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getting smaller voltage from BOEBOT board. — Parallax Forums

getting smaller voltage from BOEBOT board.

neotericneoteric Posts: 144
edited 2006-03-11 19:52 in BASIC Stamp
I need to get some voltage from my boebot board to drive the backlight on an LCD.· It can take 3.0 to 3.6V.· Do I just get a voltage regulator and put it on the prototyping board.· From searching google, it seems the way to go.· Opinions?

THANKS

Comments

  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-03-10 02:27
    Seems right to me -- but only if you're going to source it from "VIN" (NOT "VDD".)
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-03-10 05:02
    You can power it from the 5V supply.· What you need to determine is the forward voltage and current.· With that you can calculate the resistor needed to run it from the 5V supply.· If 3.6V is the forward voltage, then it would be 5V-3.6V/If where If is the Forward Current.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • neotericneoteric Posts: 144
    edited 2006-03-10 16:02
    THANKS!!!·· Wow, two options, I like it.

    According to the data sheet.·[url=mailto:60mA@3.6V]60mA@3.6V[/url]· ·(5.0V-3.3v)/.06A= 28 Ohms· Didn't understand that until just now....

    So a 28 Ohm resistor would work?· Any advice, for my own learning, as to the benefits of a regulator vrs a resistor?· Or is that in effect what a regulator is.·







    If you care...
    Blue White option http://www.stegen.com/pdf/LCD-module_dip204-4e.pdf?osCsid=899933491644294b6f20df11bff8a94b

    Post Edited (neoteric) : 3/10/2006 4:07:06 PM GMT
  • Paul BakerPaul Baker Posts: 6,351
    edited 2006-03-10 16:25
    A regulator will precisely output a voltage as long as the input voltage to it is a certain amount above the output voltage (this is called the drop out voltage). A resistor does no such thing, however since the voltage you would be supplying the resistor comes from a regulator, it will appear to have the same effect. However if you instead connected the resistor to the supply and it varied (like a battery draining) the voltage seen by the LCD would vary. Also the resistor trick only works when you have a constant current flowing through it, which is the case for this application, but it would not work for a load whose resistance varies.

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  • neotericneoteric Posts: 144
    edited 2006-03-10 16:45
    Excellent Explanation· Thank you so much!
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-03-10 17:18
    neoteric said...(trimmed)
    THANKS!!!·· Wow, two options, I like it.
    According to the data sheet.·[url=mailto:60mA@3.6V]60mA@3.6V[/url]· ·(5.0V-3.3v)/.06A= 28 Ohms· Didn't understand that until just now....
    So a 28 Ohm resistor would work?· Any advice, for my own learning, as to the benefits of a regulator vrs a resistor?· Or is that in effect what a regulator is.
    The 28 ohm resistor will do fine at 3.3V Vf...I thought you said 3.6V which would be 23 ohms.· There is no need for a regulator in something like this but you can use one if you want.· Resistors are cheaper.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • neotericneoteric Posts: 144
    edited 2006-03-10 17:25
    Well, I am thankful for the answers.· In one place on the datasheet it says 3.6v for the bluewhite.· In another place it says 3.3v.· I believe it to be 3.3v.·

    Thanks all
  • Paul BakerPaul Baker Posts: 6,351
    edited 2006-03-10 17:28
    Always better to play it safe (choosing the 3.3 Vf value), if you are wrong, the backlight may be a little dim, if it is really 3.3V and you designed it for 3.6V you would reduce the lifetime of the backlight and may even burn it out.

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  • JonathanJonathan Posts: 1,023
    edited 2006-03-10 18:12
    What about using an adjustable vreg like the LM317? Then with a small pot you can control the inensity of the backlight.

    Jonathan

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    www.madlabs.info - Home of the Hydrogen Fuel Cell Robot
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-03-10 18:39
    Jonathan,

    ·· Paul suggested that very thing.· If he has a 3.3 that would be great, otherwise I still think the resistor is the way to go.· After all, that's how most of them are handled anyway when they have on-board connections for the backlight.· The advantage of the user powering it is that they can implement brightness and on/off control.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • marzec309marzec309 Posts: 146
    edited 2006-03-10 19:02
    Jonathan,

    You don't need the regulator to make it adjustable. You can wire the pot in between the resistor and the backlight. This would limit max voltage by the 28 ohm resistor. the pot would then add more resistance. thus the backlight would be dimmed by turning the pot.


    mike
    bmp
    bmp
    sample.bmp
    217K
  • Paul BakerPaul Baker Posts: 6,351
    edited 2006-03-10 19:12
    I would change the value of the pot to be a much smaller value (20-100&#937[noparse];)[/noparse] with 1kΩ you would only have a very small angle of useable resistance.

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  • marzec309marzec309 Posts: 146
    edited 2006-03-11 19:52
    Ya changing the pot to a smaller value would be a good idea, but the small ones are hard to find in my area.
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