GabiOsz

Hi,

I have made a silly mistake and need a little help!

Basically, I have tried soldering normal 5mm red LEDs into the track then cutting and soldering it so that I have 4 bulbs in series in 6 groups and no resistors. I thought that this would work, but having run a 12v feed they have all fried, short of buying a new light, how can I remedy my error?

I had tried 12v LEDs but these proved more unreliable than the original bulbs, the small in-line resistors were prone to snapping.

Ideally I'd like to solder new LEDs with the correct resistance but am not very good at working the resistance out.

Any suggestions gladly appriciated.

brucec59

Depends on the LEDs. Most high-brightness LEDs are 3.6V. Four in series should have worked. But some are 1.2V, as are most generic LEDs. Maybe that's what you used, and that's why they fried? Best to test them out before putting them in the housing. At least test a group of 4. Short of buying a new light, I'd say give it another go, but just be sure what kind of LEDs you're buying and what voltage they require.

Disclaimer: I've never put LEDs in a 993 light bar; I just happen to know a thing or two about electronics.

GabiOsz

Bruce, you are a star, I can solder and all that, but i glaze over when it comes to the mathmatics of resistancevalues, etc. I'm going to give it another go!

do these look ok?

http://cgi.ebay.co.uk/30x-5mm-Red-50...QQcmdZViewItem

leeshephard

just use this site to calculate what resistors you need:

http://ourworld.compuserve.com/homep...Bowden/led.htm

its pretty simple to use!

regards

Lee

TwinDaddy

You definitely need resistors. You can use the calculator that Lee mentioned, but calculate the resistor value for each series group of four, not for the whole array at once. Then use a separate resistor dedicated to each group. That allows for slight variations in the voltage drops amongst the groups, which is inevitable in the real world. Puts less stress on the system and makes it more reliable.

GabiOsz

If i have 3.2v ~ 3.6v LEDs instead of the 2v ones, 4 in series makes 12.8v +, would that work without a resistor?

cowtown

At a minimum, you want a resistor in each serial string of LEDs to limit current, so the they don't overheat and fry. LEDs will use more current the hotter they get, happily self destructing. Remember nominal voltage with the car runining is ~14V - 14.5V and pick resistors accordingly to limit current. They will be dim when tested with the engine off and brighter when it's on.

Better yet, use a switched power supply or at least an LM317 or LM350 adjustable regulator configured for current limiting. It's not much more work than using a handful of resistors, and much more predictable. This gives you consistent safe current levels whether the car's running or not. That's what I did for my rear light bar, coupled with high brightness LEDs.

Note also that red/orange LEDs that match the lens in front of them are the ones to use for maximum light output.

GabiOsz

Ok, I'm as thick as pig s*it when it comes to electronics, and the resistance calculator befuddles me. Here are the specs of the LEDs I've just ordered, can someone please tell me which resistors to get to wire (1) into each series?

Emitted Colour Red

Size (mm) 5 mm

Lens Colour Water Clear

Forward Voltage (V) < = 3.2 ~ 3.6

Reverse Current (uA) < = 30

Luminous Intensity Typ Iv (mcd) 4000 ~ 5000

Absolute Maximum Ratings ( Ta = 25°C )

Max Power Dissipation PM = 80mw

Max Continuous Forward Current IFP = 75mA

Reverse Voltage 5 ~ 6 V

Lead Soldering Temperature 240°C ( < 5 S )

Operating Temperature Range -25°C ~ +85°C

Preservative Temperature Range -30°C ~ +100°C


Thank you ever so much

cowtown

Assuming you have 14.5V and conservatively running your LEDs at 3.4V, 60ma each, you can set them up in serial strings of 4 with a 15ohm resistor. 1/4watt resistors are fine.

If you want to shoot for maximum light output (and risk short lifespan), the way to go would be strings of 3 LEDs with 51ohms of resistance on each string. Use 1/2 watt resistors here so you don't start a fire. This configuration would run each LED at 3.6V and 70ma, and I don't recommend it.

You really should measure your engine-on voltage and design accordingly. You want to run them as bright as you can within specs. If your system voltage is actually 14V, your design would change a bit.

All these problems are avoided by using an LM317 voltage regulator and 2 resistors to power all LEDS.

GabiOsz

Thanks very much, I think I'll play it safe if I go down the resistor route, but I'm going to look at the regulator, which resistors would I need with that?

I will let you know how I go.

GabiOsz

Would this regulator work? and how would you wire it in?

http://cgi.ebay.co.uk/Adj-Voltage-Re...QQcmdZViewItem

leeshephard

Gabi, just go to a local Maplin shop and buy it ...no need for ebay, and that looks expensive as well once you add P+P

or check this link:

http://www.maplin.co.uk/Module.aspx?...ID=1&source=1&

cheers

GabiOsz

Will do thanks for the tip, but I'm not quite sure how I'd wire it in. Bear in mind I'm stuck with 6 sets of 4 LEDs in series, where would the regulator fit into this and which legs do I wire? Sorry for the stupid questions.

David Schiff

another thought
no need for resitance measurments etc etc.
go to SuperbrightLEDs .com
pick your 12vleds . i have string of reds[ they come in a string] in the panel overhang for the airplane for nite work, and i illuminated all the cabinets, icebox and under berth storage on the boat with these. they also have the little bannana bulbs, double ended electrodes, for the interior light,
no afil yayabla bal
david

ACEparts_com

You need some led's with build in resistors so that they're a direct replacement. This is what i used. http://www.aceparts.com/all-led-bulb...ightness-1.htm

They make an enormous difference!

I was going to produce an LED pcb replacement but figured there wouldn't be much demand