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Author Topic: LEDs & resistors  (Read 3870 times)

Turbulent

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LEDs & resistors
« on: December 31, 2009, 02:03:19 PM »

Need a bit of help here guys.

I'm wiring up some LEDs to a 12v system but need to fit resistors.

Q1: what type of resistor do I need to Buy - the LED's are 6v.

Q2: can I fit one resistor to the wiring loop before the first LED or do I need to fit one to each LED.

Cheers.

Mark47

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Re: LEDs & resistors
« Reply #1 on: December 31, 2009, 02:11:30 PM »

This site will help with resistors. http://ledcalc.com/

Although you will find plenty more on the web.

Q2, yes you can, but something from memory makes me think If one led blows the lot will blow also. (Don't quote me on that). Someone more knowladgable will keep you right. O0
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boatmadman

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Re: LEDs & resistors
« Reply #2 on: December 31, 2009, 02:24:14 PM »

another LED calculator that outputs a wiring diag for you:

http://led.linear1.org/led.wiz
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Rex Hunt

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Re: LEDs & resistors
« Reply #3 on: December 31, 2009, 02:28:04 PM »

another LED calculator that outputs a wiring diag for you:

http://led.linear1.org/led.wiz

If you use this site it will advise you as to the power rating of the resistors, and gives the colour codes to help you fit the right value in the right place.


 :-))


Rex
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FullLeatherJacket

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Re: LEDs & resistors
« Reply #4 on: December 31, 2009, 03:15:00 PM »

That is truly brilliant!! Many thanks.
FLJ
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Turbulent

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Re: LEDs & resistors
« Reply #5 on: December 31, 2009, 03:19:11 PM »

Cheers men. I knew you'd not let me down.
Maths done & resistors ordered. :-)) :-)) :-))

Subculture

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Re: LEDs & resistors
« Reply #6 on: December 31, 2009, 03:32:25 PM »

What sort of LED's are those?

Most LED's drop no more than 2-3.5 volts across them, depending on colour and type.

Be aware that a freshly charged 12 volt battery can give over 14 volts. If you're only driving a few low wattage LED's why not just wire them into the 5 volt supply from your receiver, that way the voltage is regulated.

Damien

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Re: LEDs & resistors
« Reply #7 on: January 01, 2010, 09:10:30 AM »

What sort of LED's are those?

Most LED's drop no more than 2-3.5 volts across them, depending on colour and type.

Be aware that a freshly charged 12 volt battery can give over 14 volts. If you're only driving a few low wattage LED's why not just wire them into the 5 volt supply from your receiver, that way the voltage is regulated.

Subculture,
The spec's of the LED's will state the forward voltage, and the recommended current eg super bright Led typically 3.6v current 20mA above 30mA can kill the led.
The resistors function is to limit the current drawn to within acceptable limits, connecting directly to the 4.8v - 6v Rx pack there is nothing to limit the current
allowing the Led to draw whatever current it wants, at best this will shorten the life of the Led at worst it will let the smoke escape from the Led.

The Maths to work the restor valuu is
Battery volts minus Vf divided by current  eg; 12v - 3.6v = 8.4vdivided by .02A = 420 ohms the nearest resistor value would be 390 ohms or on the conservative side 470 ohms.
Damien.
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Subculture

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Re: LEDs & resistors
« Reply #8 on: January 01, 2010, 11:19:09 AM »

I was referring to a regulated voltage, the type that comes from a BEC. If you calculate a dropping resistor for 12 volts, and the battery is putting out 14 volts, the LED will be sinking more current than intended and this may cause problems.

Also worthwhile to avoid assuming that LED's sink 20-30mA of current. Most 3mm or 5mm packaged devices do, but I have a couple of very bright LED's that draw about 3A at 3.5 volts (Seoul P7) and put out the luminance equivalent to a 50W halogen lamp.

boatmadman

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Re: LEDs & resistors
« Reply #9 on: January 01, 2010, 11:38:30 AM »

I am led ( :}) to believe that if you overcurrent a LED it will shorten its life, conversly, if you give it a slightly higher resistor to limit the currewnt, life will be extended, but, at the cost of a reduction in brightness.

