Pic powered from USB?

[Squibcakes]

Hi Folks,

Dumb question, can a PIC (18f4xxx) be self powered from the USB supply?

What voltage is it anyway?

Cheers
J

[Melanie]

Yes, and it's 5v.

[Christopher4187]

I'd also like to add something as I don't have the opportunity to do this too much. I have some projects that use USB and I don't use a regulator, it saves (very little) on the cost of the product, and it works very well. Just incase you are going to make a big project off of the USB, most computers have a 500mA limit so if you are going to have a lot of loads, be careful not to go over that limit. Does anyone know if they will make computers with more than the 500mA limit?

[Melanie]

The USB specification states 500mA max for Bus powered devices. That's why the PC's only deliver that much. Anything over that and you have to power it yourself externally.

[Luciano]

Hi,

The following from the link below.

Best regards,

Luciano

* * *

Theory vs. Reality

USB ports do not limit current. Although 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 5 A, a wise designer shouldn't rely on that. In any case, a USB port can never be counted on to limit its output current to 500 mA, or any amount near that. In fact, output current from a port often exceeds several amperes since multiport systems (such as 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. Therefore, a 4-port system may supply more than 2 A from one port if the other ports are not loaded. Furthermore, while some PCs use 10% to 20% accurate IC-based protection, others use less accurate polyfuses (fuses that reset themselves) that will not trip until the load is 100% or more above the rating.

USB ports rarely, if ever, turn off power. The USB spec isn't specific about this, but it's 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 than an electrical fault (such as a short). Most notebook and mother-board makers are unwilling to pay for fault protection, let alone smart power switching. So no matter what dialog takes place between a USB peripheral and host, 5 V (at either 500 mA or 100 mA, or even maybe 2 A 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 items that have no communication capability. They may not be “compliant,” but they function.

From:

http://powerelectronics.com/mag/power_charge_battery_faster/

For the pdf version of the article, including diagrams:
http://powerelectronics.com/ar/Maxim.pdf

[Squibcakes]

Ok Folks,

Thanks for the info.... I'm pretty sure my pic project wont draw 0.5 Amps! But thanks for the tip.

Jazza