5v on a pc psu

[sec1223]

right chaps... i want to fit a 5v usb charging system to my caravan.

iv got 240v in and wonder if a pc psu will be ok if i use the 5v side ?.

i might draw 5 amps if i charge everything at once, so will a pc psu give over 5a @ 5v?

cheers

[kr0ne]

Main problem you will have is that, no matter how much current is available, no USB device should draw more than 100mA without first negotiating this with the "host".

Clearly a simple PSU won't be able to tell the device that it can have more than this, so charging will be incredibly slow...

You will probably find it easier just to buy a car USB charger and wire that in instead. Make sure you get one marketed as having a "chip" though, or same problem.

(They are not very expensive... few quid. )

[sec1223]

isnt the charge rate distributed by the motherboard?.. the psu provides say 5a and the mobo drops it to suit all the usb ports.

[sec1223]

a simple psu talks to nothing.. it gives 5v @5a amps out (for instance) and i want to tap into that 5v

[kr0ne]

No, current is not 'given out' - it is drawn.

If USB devices could draw as much power as they wanted then when you plugged a high current device into your computer, there would be nothing to stop it overloading the port.

The USB specs are quite clear and any device that obeys them (which should be any device bearing the USB logo) will draw no more than 100mA when it is first connected. If the device needs or wants more than this, then it must negotiate that with the host first. The host will tell it how much it can draw and the device will do as it is told.

You're right that 5V is 5V is 5V though, so it will charge your devices without damaging them - just really slowly as they will not draw more than 100mA with no "host" to talk to.

[Admiral]

I would go for something 'proper' rather than a Heath Robinson affair, caravans are pretty flammable things.

http://www.techpowerup.com/178905/start ... 0-hub.html

[Mattylad]

I have never seen anything in a USB charger that allows the device being charged to communicate with it - they are simply chargers.

[cjay]

Well, the USB specs are very clear but that doesn't mean that device manufacturers actually pay attention to them.

That costs money.

There are many devices out there that just draw as much current as they can get with no negotiation at all and some even come with a dual head USB cable to get around the USB port current limit to draw twice the current one port can supply.

External bus powered hard disk caddies are one example of a device that will use as much current as the host is capable of supplying to meet its own needs without negotiating and many come with the dual head cable, they often can't spin the disk up properly if the port has decent current limiting.

Some devices will only charge at full rate (or at all) with certain combinations of resistors on the USB data lines which the device will 'look' at to decide if it's plugged into a 'proper' charger but there's no negotiation as such per the USB Association specs, they're just working out if they're plugged into a dumb charger or USB port by measuring termination resistors (not exactly clear if they're measuring or just looking for voltage to be present on data lines).

The Mintyboost charger at Adafruit.com is a good place to look for information on how devices work out what they're plugged into.

You do need to be careful supplying current to devices that ignore the USB specs though, a LiIon battery in a cheap device (pocket battery pack charger I'm thinking) can go bang in a very spectacular way so Polyfuses are a good thing to include in any charger design.

[kr0ne]

Well try it and see...

I wired up my van with 5V power supplies that came with USB tails pre-wired and were rated at 2.1A. Various Samsung and HTC phones and a cheap Chinese no-name tablet all failed to charge at an acceptable rate, yet loading up the supply with a dumb load confirmed it was happy enough supplying more than 2.1A!

A cheap Chinese USB car charger, sold as having a "chip" but having a lower current rating, charged them all up in no time.

Of course there will be junk out there that ignores the spec, but that's hardly the point. If the intention is to charge USB powered devices, then get something that is designed for the job as it's not as simple as feeding 5V to your phone! (And make sure you use a decent, short USB cable too...)

[sec1223]

I would go for something 'proper' rather than a Heath Robinson affair, caravans are pretty flammable things.

http://www.techpowerup.com/178905/start ... 0-hub.html

nothing wrong with a bit of homebrew pal.. and that usb hub that STILL needs an external psu is 80 quid.

[Admiral]

Yes pal, it's eighty quid pal, and still needs a PSU pal, but my point is pal that a fire in a £12k caravan with your family asleep in it isn't quite worth bodging something together pal.

[Mattylad]

Is a PC not able to be left on without catching fire?
Do they not have fusing in them? I have seen many USB chargers that do not and that do catch fire.

Given only 5A is drawn then IMO a PC psu is likely to be far safer as it can supply far more, has overload protection etc.
So the idea of this is in fact not as unsafe as you are making out Admiral.

[sec1223]

Is a PC not able to be left on without catching fire?
Do they not have fusing in them? I have seen many USB chargers that do not and that do catch fire.

Given only 5A is drawn then IMO a PC psu is likely to be far safer as it can supply far more, has overload protection etc.
So the idea of this is in fact not as unsafe as you are making out Admiral.

agreed.

[cjay]

Well try it and see...

I wired up my van with 5V power supplies that came with USB tails pre-wired and were rated at 2.1A. Various Samsung and HTC phones and a cheap Chinese no-name tablet all failed to charge at an acceptable rate, yet loading up the supply with a dumb load confirmed it was happy enough supplying more than 2.1A!

A cheap Chinese USB car charger, sold as having a "chip" but having a lower current rating, charged them all up in no time.

Of course there will be junk out there that ignores the spec, but that's hardly the point. If the intention is to charge USB powered devices, then get something that is designed for the job as it's not as simple as feeding 5V to your phone! (And make sure you use a decent, short USB cable too...)

If you read the Mintyboost article you'll find out why they failed to charge at a decent rate.

[sec1223]

Well try it and see...

I wired up my van with 5V power supplies that came with USB tails pre-wired and were rated at 2.1A. Various Samsung and HTC phones and a cheap Chinese no-name tablet all failed to charge at an acceptable rate, yet loading up the supply with a dumb load confirmed it was happy enough supplying more than 2.1A!

A cheap Chinese USB car charger, sold as having a "chip" but having a lower current rating, charged them all up in no time.

Of course there will be junk out there that ignores the spec, but that's hardly the point. If the intention is to charge USB powered devices, then get something that is designed for the job as it's not as simple as feeding 5V to your phone! (And make sure you use a decent, short USB cable too...)

If you read the Mintyboost article you'll find out why they failed to charge at a decent rate.

iv failed to find out what a mint tin usb charger has got to do with a pc psu?