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5v USB power supply suitability. and Why is the prop 3.3? — Parallax Forums

5v USB power supply suitability. and Why is the prop 3.3?

TJHJTJHJ Posts: 243
edited 2009-02-25 14:00 in Propeller 1
So does anyone know if a standard USB supply(as in power from a USB computer port) is clean and consistent enough to use as the main supply for 5v chips ect. with out an additional regulator and to the 3.3v reg for the prop?

I measured every usb port in my house with everyone being slightly different. I have never measured a whole bunch of 5v regulators to see how much variance there is. So I decided to ask.

Also anyone know off hand what the technical reason the prop is 3.3v instead of the seemingly more common 5v? I am sure there is a reason I just haven't found it yet.

Thanks,
TJ

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2009-02-25 05:10
    1) The 5V supplied by the USB host is intended to be used to power 5V logic in the peripherals. Although it's fairly clean, it's generally a good idea to add some kind of overvoltage protection at around 5.5V

    2) The Prop is powered from 3.3V because of the maximum voltage of the manufacturing process used and to limit the power consumption and heat dissipation of the Prop. The Prop II will run from 1.8V internally although it will have 3.3V I/O pin drivers.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-02-25 06:12
    1) I've never seen a USB supply much over 5V, but I have seen USB voltages as low a 4.4V, depending on the load current, which is limited by the USB host. If your USB-powered system requires a dependable supply voltage, it's best to regulate the USB supply down to 3.3V and power your entire system from that, if you possibly can.

    -Phil
  • mctriviamctrivia Posts: 3,772
    edited 2009-02-25 06:47
    usb specs allow for 500mA and quite a wide voltage range on the device side. The voltage drop is due to very small gauge wires and the relatively high current. I recommend using a switching supply that can take anything from 3 to 6v in and give you a steady 3.3v out. a 3.3v regulator linear regulator will work as long as you are not trying to draw anywhere near the upper current limits.
  • TubularTubular Posts: 4,706
    edited 2009-02-25 08:25
    I'm not 100% sure on this, but I thought usb was only good for 100mA by default, and you had to negotiate (intelligently) to get the 500mA?

    Happy to be corrected if I'm wrong. Its something I'd really like to know...

    tubular
  • BradCBradC Posts: 2,601
    edited 2009-02-25 08:38
    Tubular said...
    I'm not 100% sure on this, but I thought usb was only good for 100mA by default, and you had to negotiate (intelligently) to get the 500mA?

    Absolutely correct. In theory a hub is perfectly within its rights to shut your power off if you draw more than you negotiate. In practice it never happens. Heck, I've seen a device draw 1A from a port and not even have the D+/- lines connected. It's ugly, but it happens. Same as these hard disks that use a dual headed cable, where they have one power/data port and another power port in parallel to give them 1A. Certainly not legal in the specifications, but everyone seems to be doing it.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Cardinal Fang! Fetch the comfy chair.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-02-25 08:55
    Just don't try this from an unpowered hub, though, if you've got other devices plugged into it! Also, if you're going to be drawing power from the USB port, it's always a good idea (and required by the specs) to include a soft-start circuit, so you don't introduce any nasty current transients when your device's fillter caps charge up.

    -Phil
  • BradCBradC Posts: 2,601
    edited 2009-02-25 08:58
    Phil Pilgrim (PhiPi) said...
    Also, if you're going to be drawing power from the USB port, it's always a good idea (and required by the specs) to include a soft-start circuit, so you don't introduce any nasty current transients when your device's fillter caps charge up.

    Which is precisely what happens to my USB powered protoboard when I plug it into the USB socket. The Electrolytic designed to smooth the power to the servos causes a nice instantaneous current pulse that causes most hubs or root hubs to scream "Overcurrent" and reset the port.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Cardinal Fang! Fetch the comfy chair.
  • Chuck RiceChuck Rice Posts: 210
    edited 2009-02-25 14:00
    BradC said...
    In theory a hub is perfectly within its rights to shut your power off if you draw more than you negotiate. In practice it never happens.

    I have been shut down for drawing too much current. The internal USB hubs on the Mac monitor the USB ports and will send you a NastyGram message telling you to stop. It then shuts down the port to protect itself.

    I have not seen this happen with external powered hubs though.
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