renander
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DELL USB-C charger 45 watt

Sun Aug 18, 2019 2:30 pm

Hi,

Im am new with working on raspberrys, i hope you can help me with this (probably simple) one.
I would like to connect two WD elemtes external HDD's 1TB over the usb 3.0 ports.
I think the raspberry's standard power supply is not sufficient. Will this one work or will it blow the pi?

https://www.dell.com/en-us/work/shop/ac ... d/492-bbuu

Thanks

René

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thagrol
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Re: DELL USB-C charger 45 watt

Sun Aug 18, 2019 7:11 pm

Short answer: almosr certainly not.

Long answer:

I'd expect it to require USB3 PD negotiation in order to supply the full 45w and to do so at more than 5v. Unfortunately Dell don't have that information on their website.

45W requires a current of 9A at 5v. The USB C connector on the 4B is rated for 3A plus anything more than 5v will damage your Pi.

Add to that there's no support on the Pi 4 for USB PD (Power Delivery) negotiation and the known issue with pull resistors that make smart USB C PSUs and cables identify the 4B incorrectly and refuse to send power and all bets are off.

You need to find a PSU that publishes it's full spec not just how many watts it provides.

Even if the PSU can push 9A @ 5v to the Pi, that'll probably blow a fuse somewhere on the board. That's it the Pi tries to pull that much. If the pi only needs 10W the other 35W won't be seen.

I don't have one of those Dell units and while I do have a similar spec USB C PSU for my HP netbook I'm unwilling to test it on my 4B.
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Re: DELL USB-C charger 45 watt

Sun Aug 18, 2019 7:25 pm

My Dell USBC power supply for my laptop dock does NOT work.
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ProDigit
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Re: DELL USB-C charger 45 watt

Sun Aug 18, 2019 7:34 pm

You'll need to install a powered USB HUB in between the connection points, or use a power splitter cable like such:
Image

Andyroo

Re: DELL USB-C charger 45 watt

Sun Aug 18, 2019 7:44 pm

Do not use a splitter - you can get away with them sometimes on PCs if the ports are on different hubs or the
PC can deliver enough power.

Buy an enclosure with a power supply or a powered hub that you know does not back feed power.

If you are concerned about the back feed make up a cable with data and ground only - no 5v line

A search here will turn up posts with working hubs OR try a Pi supplier for their recommendations.

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Re: DELL USB-C charger 45 watt

Sun Aug 18, 2019 7:59 pm

thagrol wrote:
Sun Aug 18, 2019 7:11 pm
That's it the Pi tries to pull that much. If the pi only needs 10W...
15W, including the 1.2A (6W) available for USB peripherals.

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davidcoton
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Re: DELL USB-C charger 45 watt

Sun Aug 18, 2019 9:21 pm

thagrol wrote:
Sun Aug 18, 2019 7:11 pm
Even if the PSU can push 9A @ 5v to the Pi, that'll probably blow a fuse somewhere on the board.
Except that the Pi4B doesn't have a fuse to blow. So if a power-hungry peripheral could cause the total draw to exceed 3A, and if the PSU can supply that much current without negotiation, then the Pi could well be damaged by overheating a track or connector.
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thagrol
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Re: DELL USB-C charger 45 watt

Sun Aug 18, 2019 9:31 pm

W. H. Heydt wrote:
Sun Aug 18, 2019 7:59 pm
thagrol wrote:
Sun Aug 18, 2019 7:11 pm
That's it the Pi tries to pull that much. If the pi only needs 10W...
15W, including the 1.2A (6W) available for USB peripherals.
Well, yeah. But that's at full load. It could draw less. I used 10W to illustrate my point not because that's what a 4B needs/wants.

Botom line: if your peripherals need more than a total of 1.2A they need to be powered seperately to the Pi. That could be from the same PSU but tapped before power enters the Pi.

Edit:
That last sentance only aplies to dumb 5v PSus. Smart, USB PD capable ones probably won't work that way.
Last edited by thagrol on Sun Aug 18, 2019 9:53 pm, edited 1 time in total.
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Re: DELL USB-C charger 45 watt

Sun Aug 18, 2019 9:46 pm

Andyroo wrote:
Sun Aug 18, 2019 7:44 pm
Do not use a splitter - you can get away with them sometimes on PCs if the ports are on different hubs or the
PC can deliver enough power.
Do NOT use a Y cable if the ports are on different hubs.

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Re: DELL USB-C charger 45 watt

Mon Aug 19, 2019 12:50 am

That PSU probablly doesn't offer more than 3A. To get the full 45W out of it would require switching up the voltage to 15V which the Pi can't do. So while it may work with the Pi it probablly won't offer any advantages over the official 5V 3A brick.

Furthermore the Pi4 has a design flaw which causes compatibility issues with "electronically marked" usb C cables. It's possible that the output cable built into that PSU may be electronically marked and hence not compatible with the Pi4. Only way to find out is to try.

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Re: DELL USB-C charger 45 watt

Mon Aug 19, 2019 11:55 am

If you've got to power it externally anyway, a Y splitter is your only option.
If it could damage the pi, then go for option 2:
A powered USB hub.

Because it doesn't matter if you use a Y splitter cable onna secondary PSU, or use the same PSU as the Pi's PSU. The voltage on the 5v output will always be higher than the voltage on the Pi's USB 5v output.

Separate USB hub seems your best option, or, use a Y splitter cable with a Diode in series on the positive lead.
It'll cause a voltage drop from the pi, causing it to provide less power, while at the same time, preventing any over voltage from your secondary circuit to go back to the pi.

The only thing common, would be common ground, which might cause some imbalance, but ground should be ground, .. zero voltage...


Anyway, powered hub is the safest option.

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Re: DELL USB-C charger 45 watt

Mon Aug 19, 2019 12:22 pm

ProDigit wrote:
Mon Aug 19, 2019 11:55 am
If you've got to power it externally anyway, a Y splitter is your only option.
No it isn't. As you say so yourself below.
If it could damage the pi, then go for option 2:
A powered USB hub.

Because it doesn't matter if you use a Y splitter cable onna secondary PSU, or use the same PSU as the Pi's PSU. The voltage on the 5v output will always be higher than the voltage on the Pi's USB 5v output.
Evidence please.

It's by no means a given that the external PSU will have a higher voltage than the Pi's USB port. A PSU can supply less than 5v and still be in the USB spec (5v +/- 5% I think) and it depends on how good (well regulated) both PSUs are

If the two PSU aren't perfectly matched you'll hit problems.
Separate USB hub seems your best option, or, use a Y splitter cable with a Diode in series on the positive lead.
It'll cause a voltage drop from the pi, causing it to provide less power, while at the same time, preventing any over voltage from your secondary circuit to go back to the pi.

The only thing common, would be common ground, which might cause some imbalance, but ground should be ground, .. zero voltage...
Nope. The two nominal +5v lines are cross connected otherwise there's no advantage to using a Y cable. If one of those is pushing 4.9v and the other 5.1v (both values are in spec) expect problems.
Anyway, powered hub is the safest option.
Yes. Or possibly a modified Y cable with one of the +5v lines cut.
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