Somebody Please Help

[ukazn]

I am planning on using some blue L.E.D's in my car (For my boot install).

I saw some in maplins that are 5V but i've been told that you need to use resistors on them.

Here is a link to the L.E.D's that i was thinking of using:

http://www.maplin.co.uk/products/module.as...2703&modulecode

1. What kind of resistors would i need to use?

2. Can anyone tell me where i can get 12V L.E.D's in a similar size.

[Tommo]

Think its a 4kohm resistor your looking for, If my maths is any good

0.003A x 4000= 12v. I'll check it out tomorrow.

You could always try Radio spares (R.S) for 12v leds

[Maz]

Hiya Ukazn, have you and Skippy got any Pics on you guys Sound install? cause i would like to see how you guys done to your boots and in the Dash, im goin to get SQ + to do my install, but is looking into different way of doing it, i want install but i still want a big boots too( asking alot i know :D :D ), but i use car for work too, so cant have a boots install to big, but im goin to have a Alpine F1 screen and the CDA7998A CD head unit too ( ordered the double din Facia from US btw thank Ukazn and Skippy for the information on this), but im still not sure what Amp to use, also goin to add Dvd changer , Cd changer, maybe 2nd Battery, Cap,also wanted some Neon under car kit and neon in boots too, any ideas want make should i go for guys? any ideas is welcome, i had my 323F installed by SQ + before any i loved it, so hopefully i can do the same with IS200 too,.... :D :D :D

[Mike246]

Nope. It doesn't work that way. Trust me, I'm a consultant!

LEDs are current-driven so don't worry about voltage across the LED. The forward current is 20mA from a 14 volt source. Mr Ohm says R = V/I therefore you need a series resistor of 700 (use 680 or 750) ohms. Power rating needs to be 0.5 watt.

[Geoffers]

Yep - Mike is dead right on this one. 4k would be way too high.

30mA is .03 not .003 which I think is where you got the 4k from, but that was based on 30mA which is the max rating for that LED. I would run it at 20mA

Not sure if you're using the diffused or clear, but 40/90mcd isn't very bright. If you want brighter they have them on the site.

PS. Mike - the Quanan sensors are fantastic.

[Mike246]

Thanks Geoffers for your confirmation.

Also for the compliment on the QUANAN - the only customers we want are satisfied ones!

[Tommo]

Sorry guys,

I was way out. Looked it up this morning fomula is:-

Rl = Vs-Vf/If therefore 12 - 5/0.025 = 280ohm resistor

using 0.025 instead of 0.03 so your not running max power all the time.

The wattage size depends on how many leds your using,

1 LED =0.175watt

10 LEDs=1.75watt

As Geoffers said there are much brighter leds in Maplins, at 40mcd you might need quite a few.

It was getting late last night sorry for error

[Geoffers]

In theory yes, but it doesn't strictly apply because you can't take into account the forward voltage drop. Unfortunately, it doesn't work like that - it's a common misconception with LEDs. You need to limit the current (Which is why you can't put several in series like xmas tree lights)

If you used a 280ohm resistor you would end up with a DED not an LED! (The magic smoke would get out - all things work on magic smoke. Once it's got out, things don't work anymore.) Try it, but wear protective goggles!!

680ohm, .5watt would get the best out of those particular LEDs

Sorry - it sounds as if I'm trying to prove you wrong here - not at all, it's just that I can guarantee the LEDs won't last long with a 280ohm resistor at around 14v, and Ukazn will be disappointed on the project and wonder where he went wrong.

[Tommo]

Hi Geoffers,

Not that well up on my electronics so i looked it up this morning in the Maplins catalogue, thats where i got this formula. was told that 12v minus the 5v would give you the 7v to work from. Also told the resistor would be put in line before the leds where fed in parallel. Is this wrong?

Would be nice to know, this ones bugging me

[ukazn]

Thanks very much peeps. Once I've finished my project there will be pictures for all to see.

[Geoffers]

Hmmm. Bit of a hard one to explain this.

When working with LEDs you should always work by the current, not the voltage. Take your supply voltage and divide by the current to be drawn.

For years I've seen the equation: R = ( Source Voltage - LED Voltage Drop ) / Amps, but it simply doesn't work long term. mainly because these are ABSOLUTE MAXIMUM ratings.

LEDs do not have resistance like a resistor does. LEDs have a dynamic resistance, that is their resistance changes depending on how much current passes through them. But it's easiest to think of them as having NO resistance. This means that if you just connect an LED to a Battery, you'll have a short circuit. (Well not exactly but I'm trying to explain it here)

Let me think of a better way to explain it and I'll re-post.

Mike - help me out on this one - I'm not explaining it very well at all!!

[Mike246]

Right. Now pay attention, Bond.

An LED is a diode that emits light. It is driven by current and the value of this depends on the chemical construction, size and colour. All of these also affect the voltage across the device which will be a set value between 1.6 and 4.2 volts. The LED will be a low resistance when forward-biased (lit). It will be knackered if reverse-biased (connected round the wrong way).

LEDs can be connected in series. They can also be connected in parallel but a current-limiting resistor must be in series with each LED.

All of the above is interesting but not important. The object is to get the current right. Too little and the LED will be dim; too much and it will be bright and burn out. A car's source voltage is 14.4 from the alternator and you need to drive it at 20mA, therefore you need 680 ohms in series with each.

[Geoffers]

Cheers Mike, just couldn't put it clearly!

To save confusion with others - I only made the comment about not connecting in series (a lot of them anyway) because with no series resistor only a very tiny variation in voltage causes a fairly sharp change in current. Also, (and less important) with the different make up between batches, etc, you tend to get the odd one or two that end up with noticebly different brightnesses.

If you connect 2 or more LEDs in parallel (without limiting resistors), the one with the lowest voltage drop hogs all the current, and it will overheat and fail prematurely. If you have a lot of LEDs in parallel, then the amount of current that the lowest LED hog will be even greater, with consequent overheating and failure. (You loose the magic smoke.)

[Monster-Mat]

where can i buy some magic smoke

[Mike246]

If it's the stuff I'm thinking of, there's a bloke round the corner to Tooting Station sits in a chromed-up 5-series. He'll sell you all you want.

[Tommo]

Everydays a school day, Think I've got my head round it at last

Cheers guys, Be nice to see some pics when these LEDs are fitted.

Just off to Tooting now