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User Rank
Iron
Re: Go John!
10/22/2012 5:16:19 PM
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@John, regarding the resistor power dissipation:

You didn't include a schematic of your light, so I had to infer from your math bit.

It appears that you are wiring the three 10W LEDs in series, and estimating 9.5V for the set, or ~3.2V each. That sounds correct, so far.

Powering that off of 12V, you need to drop (12 - 9.5) or 2.5V, as you showed.

Then, you place the LEDs in series with a power resistor, and switched 12V battery.

If all the above is true, then the (composite) resistor will see a 2.5V drop, and pass 2.5A. P=IE, for 6.25 Watts total dissipated, or ~1.6W per resistor.

If the 40W of resistor was getting to 200°C, then there it seems like must have been more than 3A in the circuit. If you were dissipating 40 Watts in 1 Ohm, then: P = I^2R, and I = sqrt(40), or 2 sqrt(10), or 6.3 Amps.

+ o------/\/\/\----->|---->|---->|------o -  Right?

Dave

User Rank
Iron
Re: Nice work John
10/22/2012 5:08:45 PM
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Thanks John, I've just checked it out.  Did you notice that they only have line voltage drivers for them?  Trust me on this, once you start playing with constant current switching power supplies, you'll never look back at those burning hot power hungry resistors and linear regulators.  ...and in spite of what I said earlier, do take  your time and get a proper BSEE from an accredited university.  There are all too many people out there who would take advantage of your ingenuity to the detriment of your youth and education.  Best, -Phil

User Rank
Iron
Re: Go John!
10/22/2012 5:00:17 PM
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@Rudy,

I'm sorry but you're way off.  You're thinking in terms of a fixed voltage.  An n watt resistor is rated for n watts no matter what's next to it (except for a heat gun.)

User Rank
Silver
Re: Go John!
10/22/2012 3:58:24 PM
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Eddy, yes I really do enjoy making these kinds of things, even before I knew about Gadget Freak.  For my next one, I'm thinking either audio or integrated computing.

User Rank
Silver
Re: Nice work John
10/22/2012 3:51:28 PM
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Also, laserdudephil, I would recommend looking at DX.com, as they have some even higher power LEDs, one of them 150W.  I'm actually looking to get one of those myself, to make a miniature spotlight.  Problem is, I have to find some even stronger goggles first, and a way to keep it cool.

User Rank
Silver
Re: Go John!
10/22/2012 3:47:44 PM
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Thanks for all the feedback, I do have one question about engineering jobs and such though, specifically, what kind of jobs are there in building stuff, and what kind of college majors exist for jobs like that?  I have recently started looking into colleges and such, and am not sure what to major in (I still have two years, of course, but it doesn't hurt to look ahead).

As for the debate over resistors, yes, more resistors means more power dissipated.  It has to dissipate about 30-35W (2.5-3A@12V).  Even though the LEDs drop the voltage, the ammout of power dissipated is still big, as when I had 40W resistors (4x10W), they got to about 200C, at which point it started melting the plastic around them.  Also, yes, that does dissipate 40W.  If you think about it, then it makes sense that the whole resistor block, be it 90W or 10W will always have 3A through it.  Since each resistor (for the 4x20W configuration) will contain 1/2 of the ammount of resistive material between the two ends, each will recieve 1.5A@12V.  If you put two in series, then each only needs to sink 1/2 the voltage.  It is then 1.5A@6V, or 9W per resistor.

User Rank
Iron
Re: Go John!
10/22/2012 2:43:50 PM
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@Rudy - Hey, no problem - I love a good technical discussion! :{)

In the series/parallel arrangment John used (3x3 of 10 W originally, and I have to assume 2x2 of 20W in the parts list), both the current and the voltage divide among the 9 resistors. 1/3 I x 1/3 E = 1/9 P in each resistor. Doesn't matter a whit, if they are all in series, all in parallel, or in groups like this.

It's all a bit overkill, though. Unless I haven't had enough coffee yet this morning, dropping 2.5V at 2.5A is only 6.25 Watts. Still a bit much for a single 1 Ohm/10 Watt resistor, but should be no sweat for a single 1 Ohm/20 Watt, as in the parts list.

Dave

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Gold
Tinkering
10/22/2012 2:24:43 PM
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It's great to read about this young man "tinkering" with stuff...nice project!  John Duffy, keep up the great work, and consider an engineering or science degree, and career!

All too often, interviewing engineering personnel candidates, I found that many young degreed engineers have never worked with/on anything while growing up (or currently).  They have not had projects building gadgets, fixing things, or modifying stuff.  They say things such as they like cars, but have never attempted any work on cars.  I wonder if they can even hammer a nail, or screw-in a screw...use a tool.  Installing software and playing video games does not count to me as a project or tinkering.

By age 18, my list of projects and accomplishments was huge, a lot of practical experience...including advanced auto and motorcycle mechanics, and Hot Rodding (modifying to be powerful and fast).  Growing up, I helped my dad with numerous house and car repairs, my dad was an engineer (now retired).

User Rank
Iron
Re: Go John!
10/22/2012 2:04:43 PM
NO RATINGS
@Dave ---

I don't mean to argue, but I think you need to re-think that.  If I place two 1K, 1-Watt resistors in series, I end up with a 2K, 1-watt resistor.  The same holds true for three series 1/3-ohm, 30-watt resistors, as in John's example.  It ends up being a 1-ohm, 30-watt resistor.

If John had started with nine 9-ohm, 10-watt resistors, and placed them ALL in parallel, I agree that he would end up with a 1-ohm, 90-watt resistor.  That's not what he describes, however.

User Rank
Iron
Re: Go John!
10/22/2012 1:06:47 PM
NO RATINGS
From your video we can see that you have done some experimentation with other batteries and LEDs.  Enjoyable video and it looks like you are having fun.

What's next for you?

-Eddy

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