A bunch of questions

Discussion in 'General Electronics Chat' started by peck68, Nov 30, 2009.

  1. peck68

    Thread Starter Member

    Nov 27, 2009
    So first off, I have only been learning this stuff, well since saturday really ;)

    I got a couple of Q's to ask...


    First off, i just want to confirm this really - is this the correct way of figuring out how much resistance i need for a 30mA LED?

    R = V/A. It was a random guess which i think may be correct, that the amperage is the maximum rating on the LED? ie 30mA in this case.
    So a 9v battery will need a resistance of 300ohms to stop the LED from blowing up. However, 330 ohms is used i know because of standard values.

    If i am right please confirm me as i will be happy for figuring out :D

    Parallel LEDs
    Do LEDs have resistance? Can i safely line up 3 or 4 or so LEDs in series without losing so much power?

    Cause as i read the tutorial on this site, it recommends me not to align LEDs in parallel as it damages the LED or something along those lines.

    So if it is a bad idea to use parallel, am I also correct that adding up the total amps required will give me the required resistance? So 3 30mA LEDs will consume 90mA, therefore doing the thing i did above would require then 100Ω? < Let me know on this, i need to know from my mistakes :confused:

    I have made a little (and i mean little) schematic that i did in paint.net. As i do not know of any schematic makers.
    by the way that battery is 9vDC

    Is this the only way of having multiple LEDs together? Or is there a way that i can have them in a "parallel" sort of way?


    I have even watched the video on the site about grounding a circuit. I see it in many schematics, and also a +Ve symbol at the top of the circuit.
    No batteries are seen in the schematic either.

    Can somebody explain to me why grounding is used and why +Ve is used at the top and what i will need to use it for later on in life?


    Just to point out, i do understand ohm's law and i can work out how many amps or resistance is across a component in a circuit ;) however, i have yet to see one tutorial on putting it in use and finding how much resistance i need over a component.

  2. SgtWookie


    Jul 17, 2007
    That's basically correct, but you will need to subtract the Vf of the LED to get the right resistance.
    The generic formula is:
    Rlimit >= (Vsupply - Vf_LED) / DesiredCurrent
    So, if you have an LED with a typical Vf rating of 2.5v @ 30mA, you then calculate:
    Rlimit >= (9v - 2.5v) / 30mA
    Rlimit >= 6.5 / 0.03
    Rlimit >= 216.666... Ohms. 220 is the closest standard value. [eta] corrected value.
    They have conductance, which is sort of the inverse of resistance. But don't let me confuse you.

    Let's say that you have some LEDs that have a Vf of 2.5v # 30mA, and you want to see how many you can run in series.
    SeriesLEDs = INT(( Vsupply-1v)/Vf_LED)
    You need that 1v for the current limiting resistor.
    So from 9v, you would calculate:
    SeriesLEDs = INT((9V-1V)/2.5v)
    SeriesLEDs = INT(8/2.5)
    SeriesLEDs = INT(3.2)
    SeriesLEDs = 3
    Plugging that back into the original equation:
    Rlimit >= (Vsupply - (3 x Vf_LED)) / DesiredCurrent
    Rlimit >= (9 - 7.5) / 30mA
    Rlimit >= 1.5 / 30mA
    Rlimit >= 50. 51 Ohms is the closest standard value >= 50.

    You can run parallel strings, but each string needs to have it's own current controlled.

    3 LEDs in series will all get the same current flow through them, so they will require 30mA for the whole string, including a single limiting resistor.

    3 LEDs in parallel will require 30mA per string and each their own current limit resistor.

    Please attach the schematic using the Go Advanced/Manage Attachments buttons. Your link didn't work for me.

    The easiest way is to run them in series.
    If you need more LEDs than you can do in 1 string, you can parallel strings - but each string must have it's own resistor.
    Simply, "ground" is a 0v reference point. If there were no "ground" in a circuit, the voltages might be at an arbitrary level that could be a terrifically large numbers. It is much easier to say that the battery is 9v when referenced to ground (0v) than some ridiculously large numbers.
    Really? Our E-books are full of such examples. And the 1st part of my reply to you was a straight implementation of Ohm's Law.
    Last edited: Nov 30, 2009
  3. peck68

    Thread Starter Member

    Nov 27, 2009
    Ahh brilliant thanks a bundle for clearing this one up :D

    However, one thing i want to add...

    I did 6.5/0.03 and the answer came to 216.7? :confused:

    File attached by the way
  4. SgtWookie


    Jul 17, 2007
    That's odd - I must've typoed the calculation. :confused: 216.666... is correct.
    In that case, 220 Ohms would be the closest >= to 216.666...
  5. peck68

    Thread Starter Member

    Nov 27, 2009
    Ah, no worries - no wonder i was getting a little confused over that

    Anyway cheers for helping me out :D


    Oh and also another thing, any way of making clear schematics (software etc) without doodling on paint to make something not even that clear?
  6. SgtWookie


    Jul 17, 2007
    There are a number of freeware packages around.

    Cadsoft Eagle has a freeware version for home use; it has a few limits (just 1 schematic per project, max board size 3"x4",2 layers, but is very good within it's limits. A bit quirky to learn. Tutorial is available on sparkfun.com

    Linear Technology's LTSpice is a very powerful and free schematic capture and SPICE simulation program. Available from their website, linear.com

    Texas Instruments has Tina TI available for a free download on their website, ti.com

    I'll use an old version of Circuitmaker Student for quick simulations here, but it's no longer supported and won't run on Vista or Windows 7.
  7. Audioguru


    Dec 20, 2007
    I use Microsoft Paint for all my very clear schematics. I copy and paste parts from datasheets and from other schematics. Straight lines are made with the Shift key held down.

    But I never copy and save a JPG file type because it is too fuzzy.

    This schematic was made with Microsoft Paint:
  8. Wendy


    Mar 24, 2008
  9. SgtWookie


    Jul 17, 2007
    AudioGuru is so old-fashioned that he whittles his calculators out of wood. :)
  10. Audioguru


    Dec 20, 2007
    I still have the slide-rule that I used in school. Then I got an LED display calculator that ate 9V batteries. I added a supply bypass capacitor then each battery lasted 5 times longer.