Resistors to pos or neg side of led? two answers??

Discussion in 'General Electronics Chat' started by rfhelp, Mar 30, 2009.

  1. rfhelp

    rfhelp Thread Starter Active Member

    Joined:
    Mar 23, 2009
    Messages:
    59
    Here is what I read on another tutorial page:

    "I looked through all the resistors, found the one that was brown, green, brown, gold, and wired it in line on the positive electrode of the LED. (Whenever using a resistor on an LED it should get placed before the LED on the positive electrode").


    On here I am told to connect to the neg side? please advise.
  2. Externet

    Externet Distinguished Member

    Joined:
    Nov 29, 2005
    Messages:
    522
    Location:
    Mideast USA
    A LED has no positive nor negative. Has anode and cathode. That tuturial is not very good to start.

    The resistor --/\/\/\/-- goes at ANY side of the led --A|>|K-- ; but the cathode (K) of the led should path to the negative (gnd) of the supply, anode (A) towards the positive (+)

    +--------------/\/\/\/---------A|>|K-----------------gnd

    or

    +--------------A|>|K----------/\/\/\/----------------gnd

    Miguel
  3. kakin

    kakin New Member

    Joined:
    Mar 24, 2009
    Messages:
    9
    depends on what you want the resistor to do.

    for example: you have two leds and connect the resistor to voltage and the other side to both leds. no problem here, but if you connect both leds to voltage and both cathodes (negative side) to the resistor then it won't work as nicely.

    One of the leds may be dimmer or not on at all, because the current through first is causing a voltage in the resistor, making the second led "see" a smaller voltage or non at all (remeber voltage is a difference between two points)
  4. mik3

    mik3 Senior Member

    Joined:
    Feb 4, 2008
    Messages:
    4,846
    Location:
    Cyprus, but now in UK (GMT+0)
    It doesn't matter if place the resistor before or after the LED because is the amount of current the resistor limits as not to burn the LED. In a series circuit the current is the same for all components and because the LED and the resistor are in series you can connect them as you want.
  5. thatoneguy

    thatoneguy AAC Fanatic!

    Joined:
    Feb 19, 2009
    Messages:
    6,356
    Location:
    Midwest USA
    <smartass>
    It depends if current is moving from negative to positive or positive to negative. (Sorry, it was another thread...)
    </smartass>

    In reality, no, it doesn't matter which side the resistor is on. The two components together make up a Series Circuit, and current is the same at all points, but voltage changes.

    To get the correct value of resistor, Use this: LED series parallel array wizard

    Rule O' Thumb:
    LEDs with clear lenses (super-bright) are usually 20mA
    The colored lens LEDs are usually 10mA

    If you have the spec sheet, currents may be higher, but that rule of thumb gets most lit very brightly.
  6. rfhelp

    rfhelp Thread Starter Active Member

    Joined:
    Mar 23, 2009
    Messages:
    59
    They are clear and very bright. I picked up some 1k, and 3k, resistors today to dull them down


    Emitted Colour : Green
    Size: 5mm
    Lens Color : Water Clear
    Forward Voltage (V) : 3.0~3.6
    Forward Current (mA):20
    View Angle: About 25 degree.
    Static Sense:Yes
    Luminous Intensity: 15000mcd
    Life Rating : 100,000 Hours
    Free Resistors:
    Supplied with free resistors for DC 12V
  7. rfhelp

    rfhelp Thread Starter Active Member

    Joined:
    Mar 23, 2009
    Messages:
    59
  8. SgtWookie

    SgtWookie Expert

    Joined:
    Jul 17, 2007
    Messages:
    22,096
    Location:
    In the vast midwest of the USA; CST
    Super-bright LEDs can cause permanent vision damage if you look directly at them for very long. They'll burn blind spots in your retina without you even knowing - until you look at something else. Better to err on the side of caution, and run them at low current.

    As has already been said, it doesn't matter which "side" of the LEDs the resistor is on.

    I've developed a preference of having the resistors on the anode side of the LED where possible, as accidentally shorting the LED's anode or cathode to ground won't hurt either the LED or the resistor. If the LEDs are on the more positive side of the resistor, accidentally shorting the cathode of the LED to ground will result in a fried LED.

