ezzysi
Posted a lot
Posts: 1,189
|
|
Oct 18, 2009 10:36:31 GMT
|
Sorry to ask this on a sunday morning but i'm hoping there's someone on here that understands the dark arts of electronickery..... I've bought some led bulbs that are rated at 12v but I'm using them on a 14v supply and want to wire a resistor in the feed wire to stop them burning out.. The bulbs have 6 led's in each and draw about 20ma I'm guessing?? Have tried to find the answer myself but mind is getting puddled. In payment for this favour I present some sunday morning rat pic'age   ![]() <a href="http://media.photobucket.com/image/ratrod/jmefcrooks/ratrod.jpg?o=1" target="_blank"><img src="http://i224.photobucket.com/albums/dd200/jmefcrooks/ratrod.jpg" border="0"></a>[/img]   |
| |
1991 Mk2 Golf Gti 8v
2005 Passat tdi (daily)
1971 Mk1 Escort
2004 Touran (her's)
|
|
|
|
wez
Part of things
Ratty Fords FTW
Posts: 617
|
|
Oct 18, 2009 10:43:07 GMT
|
hmm, is this not ohms law thingy, i recall from school electronics lessons. have you searched online for an ohms law calculator - you can prob just put in the requiered numbers and out pops your resistor colour code  SAying that, when i built my camper van, i installed some 12v LED downlighter bulbs (They look like halogen ones from a house but have loads of LEDs in them insteed) The bulbs were 12v, but they've been rigged up in my camper for bout a year now, and that pushed about 14.6v when engine running and i've had no probs at all, and theyve been on for hours at a time |
| |
Maximum signature image height = 80 pixels
|
|
rysz
Club Retro Rides Member
Posts: 2,554
|
|
Oct 18, 2009 10:53:04 GMT
|
You need a voltage divider. This is two resistors in series from live to Earth, then from the junction of the 2 resistors, you take your 12V. See here: en.wikipedia.org/wiki/Voltage_dividerHope that this is of some guidance! Rysz. |
| |
|
|
wez
Part of things
Ratty Fords FTW
Posts: 617
|
|
Oct 18, 2009 10:53:41 GMT
|
|
Just worked it out I think, a 200ohm resistor is what you require... Red Black Brown Gold is the colour, if you need one at all, like i say my camper 12v lights are fine on over 14v
|
| |
Last Edit: Oct 18, 2009 10:57:38 GMT by wez
Maximum signature image height = 80 pixels
|
|
|
|
|
Oct 18, 2009 10:55:52 GMT
|
|
I have to say that I wouldn't bother. If they're rated for 12v then that means they're probably intended for some sort of automotive application, and all "12v" car systems nominally run upwards of 14v all the time anyway. I would be willing to bet that the bulbs can handle it just fine.
|
| |
|
|
|
|
|
Oct 18, 2009 13:18:21 GMT
|
|
Resistance = Potential Difference (Voltage) / Current
|
| |
|
|
ezzysi
Posted a lot
Posts: 1,189
|
|
Oct 18, 2009 13:18:43 GMT
|
I have to say that I wouldn't bother. If they're rated for 12v then that means they're probably intended for some sort of automotive application, and all "12v" car systems nominally run upwards of 14v all the time anyway. I would be willing to bet that the bulbs can handle it just fine. That was my thoughts tbh, but after doing some looking around another forum (passatworld) there all finding that they fail after a few months.... There is a supplier that does 14.4v ones specifically for this problem but there 10x the price!!!! |
| |
1991 Mk2 Golf Gti 8v
2005 Passat tdi (daily)
1971 Mk1 Escort
2004 Touran (her's)
|
|
bxer
Part of things
Posts: 457
|
|
Oct 18, 2009 14:45:11 GMT
|
|
You'll want a resistor in series first off, that's the usual way with LEDs
When two things are in series, same current goes through both.
So... you're starting with 14v, you want 12v for your LEDs, that leaves 2v for the resistor to drop.
Knowing (or guessing) 20mA, aka 20/1000 A aka 0.02 A, apply Ohm's law which in one form is R = V / I:
R = 2 / 0.02
You need a 100 ohm resistor.
|
| |
|
|
|
|
|
Oct 18, 2009 16:48:26 GMT
|
|
^What bxer said.
However, make sure that you know what the current is - a guess of 20mA isn't really good enough for the resistor value. It's worth looking it up with the manufacturers or suppliers.
|
| |
|
|
kabman
Part of things
Posts: 348 
|
|
Oct 18, 2009 16:58:06 GMT
|
|
You need a resistor in series that has one sixth the resistance of the LED.
|
| |
|
|
|
|
|
|
|
Oct 18, 2009 18:31:44 GMT
|
You need a resistor in series that has one sixth the resistance of the LED. Not that simple, I'm afraid - LEDs drop a certain amount of voltage and have an equivalent series resistance. V_drop + (I x R_equiv) = Voltage across LED. Without knowing the current draw there's not much to do - you could measure the current when the LED is connected to a know 12V source though. |
| |
|
|
ezzysi
Posted a lot
Posts: 1,189
|
|
Oct 18, 2009 20:12:58 GMT
|
Muchos thankyous for the reply's, I knew someone on here would know. Makes perfect sense now bxer, I'd confused myself with all the weird and wonderful formulae i'd found on the t'interweb  I'll run 12v through it when it arrives and measure the current draw to be safe. Cheers all ;D |
| |
1991 Mk2 Golf Gti 8v
2005 Passat tdi (daily)
1971 Mk1 Escort
2004 Touran (her's)
|
|
kabman
Part of things
Posts: 348
|
|
Oct 19, 2009 20:55:30 GMT
|
You need a resistor in series that has one sixth the resistance of the LED. Not that simple, I'm afraid But it is that simple  The LED and the resistor are in series so the current through each is the same and the voltage across each is proportional to it's resistance. If you want six times the voltage across the LED as across the resistor (ie 12V and 2V for a total of 14V) it needs to have six times the resistance. |
| |
|
|
|
|
|
Oct 20, 2009 16:31:14 GMT
|
|
But the resistance of a diode can't just be measured - it varies depending on the operating conditions.
|
| |
|
|
|
|
|
Oct 20, 2009 17:40:43 GMT
|
|
Those 12V LED's already have an inbuilt resistor that performs two functions- 1- to bring 12V across the legs down to the 3V required by the diode chip inside to operate without becoming toast, and 2- to limit the resistance somewhat- it won't draw more than 40mA. They are designed with automotives in mind.
I ran plain-jane Maplin "12V" LED's for years in a car that ran at 14.5V with no problems whatsoever (in fact, they got fitted in 1997, and the car is going to the scrapper with them still in, still working) Just don't connect them backwards, as the cheap ones are rated only to about 8V reverse voltage and go PHUT with an impressive little flash with 12V across them. Most of those ones you can't hook up backwards or have protection.
--Phil
|
| |
Last Edit: Oct 20, 2009 17:45:53 GMT by PhilA
|
|
