deliver full power. So there should be no need to use two ports (unless you need to halve the milliohms of contact resistance in the USB connectors)

barrychapman
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Joined: Thu Oct 25, 2018 2:55 am

Re: Power demands

Wed Oct 02, 2019 2:39 am

Paul Hutch wrote:
Tue Oct 01, 2019 4:38 pm
Even the most power hungry Raspberry Pi setup should use less than 20 Watts. So if the RavPower USB power supply gives 20W per port you should only need one port.
(Pi4 power adapter is 15.3W https://www.raspberrypi.org/products/ty ... er-supply/).

What length and gauge of wire are you using for power? The Pi4 power adapter uses a 1.5m 18AWG cord to ensure adequate power.
Imperf3kt wrote: It should work as long as the ravpower device has one regulator between two ports.
If there is only one regulator circuit for both ports then either port can deliver full power. So there should be no need to use two ports (unless you need to halve the milliohms of contact resistance in the USB connectors)

I am using a USB 3.1 cable retrofitted and shortened for this application. I have heard horror stories of slight voltage variance causing the competing power sources to literally kill each other. In my mind it probably isn't worth it.

The only times I was getting low voltage alerts was when I was using a sub-standard PSU, and had my LTE, GPS, keyboard all plugged in. Haven't really had a problem on the single port with just the GPS and LTE module activated. I do have the onboard WiFi and BT disabled too.

In response to your last point, I think there are two regulators. From what I read, each port on the device delivers 15 or 20W @ 5VDC, but I would verify first. It is a RavPower USB 12V -> 5V accessory power supply
Last edited by barrychapman on Wed Oct 02, 2019 2:43 am, edited 1 time in total.

barrychapman
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Re: Power demands

Wed Oct 02, 2019 2:40 am

itimpi wrote:
Mon Sep 30, 2019 8:12 pm
Why do you think you do not want a Y-Cable? The standard use of a Y-Cable is to take the power from 2 USB ports into a single device that requires more power than a single USB3 port can provide. In the past this was often used for 2.5” hard drives although modern ones are less power hungry and thus do not need Y-cables.
because even a slight variance in voltage between the ports could cause them to do battle til the death, not what i want to have happen. It could cause the Pi to get caught in crossfire, or worse set the bus on fire.

Neither of which are favorable scenarios

barrychapman
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Joined: Thu Oct 25, 2018 2:55 am

Re: Power demands

Wed Oct 02, 2019 2:43 am

Imperf3kt wrote:
Mon Sep 30, 2019 9:42 pm
It should work as long as the ravpower device has one regulator between two ports.
If it has one regulator per port then there may be conflicts between the two supplies while they "fight" each other if the voltages are slightly different
Sorry just saw this post for some reason. This is exactly my concern. Slight variances could kill both PSU's

Paul Hutch
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Re: Power demands

Wed Oct 02, 2019 3:33 am

barrychapman wrote:
Wed Oct 02, 2019 2:39 am
I am using a USB 3.1 cable retrofitted and shortened for this application.
Check the gauge of the wire, many USB 3.1 cables have thin power conductors (26 AWG or smaller). A 0.5m 26AWG cable will drop 0.4V @ 3A which with a nominal 5V supply will definitely be a problem. Even a 24AWG 0.5m cable will drop 0.25V @3A and when you add in the voltage drop of the connectors it often is a problem.

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