L.E.Ds as side lights

Discussion in 'General Electronics Chat' started by Webby, Nov 1, 2008.

  1. Webby

    Thread Starter Active Member

    Jun 15, 2008
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    Hi, I would like to purchase some L.E.Ds which I ve found on ebay to use as side lights on my vehicle is there a way to calculate the resistor value to be used in my circuit. The vehicle is 12v supply fed.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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  3. Webby

    Thread Starter Active Member

    Jun 15, 2008
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    Thanks for that interesting read, for a safe value a 1k[​IMG] resistor should be suitable for the LEDs to work being as my supply is 12v.
     
  4. scubasteve_911

    Senior Member

    Dec 27, 2007
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    Don't forget power dissipation, which is also described there. It will let you know what 'wattage' you need.

    Steve
     
  5. Webby

    Thread Starter Active Member

    Jun 15, 2008
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    Hi Steve, is it correct in saying an L.E.D can draw upto 20 milliamps or less? Being as there working voltage range is between 2-4v.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Voltage in a vehicle can vary quite a bit. Under light accessory load with a fully charged battery, you might measure around 13.4V; if there are a number of accessories on and the battery is discharged, you may measure upwards of 14.4v.

    If you're using a simple resistor as a current limiter/voltage dropper, you'll need to plan for the maximum voltage that might be seen in the system. As a result, the light output at lower voltages will be less.

    There are two basic ways to correct this problem; one is to use a voltage regulator to supply a fixed voltage across a number of LED "strings", and use current limiting/voltage dropping resistors in each string, OR use active current limiters in each string.
     
  7. John P

    AAC Fanatic!

    Oct 14, 2008
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    If you want to be a bit more sophisticated about it, put the LEDs in groups wired in series, and use one resistor per group. Or better still, use an electronic constant current circuit. Or best of all, a switching regulator circuit.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    You need to go by the manufacturer's specifications.
    Usually, they will give you a Vf (forward voltage) for a single current.
    They may give minimum, typical, and maximum values.
    For instance, a red LED might have
    Vf: 1.9V(Min) 2.1V(Typ) 2.4V(Max) @ 25mA
    You can't buy LEDs that have all identical Vf values. Everyone would only buy the lowest Vf LEDs. If you measured 100 LEDs out of the same batch for Vf at a given current, a plot of the results would look like the traditional "bell" (Gaussian) curve.

    So, when you are calculating your resistors, use the "Typical Vf" specification.
     
  9. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    You don't need to worry too much about the maximum current rating as modern white LEDS are very robust and can withstand a fair amount of over-current. I've been using ebay 10mm white LEDs as sidelights at around 30-40ma for nearly a year now without failure.

    Tempting as it is, don't ever look directly down the beam of high brightness LEDs, they really can damage your eyes.
     
  10. Webby

    Thread Starter Active Member

    Jun 15, 2008
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    Thanks for the info guys.
     
  11. Webby

    Thread Starter Active Member

    Jun 15, 2008
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    Just done some research my side lights are 5W bulbs times 2 =10/12 which gives me just under 1amp current draw from this calculation can I work out the resistance in order to give me some kind of indication of a resistor value?

    Thanks Again
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    Your existing side lights are incandescent. They require far more power than LEDs do. LEDs and incandescent bulbs are very different critters. ;)

    You'll have to go by the ratings supplied with the LEDs.
     
  13. Webby

    Thread Starter Active Member

    Jun 15, 2008
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    Cheers SGT Ill take a look at the L.E.Ds and post back. What should I be looking for on the L.E.Ds if there are any scribings.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    LEDs as components generally do not have markings. Basically, there are only clues as to which lead is the anode (+ side) and which is the cathode (- side). Usually, the cathode lead is shorter, and/or there is a flat spot on the rim of the LED.

    You will have to find out the rating from the packaging of the LED. Failing that, you will have to ask that question from the person selling the item. If you do not like the answer (ie: you don't get complete information) then do not buy the product.

    For a 1st project, I suggest that you buy from an authorized distributor, such as Digikey, Mouser, or Newark. Digikey is quite "hobbyist friendly", there are no minimum order $, part specifications/datasheets are available online, and they will ship small orders in 1st class mail which will save you money.

    Since your side marker lights are definitely safety-related, I urge you to use quality components from an established vendor.
     
  15. Webby

    Thread Starter Active Member

    Jun 15, 2008
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    Hi SGT just a query when I have purchased a suitable resistor to power my L.E.Ds am I best wire it in series power supply side or wire it in on the earth side of the L.E.D?
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    Well, a resistor does not power an LED; it functions as a current limiter to prevent excess current from flowing through the LED.

    It really doesn't matter whether the resistor is on the high side (more positive) or low side (more negative). However, my general preference is to use the current limiting resistors on the high side.

    There are circumstances where one might want them on one side or the other, such as if switching in different values of resistors using MOSFETs, for example. One such circuit is attached. If U1 is off, no current flows. The other two MOSFETs provide for selectable current flows through the LED, up to about 47mA.
     
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