Ian
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Circlip

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Re: LEDs & resistors
« Reply #10 on: January 01, 2010, 11:48:06 AM »

Don't know if these people will supply any of the fancy timing chips, but the single LED flasher units are what are fitted to More Reasons free "Flashy" signs. 12 units per sign, thanks uncle Ken.

  http://www.rocketproduction.co.uk/about.aspx
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Subculture

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Re: LEDs & resistors
« Reply #11 on: January 01, 2010, 12:28:23 PM »

I am led ( :}) to believe that if you overcurrent a LED it will shorten its life, conversly, if you give it a slightly higher resistor to limit the currewnt, life will be extended, but, at the cost of a reduction in brightness.

Ian

Yes. Although unless you are very unlucky indeed, an LED used in a model should long outlast it's owner in lifespan terms.

malcolmfrary

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Re: LEDs & resistors
« Reply #12 on: January 01, 2010, 02:08:39 PM »

Exceeding the maximum current will result in early replacement.  Best thing is to do the arithmetic and keep the current below the max.  LEDs that are quoted as "6v" or "12v" usually are normal 2 volt or thereabouts types with the appropriate resistor built in.  To run a 6 volt one off 12 volts it will need another external resistor to lose the excess voltage.  A large collection will need one resistor each OR a voltage regulator to calm things down.  A 5 volt BEC would do nicely for quite a large collection of 6 volt LEDs, giving a small reduction in light output (not usually a bad thing on a model, anyway) and an infinitely extended life-span.
The BEC in this case acts like a self adjusting resistor for the whole system that it is supplying.
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Turbulent

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Re: LEDs & resistors
« Reply #13 on: January 01, 2010, 02:14:50 PM »

Exceeding the maximum current will result in early replacement.  Best thing is to do the arithmetic and keep the current below the max.  LEDs that are quoted as "6v" or "12v" usually are normal 2 volt or thereabouts types with the appropriate resistor built in.  To run a 6 volt one off 12 volts it will need another external resistor to lose the excess voltage.  A large collection will need one resistor each OR a voltage regulator to calm things down.  A 5 volt BEC would do nicely for quite a large collection of 6 volt LEDs, giving a small reduction in light output (not usually a bad thing on a model, anyway) and an infinitely extended life-span.
The BEC in this case acts like a self adjusting resistor for the whole system that it is supplying.

That sounds like aplan, I'm putting them in a bait boat to replace existing coloured - not very bright 12v ones, the replacements are very bright clear ones & as there are 2 ESC with BECs inside the boat this would seem like a reasonable place to run them from - 3 off of each ESC.

SteamboatPhil

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Re: LEDs & resistors
« Reply #14 on: January 02, 2010, 10:21:28 PM »

Component shop have a helpful leaflet on LED's and resistors.

Ane we all know what LED stands for don't we

LITTLE EEKY DOT

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Damien

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Re: LEDs & resistors
« Reply #15 on: January 04, 2010, 08:25:14 AM »

The regulated voltage from a BEC will be 5 or 6 volt the current is not regulated the voltage regulator will have a current rating to cover the Reciever & a max of 6 to 8 servos a boat with 2 03 servos will probably accomodate a few Led's, resistors will still be needed. Following the formula 5v supply will need a 68r and 6v will need a 120r to ensure the longevity of the Led's

Perhaps Mr Full leather jacket can tell us what the current limit for the Bec on the Action electronics ESC's is, it certainly won't the 15+ Amps available to the motor we'll then have a means of approximating how many servos & Led's we might be able to use this way.

Damien.  

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FullLeatherJacket

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Re: LEDs & resistors
« Reply #16 on: January 04, 2010, 10:15:20 AM »

Perhaps Mr Full leather jacket can tell us what the current limit for the Bec on the Action electronics ESC's is, it certainly won't the 15+ Amps available to the motor we'll then have a means of approximating how many servos & Led's we might be able to use this way.

We don't fit BEC regulators to our ESCs, Damien. The separate P19 BEC and P92 Power D/B 5v regulators are rated at 1A continuous, while the 6v versions are 1.5A. I'm not convinced that the very thin copper tracks in some receivers will actually be able to handle up to 1 Amp so I'd recommend using  a P19 unit, powered from the main 12v battery and running the LED circuit(s) independently of the Rx. "Belt-and-braces" job.

Just a thought; suit yourselves.

FLJ
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Damien

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Re: LEDs & resistors
« Reply #17 on: January 04, 2010, 11:00:15 AM »

Thanks FLJ, I hadn't considered that consequence & confirms my belief that it's risky to use the Bec to power anything other than the Rx,
As FLJ says it's up to each person to make the individual choice.

Damien
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