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Author Topic: led  (Read 2144 times)

hdserv

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led
« on: March 06, 2013, 11:28:50 AM »

I have some 5 volt led's from maplin do I need resistor If I conect themto 3 AA sise batteries 4.5 vols
Jim 
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Tug-Kenny

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Re: led
« Reply #1 on: March 06, 2013, 12:27:06 PM »


If it's what it says on the tin then it would alright.

I would try one with a resistor  (say 100 ohms) and see if it lights up first. If it doesn't work, or if it's weak then the 5 volts would probably be right.  They might be already fitted with a resistor as LED's only run on 2 volts.

Hope this helps

ken
 
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thething84

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Re: led
« Reply #2 on: March 06, 2013, 08:46:27 PM »

if they are 5v resisters then they will be fine on 4.5 v. just as already mentioned they will have a resistor built in. they will be a little duller than they should be due to the lower voltage.
 
What AA batts you connecting to. As IIRC AA batts are 1.2v. Therefore 3 would equal 3.6V. I would use 4 at 4.8V.
 
James
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GAZOU

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Re: led
« Reply #3 on: March 06, 2013, 08:52:16 PM »

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Howard

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Re: led
« Reply #4 on: March 06, 2013, 10:32:03 PM »

Thought LED voltage depended on colour not all are 2volt.
                   Regards Howard.
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malcolmfrary

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Re: led
« Reply #5 on: March 07, 2013, 11:07:21 AM »


If you hunt down the specifications, there are variations between types, this covers different colours and brightnesses, and the current needed to get that brightness, but generally, a raw LED will survive having its ballast resistor calculated as if it was going to be a 2 volt device.  If an LED is sold as a "5 volt" device, it should be safe to assume that it is a 2 volt LED with a built in resistor, or an equivalent bit of circuit to restrict the current to a safe value.  Testing it with an extra resistor should do no harm, and could save a minor tragedy.
There are plenty of applications about, mostly small torches, that rely on the internal resistance of the battery to provide the current limiting, and for different colour LEDs you can expect different currents for the same brightness.
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Steve Dean

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Re: led
« Reply #6 on: March 07, 2013, 08:15:27 PM »

Let me try and make this very clear for you. If you are certain that the Maplins LEDs you have are the 5 volt versions, then have a built in current limiting resistor and therefore DO NOT need an external resistor as long as you do not go over 5 volts. You can use 4 x 1.2v batteries connected in series (i.e 4.8v) or 3 x 1.5v batteries in series (i.e. 4.5). Alternatively a 4.8v receiver style battery pack will be OK.


These LEDs can also be used on a higher voltage than the 5v specified, BUT, then you will need to connect an additional current limiting resistor. The value of the resistor will be dependent on the voltage used. If you want to do this then post a note on the forum and I'll tell you how to do the simple math.


Best regards


Steve Dean



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hdserv

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Re: led
« Reply #7 on: March 08, 2013, 10:25:14 AM »

Morning Steven Dean I went to Maplins yesterday could not get 5 volt led's but bought 3 volt  led's instead
I would like to run these led's of the receiver battery your help in choosing the right resistors would be appreciated
Jim
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malcolmfrary

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Re: led
« Reply #8 on: March 08, 2013, 10:32:44 AM »

Morning Steven Dean I went to Maplins yesterday could not get 5 volt led's but bought 3 volt  led's instead
I would like to run these led's of the receiver battery your help in choosing the right resistors would be appreciated
Jim
If you tell us the maplin part number, there may be a data sheet which will help a lot, telling things like current required.
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Steve Dean

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Re: led
« Reply #9 on: March 08, 2013, 04:28:03 PM »

Agree with Malcolm, can you please tell us the Maplin part number. I've just checked on their web site and they do not list 3V LEDs ...... are you sure you don't just mean 3mm ?


