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Powering the Prop directly from a 5v system? — Parallax Forums

Powering the Prop directly from a 5v system?

MarkSMarkS Posts: 342
edited 2008-07-22 05:17 in Propeller 1
Wouldn't it be possible to tie the Vcc pins to a 5v line via a voltage divider? It *sounds* like it would work, although I am undoubtedly missing something (current draw possibly). I cannot see a direct need for this since an additional regulator adds only three components, but if the Prop is the only 3.3v component in a system, it might have its uses.

Comments

  • PraxisPraxis Posts: 333
    edited 2008-07-21 15:29
    Hi Marks,

    Sorry to say that unless you plan to use a zener diode to clamp the voltage at 3.3 volts this idea will not work. Use a LDO (low drop out) regulator to get the 3v3.

    Cheers
  • Mike GreenMike Green Posts: 23,101
    edited 2008-07-21 15:32
    A voltage divider provides very poor voltage regulation. It provides a stable output only when the current drawn from the "tap" is fixed or nearly so or the current from the "tap" is very small in relationship to the current through the voltage divider itself. Make up a spreadsheet or use a plotting program to see how the voltage at the "tap" will vary with the current drawn from it.

    The Propeller draws widely varying current depending on the clock speed, how many cogs are running, how much I/O current is needed, etc. The mostly likely scenario is that you'll destroy the Propeller from too much voltage. I strongly suggest that you try out ideas like this on paper. It's easy enough to do. The Propeller datasheet has most of the information you'd need.
  • hippyhippy Posts: 1,981
    edited 2008-07-21 17:23
    How does running the Prop from 5V through a number of diode drops fare, or is that equally problematical ?
  • LeonLeon Posts: 7,620
    edited 2008-07-21 17:25
    I've done that a couple of times. It works OK.

    Leon

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Amateur radio callsign: G1HSM
    Suzuki SV1000S motorcycle
  • PraxisPraxis Posts: 333
    edited 2008-07-21 17:29
    I have done it as well, but better to use a regulator.
  • Mike GreenMike Green Posts: 23,101
    edited 2008-07-21 17:43
    I really wouldn't recommend it. The 1N914 diode for example has a forward voltage ranging from about 0.6V to about 1.3V over a current range of 1mA to 500mA. The Propeller at full speed (80MHz) takes about 10mA per cog with the current drawn dropping markedly if the cog is waiting for something. At RCFAST speeds, this drops to roughly 2mA per cog. You could certainly have forward voltages ranging wildly over the 0.6V to 1V range as the current drain goes up and down with cogs waiting and running and with I/O pins switching on and off. You'd need three diodes in series to prevent damage to the Propeller (5V - 3 x 0.6V = 3.2V) with the voltage dropping much too low at high drain (5V - 3 x 1V = 2V).
  • PhilldapillPhilldapill Posts: 1,283
    edited 2008-07-21 18:02
    One of the many times I fried my propeller, I ran out of 3.3V regulators, so I made a voltage divider set to 3.3V, then used a single opamp as a voltage follower with the 3.3V as an input. Works just fine. Besides, isn't that basically what a regulator is? A voltage reference and a voltage follower?(plus some thermal stuff and what not)
  • LeonLeon Posts: 7,620
    edited 2008-07-21 18:25
    When I said I'd used a couple of diodes, it wasn't with a Propeller. The load didn't vary much.

    Leon

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Amateur radio callsign: G1HSM
    Suzuki SV1000S motorcycle
  • PraxisPraxis Posts: 333
    edited 2008-07-21 18:31
    A zener, a resistor and a npn transistor if you don't want to fork out for a regulator.
  • MarkSMarkS Posts: 342
    edited 2008-07-22 01:55
    I have absolutely no desire to try this. The idea popped into my head so I thought I'd see if it would work.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2008-07-22 03:17
    It never hurts to ask! Linear regulators are so inexpensive and easy to apply, though, that there's hardly ever — but I won't say "never" — a reason to cobble something from discrete parts.

    -Phil

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    'Still some PropSTICK Kit bare PCBs left!
  • MarkSMarkS Posts: 342
    edited 2008-07-22 05:17
    Phil Pilgrim (PhiPi) said...
    It never hurts to ask!

    That's what I was thinking. If it would have worked then I could save a dollar or two. I'd much rather ask than fry my Prop.
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