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Author Topic: what size resistors  (Read 5516 times)

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Re: what size resistors
« Reply #25 on: April 09, 2014, 03:27:44 AM »

The subject of the resistor value to use with LEDs seems to be a recurring topic despite the fact that I've written two articles in Marine Modelling International on this topic and another contributor has an article in the current issue. OK, so some folks don't buy magazines so let's make this real easy for those people who are terrified by some very simple math.


No need to get all bogged down in Ohms Law and lots of techie stuff that people like myself and Dave at Action deal with every day.


Here's the simple way. The important aspect is the forward voltage of the LED. This varies with the colour. For most red LEDs this typically 2 Volts and most other colours it's between 2.4 and 3.3 Volts.


Now here's a really simple formula:


(Supply Voltage - Forward Voltage) x 50


Therefore if we are using a 12 Volt battery and a Red LED with a voltage drop of 2 Volts, then the formula looks like this:


(12 - 2) x 50


As the first part of the formula is in brackets, then you do that bit first, i.e. 12 - 2  = 10. Then multiply the answer by 50. i.e. 10 x 50 = 500


Therefore you need a 500 Ohm resistor. As this is not a preferred value, then you use a preferred value that is the next value up. This is 510 Ohms.
You will quickly learn that LEDs are very bright, so therefore use a higher value resistor, something like 680 or 1K (1000) Ohms.


To help you get the hang of the formula we'll change the battery to 6 Volts. Then we get the following:


(6 - 2) x 50


Which is 6 minus 2, i.e. 4. Then multiply by 50 and the answer is 200. To stop the LED being too bright use something like 270 Ohms or even higher.


Next time you're at a show pick up a Component Shop catalogue and you will find this formula and some examples in the back of the catalogue.
The catalogue is FREE and their LED prices are very competitive.


Hope this helps you all.


Steve.

On the same note, leds maintain their colour . meaning where an incandescent light will fade and turn yellow because its a heated filament, leds as you know are semi-conductors built around parameters, a guy wont be able to produce that warm mast light look unless he chose's warm white or amber leds.
Just thought Id throw that in ,just in case the ext question is yellowing and ultra bright white led, a guy may have to put varnish on it and build it up ,maybe even sand the outside of the led to give it some hue ,if its a clear led.

dosent make sense ...does it.. :-X
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Geoff C

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Re: what size resistors
« Reply #26 on: April 09, 2014, 09:58:02 AM »

However, We seem to have drifted off resistors for smoke units and gone to resistors for leds, as so often happens.     Thanks for all the relevant smoke info, it's much appreciated.      If and when I put leds in my smoke unit I have all the info I need, many thanks indeed.        Regarding lights, has no-one heard of grain of wheat/rice bulbs yet?      I use 6volt bulbs powered by 4.8volts ( 4 AA batteries )  and the light is perfect and they'll never blow.      How many tug models have we seen with masthead and side lights as bright as searchlights ?     Too many I fear.     best regards, Geoff.
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Geoff C

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Re: what size resistors
« Reply #27 on: April 09, 2014, 12:15:47 PM »

6volt bulbs on 4.8volt battery pack,   regards,  Geoff.
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Re: what size resistors
« Reply #28 on: April 09, 2014, 02:51:07 PM »

Yeah it happens {:-{

have to remember though, conversations do run along.

In regards to resistors for a smoke unit.

while resistor's are handy,

I mentioned before ,about using a full wave bridge rectifier to run a fan the  of an ESC.

I mentioned getting that part from an old hair dryer ,

when your done scrapping that hair dryer ,

you will be left with a bunch of bits , a switch,a thermal switch, motor and ducted fan, and the heating element wire for the hair dryer. nichrome wire..
A guy can experiment with what length of wire will produce the smoke he wants.

then its just a matter of measuring the resistance of that length of wire ,to make replacements.

simple..all a guy needs is an old hair dryer to begin bodging  a smoke unit 

http://en.wikipedia.org/wiki/Nichrome

grain of wheat bulbs are incandescent .there life span and current requirements

like any incandescent bulb ,everytime you turn it on you risk blowing it ,

Leds are solid-state and low current requirements ,can last 20years, and more than 500,000 power on cycles, can run upto 750,000hrs.

a guy could easily get this out of and incandescent bulb. Just don't cycle it of or on.

I have a Compact flouresent (different beast I know)bulb on my back porch ,13watt ,it been there 6years , I don't turn it of , I vee replaced every other bulb from that same package ,except for that one, no cycles no wear and tear .
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Re: what size resistors
« Reply #29 on: April 09, 2014, 02:58:47 PM »

The thread actually started off about LED's and somehow drifted onto smoke units !!
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Re: what size resistors
« Reply #30 on: April 09, 2014, 03:00:53 PM »

But ,you may want to put a power indicator led on your smoke unit :-))

Its all good .
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Re: what size resistors
« Reply #31 on: April 09, 2014, 03:43:42 PM »

Still looking for Red and Green oil for my old steam tug
running lights.
Used to get Aladdin Pink, Regent Green .
Police and lifeboats used ESSO Blue.   %% %%

Ned
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Geoff C

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Re: what size resistors
« Reply #32 on: April 09, 2014, 05:57:39 PM »

Thanks everyone for all your comments, kind regards, Geoff.
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Re: what size resistors
« Reply #33 on: April 09, 2014, 06:29:38 PM »

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