Also, you say you want to run the LED(s) off a receiver battery pack. Can I just be sure that you are NOT intending to run off the same pack that is powering the receiver. This is not a good idea. Whilst a single LED will use circa 20mA ( and the more you run the higher the current), it is not good practise to have things pulling down the voltage on the RX pack. Servo's are the biggest drain of current and it is principally these that cause the receiver battery to go low. Therefore, you don't want anything else taking current. Remember that when the receiver battery voltage gets very low that some receivers (particularly on 2.4Ghz) will try and re-initialise and for several seconds you will not have control of the model. Sometimes, this condition is referred to as a 'brown-out'.


Please don't be frustrated by all this techie talk .... we'll get you sorted.


Cheers


Steve
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Bob K

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Re: led
« Reply #10 on: March 08, 2013, 04:53:01 PM »

Component Shop do a good range of LEDs for model boats.
To get all the info you need follow this . . .
 
Go to Component Shop http://www.component-shop.co.uk/
Select the menu "LEDs", then sub menu "Standard".
Click on the image of the type you want.  A pop up window appears with all its tech info.
 
Using this info (ie forward voltage and mA) go to this handy LED resistor calulator
http://led.linear1.org/1led.wiz
 
Fill in the boxes with your figures and presto it creates a circuit diagram with resistor value, wattage, even the colour bands on the resistor.
 
Simples
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hdserv

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Re: led
« Reply #11 on: March 10, 2013, 11:44:57 AM »

Hi all had a look at Maplin's data sheet the green and white led's have a max forward voltage of 3.5 volts the red led  have a max forward voltage of 2volts
What would be the best way to connect these led's using a receiver type battery
that you for looking at my post and answering them
Jim
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Steve Dean

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Re: led
« Reply #12 on: March 10, 2013, 12:25:05 PM »

Hi Jim,


If we presume when you state 'a receiver battery pack' you mean a standard 4.8V pack then the resistors you need are:


For the LEDs with a 3.5 forward voltage the answer is 65 ohm ...... suggest you use a standard 100 ohm (Brown/Black/Brown).


For the LEDs with 2 forward voltage the answer is 140 ohm ..... suggest you use a standard 150 ohm (Brown/Green/Brown).


It is normal to use the next standard size up when selecting the resistor. You may well find that the LEDs are too bright .... if so use a higher value resistor. This is particularly the case with ultra bright LEDs.


The formula for calculating the resistor value is:


R (Resistance) = (Supply Voltage - Forward Voltage Drop of the LED) / Forward Current of the LED


Therefore using a 4.8V supply, a Forward Voltage Drop of 3.5V and a typical Forward Current of 20mA the equation would look as follows:


R = (4.8 - 3.5) / 0.02


Therefore:


R = 1.3 / 0.02


Therefor:


R = 65


As 65 ohm is not a standard value then we increase it to the next higher standard value .... which is 68 or 100 ohm.


As the previous respondent stated you can go on the Component Shop web site, where there is a simple calculator to do this for you if you don't fancy doing the math yourself. Component Shop do LEDs at very good prices and do most of the model boat shows.


Pleased to help you.


Steve.


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hdserv

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Re: led
« Reply #13 on: March 10, 2013, 12:40:43 PM »

thank you steven
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malcolmfrary

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Re: led
« Reply #14 on: March 10, 2013, 07:35:12 PM »

The other good reason for using a higher value resistor is that the supply voltage might well be higher - a freshly charged 4.8 volt pack could well offer over 5.5, and if RX power is from a BEC, its going to be regulated at 5 volts.
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More Coffee

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Re: led
« Reply #15 on: March 15, 2013, 11:25:52 PM »

A volt above with a resistor on a led wont hurt ,as long as the resistor your using isn't running the led to it max potential. which most calculators don't... Ive run leds  up to 18 and 27 volt with a 470ohm resistor ,on its not proper ,the resistor gets really hot.. but it can be done
So don't get to paranoid or concerned,unless a person has intentionally ran a led out for maximum brightness and shortest life,for something like a spot light
a led can actually run to its max potential and a little beyond ,depending on the duty cycle.
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