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PostPosted: 06.08.2008 01:40 
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Location: Texas
Yes Yes, I know for most (Grin if not ALL of you out there) this is a 100% trip Back to Basics, but I would rather be 100% FOR SURE than even 1% unsure. ( I don't like getting shocked ) And even though this will not give me a jolt, it is still a good practice for me to get into...


So here goes...


I am working on yet ANOTHER project, and on http://ledcalc.com/#calc the basics go like this.

- Parallel -

9 vdc Power Supply
3.5 Voltage Drop
23 milliamps
6 LED's (Blue)

Then the numbers come up as:

39.86 ohms 47 ohm - Nearest 1 watt - recommended for Resistor

19.5 is actual Power Dissipated by LED - 68 mw Power Dissipated by Resistor - 644 mw

Then Circuit's current consumption = 117 ma



For this 6 Blue LED setup as I have it, I initially set it up on the breadboard with 1/8 or 1/4 watt 35 ohm resistor (I did this because I started out with a Voltage drop of 3.7 (so they would be brighter) and a 25 milliamp also so they could be brighter, I was not sure which one controlled it so I tried to cover both bases, was this right ??? )

Anyway, I used a 1/8th or 1/4th watt resistor and of course it started getting warm REAL QUICK, and then it got to the point of being HOT to the touch.

Now for my 2nd question... (Grin) And this is more important to me than the first one above...

- Can I use 4 - 1/4 watt resistors to "make" a 1 watt resistor, and the same goes for 2 - 1/2 watt resistors?

- If I can, do I put them in series?

I am asking this because I NEED to get this done to make sure that the resistors will be "OK" if I accidentally touch them. I have called the local Rat Shack and of course NONE of the 11 I have close me has a 1 watt 47 ohm resistor.

Dan


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PostPosted: 06.08.2008 08:06 
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You should use the Guru to calculate the resistors. Making the circuit all-parallel is not the way to go. This again reminds me to take that parallel calculation off from the ledcalc.

As you can see from the screencap, the total current draw is almost halved. This means that your battery lasts almost twice as long as with all-parallel circuit. Also, if one led burns out, the rest will not follow it as the led packs have their own resistors, thus making them independent from each others.


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PostPosted: 06.08.2008 10:25 
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I was thinking of doing it that way I really was, but as it turns out I have two sections I am dealing with. Both of these sections are somewhat far away from each other (which don't really matter) but each of the "trunks" have 3 LED's each. It is a SUPER TIGHT space that I'm working with for the housing of of it, and REALLY, I barley have room to even put the one resistor. (That was the big thing about the 1 watt resistor vs. 4 - 1/4 watt ones)

I KNOW I could still use it as in your image, but things have went WAY to far (already) to change. Any way I am not really expecting that much life out of it as it is (for now) I am jsut wanting to see how long it will last with the 9v.


Dan



Now as far as the LED CALCULATOR , PLEASE do not change it... PLEASE PLEASE PLEASE... Grin... UNLESS...
You make it where ALL or at least some of the other boxes below can be changed which would in turn change the others.

What I mean is when I put in a value of 23 milliamps (as in the example) and it comes back with
Actual single LED Current = 20 make that a variable where I go into a different mode of ledcalc and am allowed to change the number 20 into a 23 and then press a button and ledcalc would in turn change the other stuff to MAKE it be a 23 Desired LED Current. This would give some room to play with stuff so I could out MANY MANY more questions to ask you... Grin...


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PostPosted: 06.08.2008 11:02 
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If I were to make the Actual LED current as a variable as you suggest, the calculator would not be as useful. There is a reason why I give you a resistor with 100 ohms in it when the raw calculation gives a 97 ohms as a value. 97 ohms is something that you can not buy from the store. 100 ohm is a standard sized resistor.

Exact Calculated Resistance indicates what the resistor should be if you really want that 23 milliamps. It is there to give that raw value. All the other calculations after that are handled with the components that you can buy from the store.


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PostPosted: 06.08.2008 11:32 
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Joined: 26.05.2007 10:06
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Location: Texas
That does make sense to me... I do think many times when I am getting something together and I put in all the numbers "what if" this was this instead of this and so on. I do 100% TRULY love the ledcalc and honestly could not do a whole lot without it, but when it comes to the final numbers once I have entered in what I need to and I see them at the bottom, I can't help but wonder what if I were to use this resistor and not the one it chose for me. What would happen then. Yes I am sure I would have FRIED allot more things than I already have, but I like to see what effect would take place by changing one of the variables.

The ledcalc is GREAT the way it is... PLEASE don't change it... Grin... Can you imagine all the people who would be mad... (AT ME !!!) Grin...


Dan


So on a bit of a different note, the LED's Actual single LED Current is what control's the brightness of the LED right... I know ( I think anyway) that the higher it goes the brighter it is the SHORTER lifespan it has, but am not 100% for sure...


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PostPosted: 06.08.2008 12:48 
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Higher the current flowing through the led -> higher the brightness but there is a limit. Too much current will heat up the innards of the led too much and it will die. Even if you don't fry it instantly, you can shorten the life span of the led by letting it run too hot.


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