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what about voltage dividers to get 3.3 from 5 — Parallax Forums

what about voltage dividers to get 3.3 from 5

mickalmickal Posts: 75
edited 2010-12-25 08:42 in General Discussion
Happy Christmas.

I would like to know if I could power an IC at 3.3 Volts from 5V with just a 1k and 2k resistor.? Is this unstable or somehow dangerous to a prop IC ?
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Comments

  • LeonLeon Posts: 7,620
    edited 2010-12-24 21:59
    Allow for Propeller current and do the arithmetic!
  • Martin HodgeMartin Hodge Posts: 1,246
    edited 2010-12-24 22:40
    mickal wrote: »
    Is this unstable or somehow dangerous to a prop IC ?

    In a word, yes. But to do mean power the IC through it's VCC pin, or provide a 3.3v input signal? Your schematic shows it as an input.
  • mickalmickal Posts: 75
    edited 2010-12-25 01:12
    sorry , just a bad diagram . i meant to power it. i will use a regulator as before. just asked because ikt only just struck me. i am not sure any arithmetic will help when i make the rules :)
  • mickalmickal Posts: 75
    edited 2010-12-25 01:26
    i just5 mfinished a 1500 page library book on DC circuits and all that divider, multi voltage source kirchoff's laws , current direction.I x R = V did enjoy the sums dozens of them but i did it to help me with microcontrfollers. Glad i did but i think i would do better to study code and datasheets.

    that is why i started using YENKA. to model the current flows i was getting so worked up with :) but its good for simple diagrams too.
  • ercoerco Posts: 20,260
    edited 2010-12-25 01:30
    A regulator or even several diodes in series is much better than a resistor voltage divider over a range of current demands.
  • mickalmickal Posts: 75
    edited 2010-12-25 01:36
    yep ive heard the diode trick in the forum,thx.im waiting on a prop and a regulator from parallax. ooh and a usb prop too :)
  • localrogerlocalroger Posts: 3,452
    edited 2010-12-25 08:42
    The propeller's current demand is far too variable to drop the voltage with a passive resistor. The resistance you'd need during bootup, with one cog running at rcfast, would be too high and the voltage would drop too low when you start more cores and switch to PLLx16 for 80 MHz. A linear voltage regulator like a 7805 is basically a resistor that adjusts its value according to the current demand so that you always get the right output voltage no matter how much current is being drawn. Another option is to drop the voltage with a few diodes, because unlike resistors they have a fixed forward voltage drop (generally around 0.7 volts) no matter how much current is flowing through them.
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