    Sometimes it's not possible to have the resistors nearest +V, as when wiring up a common anode 7-segment LED display. But, with something like dashboard trouble lights, that's not much of a concern.
  9. rfhelp

    rfhelp Thread Starter Active Member

    Joined:
    Mar 23, 2009
    Messages:
    59
    I was told that you can see which is the anode and cathode by the size of the metal in the glass. the larger is the anode(+) and the small is the cathode(-)

    I only say (-) and (+) in respect to which leads they are connected to. I realize the led itself is not (-) or (+)
  10. SgtWookie

    SgtWookie Expert

    Joined:
    Jul 17, 2007
    Messages:
    22,096
    Location:
    In the vast midwest of the USA; CST
    Actually, it varies - but usually it's the other way around.

    For lack of a better illustration offhand, have a look at this page:
    http://www.kpsec.freeuk.com/components/led.htm

    Note this line:
    "If you can see inside the LED the cathode is the larger electrode (but this is not an official identification method). "
  11. rfhelp

    rfhelp Thread Starter Active Member

    Joined:
    Mar 23, 2009
    Messages:
    59
    oops, yeh, thats what I meant. I have checked the supers and dialed them down by 5k and still too bright for my liking so I am looking for some low intensity leds with a much wider viewing angle. Flat tops would be good. the supers are just too focused.
  12. rfhelp

    rfhelp Thread Starter Active Member

    Joined:
    Mar 23, 2009
    Messages:
    59
    found some 4 and 5000 mcd flat top wide angle lights. From what I understand about leds these will be far less intense.

    The lights I have right now are 15000 mcd. What in your estimation does 5k of resistance bring the 15000 leds down to?
  13. thatoneguy

    thatoneguy AAC Fanatic!

    Joined:
    Feb 19, 2009
    Messages:
    6,356
    Location:
    Midwest USA
    My normal use of "Superbright" LEDs is to grind the lens (rounded nose) flat, then rough up the entire surface with 600 grit sandpaper, so it has a "Frosted" look. Internally, it still reflects, so the entire cylinder lights up fairly evenly at the end and first part of sides with nice soft even light.

    Once they are frosted and flat, I drive them with a 1k resistor, they are perfect for most things an LED is used for, other than a flashlight. In a white case, they blend in very nicely.
  14. SgtWookie

    SgtWookie Expert

    Joined:
    Jul 17, 2007
    Messages:
    22,096
    Location:
    In the vast midwest of the USA; CST
    The amount of light emitted is somewhat linear (porportional) in relation to the the amount of current flowing through them.

    The problems with the LEDs you have now is not only are they really bright, but also very narrowly focused. That might be OK for a sign that you'd wanted to read from head-on quite a ways away, but not so great for an instrument panel.

    "Top hat" type LEDs, on the other hand, may have too wide a dispersion angle. The perceived intensity will diminish roughly as a function of the square of the distance; they'll appear 1/4 as bright from 2 feet away as they would at 1 foot away. You might find something in a range of 70° to 100° dispersion works reasonably well.
  15. rfhelp

    rfhelp Thread Starter Active Member

    Joined:
    Mar 23, 2009
    Messages:
    59
    Interesting, never thought of that. what do you grind them with?
  16. rfhelp

    rfhelp Thread Starter Active Member

    Joined:
    Mar 23, 2009
    Messages:
    59
    Being they are on a bike and about arms length away pointing directly eyeward. Do you think the tophats will be too dim?
  17. SgtWookie

    SgtWookie Expert

    Joined:
    Jul 17, 2007
    Messages:
    22,096
    Location:
    In the vast midwest of the USA; CST
    They might be.

    I've re-focused LEDs before by using a cloth wheel mounted on a bench grinder with some rouge applied. It's a bit tricky. Helps to have the LED mounted in a holder, and snapped into a hole drilled endwise in a dowel rod. They don't launch into orbit quite so frequently then. :rolleyes:
  18. rfhelp

    rfhelp Thread Starter Active Member

    Joined:
    Mar 23, 2009
    Messages:
    59
    that works great. I just touched on my 12" disk sander, took the top right off, took some sand paper to the top mm or two and they look great even with the 465ohm resisters.

    Don' t have to buy the new ones now, thanks.
Similar Threads
Forum Title Date
General Electronics Chat resistors on both sides of L.E.D.? Apr 8, 2009
General Electronics Chat TRANSFORMER RESISTORS Oct 20, 2014
General Electronics Chat SMD Resistors Sep 13, 2014
General Electronics Chat Is it resistors I need? Diodes? If so, what type?? Sep 8, 2014
General Electronics Chat Parallel Resistors circuit Sep 6, 2014

Share This Page