noob, led resistor
Printed From: the12volt.com
Forum Name: Lights, Neon, LEDs, HIDs
Forum Discription: Under Car Lighting, Strobe Lights, Fog Lights, Headlights, HIDs, DRL, Tail Lights, Brake Lights, Dashboard Lights, WigWag, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=126835
Printed Date: May 10, 2024 at 6:23 AM
Topic: noob, led resistor
Posted By: shirker
Subject: noob, led resistor
Date Posted: April 04, 2011 at 7:27 AM
Hi,all....first post,and its a real yawner for most of you,but I aint that bright....hence why I need the LED lights!So....
bought a pair of 24 LED (white) strips from e bay,12v,to go on my LandRover Discovery 1998 diesel.The seller has NO info on the items other than that they are 12 volt.They appear to be 3mm done LEDs in a series parallel set up encased in clear flexi plastic,with two wires hanging out.I would think they are in groups of three as thats the most you can do with whites (isnt it?),but not sure,may be 3v's in 4's....hard to tell.
Anyway,what I dont know is the wattage to limit the current to.Not having any of the specs means I cant calculate myself.I reckoned a resistor of either 470 or 560 K ohm,but unsure of the amperage to use.
I have old boards around that I salvage bits from and while I can use calculators to work out impedence I dont know how to decode the things as to amps,other than on wirewounds where its written !
The alternator will be putting out about 13 and a half or more.
Any advice on this would help me to put these in as day-running lights on the sidelight circuit.
Replies:
Posted By: oldspark
Date Posted: April 04, 2011 at 9:50 AM
They are suited to 12V. You don't need resistors. That 12V should mean typical auto 12V - ie, up to 15V or more.
LEDs really arn't that critical. Most design with 1.7V LEDs as if they are 2V LED else even ignore their voltage totally in calculations (for 12V and higher).
Although the latter (ignoring their voltage) lowers their current (increased voltage drop required hence larger resistor), it has little effect - especially brightness.
(Oops - we are talking current, not brightness, but LED currents have a reasonable variation and specs are conservative. Besides, 2V to 1.7V is like 14.1V to 12.0V - an most auto systems have a long-term max of 14.4V )
[ Please - any Nordics that read this and disagree, please shut the Fup unless you (1) understand the issue; (2) are prepared to discuss rationally, & (3) not bug out of discussions for personal reasons. ] (I think tribal English comrades understand that...)
Posted By: shirker
Date Posted: April 04, 2011 at 11:00 AM
Nice one mate ! Thanks for taking the time to reply and set that out for me,in a way that I can understand.Makes sense.Now to go wire em in.....then get back on e bay and buy some red ones for the back !
Much appreciated
shirker
Posted By: oldspark
Date Posted: April 04, 2011 at 11:27 AM
CAVEAT: Unless they are cheap or a bad "design"...
(Or I am wrong...)
But you should be okay - especially if you DO understand....
Many will use (say) six (or seven) 1.7V (2V) LEDs straight onto 12V with no resistor, or 3 (maybe 4) 3.2, 3.4, 3,6... (4V) LEDs with no resistor. And I haven't heard of problems... (Maybe they are too embarrassed & stay quiet?) But I wouldn't expect any - except maybe for sustained "high" over-voltage (say 15-16V???).
If they are cheap, so what?
If expensive - maybe just try and see....
Of course, if they blow, it must be THEIR quality - NOT MY advice!
No - hang on - it's YOUR charging system! Yeah - that'll be it...
Regards, and thanks for understanding.
And for Pete's sake - if the do go/blow, REPLY with appropriate curses and insults.
Posted By: shirker
Date Posted: April 04, 2011 at 3:15 PM
Mate,they are dirt cheapies so Im not expecting miracles here ! Wanted to try a few things out before investing in proper kit.
thanks again
PS wont hold you responsible unless it is all fantastic,then I will tell everyone it was your idea...OK?
Posted By: oldspark
Date Posted: April 04, 2011 at 5:47 PM
.... your PS... OF COURSE!
I have joined the ranks of the norm - all credit and no blame/responsibility.
(And it is truly Rank!)
It should be fine, but there is always (the possibility) of an exception...
A crap, responsibility is returning....
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