FreeTrack Forum

Salut.

Je souhaite alimenter mes leds en USB (5v c'est nikel).
J'ais déja les résistances, mais voilà que mon revendeur du coin me dis que c'est impossible pour lui de me commander cette saleté de PTC en 0,6A...  

Est-il vraiment indispensable, si l'on est sur de ne pas faire de cours circuit, ou a-t-il une autre utilité (protection anti-statique, masse, je-ne-sais-quoi-d'autre...)

Enfin voilà, je ne sais rien à porpos de ce style de fusible, son utilité et sa conception (sa fonction réelle quoi...)

Pour moi un fusible, c'est un fil qui crame avant le matos ou les canalisations (ce que j'ais apris en cours... Mais là apparement c'est plus subtil ^^)


A plus !

Kal.

PTC fuses never 'break' like sacrificial fuses, they simply act like a switch and open when current goes above a threshold vlaue and close when the current drops below it.

If you can guarantee that there is not, nor will there ever be, a short circuit in your hardware then you won't need a fuse. This guarantee is hard to make with a DIY setup and if you're wrong you can damage/destroy your motherboard.

PTC fuses are preferable over other fuses because they're small and don't need a mounting, are sensitive to small currents and don't have to be replaced.

Ok, Thx.

If I understoud all, it's like this:
http://www.conrad.fr/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10001&storeId=10001&productId=206454&langId=-2&parent_category_rn=19005&ItemHighLightId=206472

Data sheet here. (EN)


In fact, it's just a fuse who dont need to be changed after an hardware fail ?
Just shut down the power and replug, and it will work fine ?

But for me, what's the better ?
The "551228-62", or the "551236-62" ?

I'll use the 4point cap, or the 3point cap at the 2.1 release, so the total current in assembly is about 140.1 mA for 3 leds, and 186.8 mA with 4 leds. (with a Resistor value of 75 Ohms 5%). (same assembly than the LED Assembly Wizard)


Bye !

Kal.

la norme USB ne limite pas l' intensité max à 100 mA ???

Heu, trés bonne question...  

http://www.commentcamarche.net/pc/usb.php3

C6_LeBorgne @ 20/09/2007 - 18h32 a dit:

la norme USB ne limite pas l' intensité max à 100 mA ???

"USB ports do NOT limit current. Though the USB spec provides details about how much current a USB port must supply, there are mile-wide limits on how much it might supply. Though the upper limit specifies that the current never exceed 5A, but a wise designer should not rely on that. In any case, a USB port can never be counted on to limit its output current to 500mA, or any amount near that. In fact, output current from a port often exceeds several Amps since multi-port systems (like PCs) frequently have only one protection device for all ports in the system. The protection device is set above the TOTAL power rating of all the ports. So a four-port system may supply over 2A from one port if the other ports are not loaded. Furthermore, while some PCs use 10-20% accurate IC-based protection, other will use much less accurate poly-fuses (fuses that reset themselves) that will not trip until the load is 100% or more above the rating.
USB Ports rarely (never) turn off power: The USB spec is not specific about this, but it is sometimes believed that USB power may be disconnected as a result of failed enumeration, or other software or firmware problems. In actual practice, no USB host shuts off USB power for anything other that an electrical fault (like a short). There may an exception to this statement, but I have yet to see it. Notebook and motherboard makers are barely willing to pay for fault protection, let alone smart power switching. So no matter what dialog takes place (or does not take place) between a USB peripheral and host, 5V (at either 500mA or 100mA, or even maybe 2A or more) will be available. This is born out by the appearance in the market of USB powered reading lights, coffee mug warmers, and other similar items that have no communication capability. They may not be "compliant," but they do function."


http://www.maxim-ic.com/appnotes.cfm/an_pk/3241

Ok, thx for reply !

So i'll maintain my usb power.
When I would have the fuse of course

Bye !