# LED resistor substitution question?



## dynoflyer (Oct 8, 2006)

I'm making LED spots using a 9V wall wort that puts out 500 mA. I want to use 6 Blue LED's and the wizard tells me to use 150 ohm resistors. . . .I only have 180 ohm resistors and 100 Ohm resistors on hand. 
Can I substitute either the 180 or the 100 ohm resistors without blowing the LED's? Will the spot still work and be bright? 
I could go to Radio Shack but I'm curious what happens if I make a substitution. 
Thanks, 
df


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## Doc Doom (Aug 28, 2008)

The 180s will work, but the LEDs won't be as bright. Use the 100s and you might let the magic smoke out.


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## Otaku (Dec 3, 2004)

You can always use a higher value resistor, but the LED will be a bit dimmer. A 100 ohm will overdrive them a little, but I do that to all my LED spots and haven't had a failure as yet. Make sure that the wart is regulated before trusting that resistor value, though. Lots of warts will run higher voltages until the mA rating is met or exceeded. If yours is running above 9VDC (like 12-14VDC) you might consider using a LM7809 regulator in the circuit.


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## dynoflyer (Oct 8, 2006)

Otaku and Doc Doom, 
Thanks for the prompt replies.



Otaku said:


> Lots of warts will run higher voltages until the mA rating is met or exceeded.


I learn something new everyday! I didn't know that, thanks Otaku.

I put the multimeter on the 9V wall wort and it reads 14.28V. So, playing with values in the wizard. . I plug 13V into the wizard and it tells me to use the 180 Ohm resistors I already have. I'm assuming I'm not going to come close to using all 500mA output even running three LED spots w/6 LED's each. At worst they'll be a bit dimmer if the current drops to less than 13V.

Doc Doom - Thanks for the hint about the magic smoke. I always keep the magic smoke in as long as possible :smoking:


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## Otaku (Dec 3, 2004)

Six LEDs per spot? What's the forward voltage on the LEDs? Assuming that they're ultrabrights, you may be looking at ~3 volts per LED for a total series requirement of ~18VDC per spot. A 9 volt wart won't light the spots. IF the LEDs are wired in parallel inside each spot then you're OK.


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## dynoflyer (Oct 8, 2006)

I'll let you know in a minute, just about to fire them up.


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## heresjohnny (Feb 15, 2006)

I doubt the wizard had you wire them in series. If I remeber, ultrabrights draw about 20 mA each at recommended load, so you should be well under the wall wart limit. Once you have it wired up, measure the voltage again and see if it is still well above 9 volts. If so you should be okay.


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## dynoflyer (Oct 8, 2006)

Voila! Works great. 

One 9V (14.28V on the MM) 500mA wall wart driving three LED spots. Each spot has 6 blue LEDs. I just followed the schematic from the LED array wizard after putting in 13V as the source voltage and used the 180 ohm resistors. 

The LED package label 
Best Hong Kong XtraBright 5mm 35cd+ NF F/R


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## Otaku (Dec 3, 2004)

Cool! Yep, the wizard showed a parallel wiring layout.


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## heresjohnny (Feb 15, 2006)

dynoflyer said:


> Voila! Works great.
> 
> One 9V (14.28V on the MM) 500mA wall wart driving three LED spots. Each spot has 6 blue LEDs. I just followed the schematic from the LED array wizard after putting in 13V as the source voltage and used the 180 ohm resistors.
> 
> ...


I'd be interested in what the voltage across the wallwart is now


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## Doc Doom (Aug 28, 2008)

dynoflyer said:


> Doc Doom - Thanks for the hint about the magic smoke. I always keep the magic smoke in as long as possible :smoking:


One of these days I'm gonna figure out a way to get the magic smoke back in and then I'm gonna be rich!

Glad you got it figured out and working, especially with Hong Kong LEDs. I've had a dickens of a time with some I've bought online.


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## dynoflyer (Oct 8, 2006)

heresjohnny said:


> I'd be interested in what the voltage across the wallwart is now


10.3V measured between the wall wart and the LED spots.


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## jdblue1976 (Nov 18, 2010)

For future reference you can wire the 180 and 100 in parallel and get 140 ohms which is close enough.


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## spinman1949 (Jun 29, 2009)

*Jd blue*

Is that right?

Been awhile, but I think when you put two resistors in parallel you end up with less resistance than the lowest value resistor.

I found this statement on a parallel resistance calculation link.

" The equivalent resistance of the circuit shown in figure 3-45 is smaller than either of the two resistors (R 1, R2). An important point to remember is that the equivalent resistance of a parallel circuit is always less than the resistance of any branch. "

It is like having two hoses instead of one. The pressure to the first hose does not diminish because a second hose is attached. Therefore at a given pressure one hose will provide a certain volume of water per minute. GPM. The second hose though smaller IE more resistance will also still provide a certain volume of water. GPM. So the total flow or in the case if electricity, 
( current ) is higher than the amount of current if one has only one hose.

If two resistors of the same value are placed in parallel the actual net resistance is now 1/2 the value of one resistor. Add another resistor and the value is now 1/3. Etc. It would be like taking the voltage and increasing it 3 fold, if you had only one resistor. So a circuit with 36 volts wired to a 12 ohm resistor would have the same current flow as a 12 volt circuit wired to three 12 ohm resistors in parallel.


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## Otaku (Dec 3, 2004)

You are correct - I use this calculator:

http://www.sengpielaudio.com/calculator-paralresist.htm

when I need to use a non-standard resistor value.


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## spinman1949 (Jun 29, 2009)

*Good link*

Thx Otaku.

That is a very helpful tool.


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## jdblue1976 (Nov 18, 2010)

spinman1949 said:


> Is that right?
> 
> Been awhile, but I think when you put two resistors in parallel you end up with less resistance than the lowest value resistor.
> 
> ...


You're correct. I took the specific case of two resistors of the same value in parrallel and generalized it, which of course is wrong.

The actual formula for the effective resistance of two resistors in parrallel is to multiple the two values together and divide that by their sum.


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## lostskeleton (Aug 30, 2011)

Yes a pretty good rule of thumb is the total resistance of the parallel circuit will be just slightly more than half the lowest resistance.

But I have to wonder what a power supply that is listed at 9V is doing putting out over 14?... that might not be a good thing.


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## pshort (May 6, 2008)

The output voltage rating of wall-warts can be misleading.

Some wallwarts are made up from a transformer and a full-wave rectifier. The output waveform is pulsating DC, basically a repeating half sine wave. The rated voltage is a RMS value at the full load. Under no-load the RMS value is somewhat higher than the rated voltage, and the value that you see with a DC voltmeter is 1.414 times that voltage. So, the unloaded RMS output voltage of a 9V wall-wart is perhaps 10V, and the value that you would see with a DC voltmeter is around 14V.

Complicated, I know.


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## lostskeleton (Aug 30, 2011)

Not too complicated, that smoothing cap and peak voltage will get you all the time. Thats why we need to use a nice regulator to make sure we keep the right voltage level and not burn those little LEDs up.

And good call pshort!


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