L.E.Ds as side lights

Thread Starter

Webby

Joined Jun 15, 2008
66
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.
 

Thread Starter

Webby

Joined Jun 15, 2008
66
Thanks for that interesting read, for a safe value a 1k
resistor should be suitable for the LEDs to work being as my supply is 12v.
 

Thread Starter

Webby

Joined Jun 15, 2008
66
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.
 

SgtWookie

Joined Jul 17, 2007
22,230
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.
 

John P

Joined Oct 14, 2008
2,025
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.
 

SgtWookie

Joined Jul 17, 2007
22,230
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.
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.
 

blocco a spirale

Joined Jun 18, 2008
1,546
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.
 

Thread Starter

Webby

Joined Jun 15, 2008
66
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
 

SgtWookie

Joined Jul 17, 2007
22,230
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?
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.
 

Thread Starter

Webby

Joined Jun 15, 2008
66
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.
 

SgtWookie

Joined Jul 17, 2007
22,230
What should I be looking for on the L.E.Ds if there are any scribings.
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.
 

Thread Starter

Webby

Joined Jun 15, 2008
66
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?
 

SgtWookie

Joined Jul 17, 2007
22,230
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?
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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