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USB power hack into the next level... — Parallax Forums

USB power hack into the next level...

ejwongejwong Posts: 21
edited 2011-09-11 08:57 in Propeller 1
i did this
usbpowerhack.jpg
using this
USB-21mm_1.jpg

and its working fine.

CHEERS!?
usbpowerhack.jpg
500 x 500 - 79K
USB-21mm_1.jpg
1024 x 812 - 50K

Comments

  • Duane DegnDuane Degn Posts: 10,588
    edited 2011-08-25 11:18
    But do you really get 5V on the 5V rail after the LM2937 regulator? I bet not.

    Plus, it would be nice to have the power from the same USB cable used to program the board (I know I've seen some forum member do this).

    Duane
  • ejwongejwong Posts: 21
    edited 2011-08-25 11:54
    checked the voltage after the regulator, got 4.7volts. yea it drops but not that big of a deal.
  • PublisonPublison Posts: 12,366
    edited 2011-08-25 12:30
    Good luck with that. I guess you don't understand how a regulator works. It must have a higher input voltage to be able to regulate to it's specified output voltage. In this case it's a LDO regulator which needs an input of AT LEAST 5.5 Volts. USB Spec is between 4.75 and 5.25 volts. You may be getting enough voltage right now, but put a couple of components on the board, and the current draw will take that voltage below specs, and you will be wondering why your circuit is going haywire.
  • TubularTubular Posts: 4,726
    edited 2011-08-25 13:35
    ejwong,

    Well done for giving it a try. 4.7 volts is more than I would have expected given the 3v3 current is also all passing through that 5v regulator.

    Are you running a reasonably current hungry program when you get that 4.7v? Something with VGA or TV and pll16x?

    It looks like the 5v rail is only used to supply power to keyboard and mouse, and the "monitor wake up" signal on pin 9 of the VGA. All of those will probably tolerate lower than 5v.

    I have to admit I once tried to source the same cables from ebay. The cables that turned up actually had much smaller diameter DC plug barrels - about nokia phone sized I guess.
  • ejwongejwong Posts: 21
    edited 2011-08-25 13:56
    i havent tried anything heavy yet, just a couple of stuffs like flashing the leds. well i tried observing if it drops anymore during reset or boot ups but it doesn't so i guess the drop is caused by the regulator.

    PS: regulators doesn't just go poof if you feed it something below it's regulating line(i.e. feeding 4volts into a 5v regulator), at worst it`ll just eat about 0.2-0.5 volts.
  • frank freedmanfrank freedman Posts: 1,983
    edited 2011-08-25 14:10
    ejwong wrote: »
    i havent tried anything heavy yet, just a couple of stuffs like flashing the leds. well i tried observing if it drops anymore during reset or boot ups but it doesn't so i guess the drop is caused by the regulator.

    PS: regulators doesn't just go poof if you feed it something below it's regulating line(i.e. feeding 4volts into a 5v regulator), at worst it`ll just eat about 0.2-0.5 volts.


    Have you tried this trick with 5V TTL logic on the board? This may only work for LV or some CMOS type logic. The tolerances for the usual TTL suspects probably will not like 4.7V very well.

    Frank
  • Cluso99Cluso99 Posts: 18,069
    edited 2011-08-25 18:28
    The 5V regulator will not be damaged. However, it will not regulate and there will likely be lots of noise (ripple) on the line. This may or may not matter depending on what is attached to the 5V line.

    IIRC DigiKey do make a cable that does this.
  • ejwongejwong Posts: 21
    edited 2011-08-26 07:18
    wouldn't it run fine? well from the beginning, the prop is designed to run on 2.7-3.6volts so having it running @ 4.7volts shouldn't be too much of an issue... well the extra parts could have an issue but it shouldn't be too bad.

    Edit: well the filter below should be sufficient to filter any big ripples, and sudden voltage dips could be backed up by the inductor and capacitor.
    filterbank.jpg
    ah do note, if you reversed the input and the output, it becomes a voltage booster if the input is pulsed.
  • idbruceidbruce Posts: 6,197
    edited 2011-08-26 08:41
    ejwong

    I have done something similar, in a manner of just supplying 5V to a Proto Board through the 5V rail. And this should be fine for running the Propeller, however as Frank asked you and tried to tell you:
    Have you tried this trick with 5V TTL logic on the board? This may only work for LV or some CMOS type logic. The tolerances for the usual TTL suspects probably will not like 4.7V very well.

    A lot of devices use 5v logic, and you probably won't be able to interface with these types of devices, but if you stay with CMOS devices you should be fine.

    Bruce
  • ejwongejwong Posts: 21
    edited 2011-08-26 08:56
    ohh, ok, i got the point =P
  • zebcrszebcrs Posts: 12
    edited 2011-09-01 09:24
    Just jump over the first 5volt regulator, short the input and the output pin and it should work on usb power.
    Or You can just add a wire from VDD to the little cap which is connected ti pin 1 on the usb connector and skip the power input in total.
    Remember there is only 500mA from the usb port.

    Haavard
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2011-09-11 08:09
    I read an interesting article on USB Power.
    I hesitated resurrecting this subject, but there's some good information presented.

    "Don't rely on protection: design your circuits to do the proper thing at all times."
  • tonyp12tonyp12 Posts: 1,951
    edited 2011-09-11 08:57
    You did not really replace it, you moved it to a different location.
    The first circuit can handle pretty crappy and cheap 6-9v ac adapters to produce nice 5v and 3.3v

    Your cable gets it's power from a presumable well regulated 5v source and those cost more.
    yes I know if you use your computers usb port it's does not "really" cost you anything.
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