Coponents in Parallel

Discussion in 'General Electronics Chat' started by Galibore, Jul 23, 2012.

  1. Galibore

    Thread Starter New Member

    May 22, 2012
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    Hi Guys,

    I'm a programmer and don't know much about electronics, so please excuse the rookie question:

    I have a 9V battery with which I want to power my Netduino as well as a 5V 16x2 Serial LCD Screen (from Grove).

    Is this possible and if so, can someone explain to me how to wire it?

    I can power the LCD with the Netduino's 5V output, but the when I use the screen's backlight, it starts flickering very faintly. This was when I was powering the Netduino with the USB cable.

    Hope I make sense.

    Thanks in advance. Cheers.
     
  2. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    We use voltage rgulators such as LM7805 or LM317 to change 9 volts to 5 volts. Is this what you're asking about?

    ps, a rectangular 9 volt battery is rather weak in the current department. It will not come close to the 1/2 amp you can get from a USB 2.0 port.
     
  3. Galibore

    Thread Starter New Member

    May 22, 2012
    20
    0
    Thanks for the response 12.

    The LCD will be mounted on my robot, so USB is not really an option.

    So what you are saying that I have two circuits in parallel, One just connects straight to the Netduino's 9-12V barrel jack, the other has a regulator and the LCD in series? Would it not be possible to just drop the volatge using resitors?

    Thanks for your help!
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    Dropping voltage with resistors worksIF the load current is known and remains constant. (Ohms law E=IR ) so IF you know the current draw of the display, AND it remains constant, a resistor will work. A regulator, on the other hand, will accommodate not only changes in load, but variations in input voltage as well.
     
    Last edited: Jul 23, 2012
  5. Galibore

    Thread Starter New Member

    May 22, 2012
    20
    0
    I see. Thanks a lot. Then regulator it is. One more question though.

    Let's assume the display uses 0.2A of current. Then, by my calcs (which is probably wrong), the regulator would have to get rid of 4V x 0.8A = 3.2W of power. Would that not cause a lot of heat?
     
  6. Galibore

    Thread Starter New Member

    May 22, 2012
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    Or is that 4V x 0.2A = 0.8W?
     
  7. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    Now you have it, and building a high efficiency switching supply is generally considered foolish for less than 1 watt. (Assuming I can read your mind for the next question.)
    However, there are supplies available (retail) that will do that.
     
  8. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    the MC34063 is cheap. But a 9V battery is very poor choice to power a LED backlight.

    I would rather use 2x or 3x AA cells.

    And maybe a MCP1640 which also is in the 50 cents range.

    For said currents, it can use RF inductor + 1uF ceramic capacitor.

    It's tiny SMD but adapters for these exist.

    I could mail you one such chip for low cost, buying a single quantity of these still attracts high shipping charges from Microchip.

    They don't distribute single piece components via letter mail, actually.
     
  9. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    Have you measured the LCD backlight current? There are some very inefficient LED backlights which need about 100mA.

    New technology green LCD backlights use a single square high-brightness LED, currents are only in the 20mA range.

    Having a 100mA backlight, using a 9V battery is not possible.
     
  10. Galibore

    Thread Starter New Member

    May 22, 2012
    20
    0
    Haha... if ever your career goes south, you should become a mind reader. Thanks for the help.


    Umm, not sure if it makes a difference, but it's an LCD, not LED. Here's the link: Grove Serial LCD

    I've ordered an LM7805 already. Gonna give it a try. I see the LCD is rated for 80mA MAX.

    On the point of capacitors, should I add bypass cap before/after the regulator and which sizes?

    Thanks for the offer for the mailing. For now, it's ok, but I might take you up on that offer one day :)

    In South Africa, in general I am struggling to source small quantities of any component. Even caps, resistors and diodes. When I can get, I have to buy what I need from three different online stores, which costs me a fortune in shipping fees because one store doesn't stock everything.... *sigh*


    Didn't measure it, but checked the data sheet just now. So could I theoretically connect two 9V in parallel?

    Thanks for all your help guys. I really appreciate it!
     
  11. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    Follow the instructions on page 21. The capacitors go right up against the regulator chip if you can get them that close. And yes, you can connect batteries in parallel if they are the same size and equally fresh.

    ps, my long term career is air conditioning. It will go south when Florida freezes over.
     
  12. Galibore

    Thread Starter New Member

    May 22, 2012
    20
    0
    Thanks for the data sheet!

    Will read it creafully.


    Mwhaha...
     
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