What value of resistor do I need?

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Old 11-24-07, 03:47 PM
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What value of resistor do I need?

Hi guys,

I have a string of led lights on my cabana on my hot tub and the transformer blew up. The string of lights is basically 8 leds lights. I would like to hook these up to another string of 120v lights that surround the cabana.

Does anybody know the value of the resistor that I would need to put in line with the 8 leds to the 120v run?


Thanks for the help

Jason
 
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Old 11-24-07, 10:36 PM
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You need the proper transformer, as that was how they were designed and approved to run.
 
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Old 11-25-07, 08:48 AM
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OK,
the problem is that the transformer does not have any label on it, Do you know which transformer I would need then to run these 8 leds?

Thanks again
 
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Old 11-25-07, 09:52 AM
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just to expand on classicsat's post a bit.

Your lights were designed to operate on the transformer. ALtering that would kill the UL listing of the string (I don't know how that applies in Canada but here. it is a no-no).

Besides that, only 8 leds does not take a lot of power so the resistance (and subsequently the resistor) needed would be quite large. It would probably be less expensive (and provide for a proper reapair) to simply figure out the transformer needed and fix it right.

So, to the transformer:

I have had some transformers that simply had info printed on them with ink (directly on the transformer) or stamped (all too often in very small and not very deeply) somewhere on the unit as well.
Look very closely and let us know.
 
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Old 11-25-07, 10:34 AM
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You COULD do this. You would need to apply some basic ohms law calcualtions. You would need to know the designed voltage, and whether they were hooked in series or parallel with the applied voltage source. You could then calculate the total circuit current you need, and then change the applied voltage to 120, and calculate the needed series resistance to cause that amount of current flow. Given that they are probably low voltage, maybe 12, the jump in source voltage to 120 will probably need a very high wattage resistor. This is just a clumsy way to do this.
 
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Old 11-25-07, 11:37 AM
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I just found this neat little array designer. It requires the specs on the LEDs used. http://led.linear1.org/led.wiz

The calculator tends to utilize series circuits in higher voltages, which to me are inherently a problem due to the fact that if one of the LEDs in the series burns out, they all go out (think old time Christmas tree lights). Not that I am an engineer but I do not like causing a problem where there is an alternative.

btw: LED's are DC devices so typically you use a power supply and not simply a transformer. I understand LEDs will operate on AC although they will only operate of forward current flow so they will only be lit 1/2 of the time althoug as our ac freq (60 Hz) they will be lit for 1/60th of a second and then be off for 1/60th of a second. Not sure if that would be overly noticable.
 
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Old 11-25-07, 12:20 PM
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Ohm's Law

Conventional LED's each drop about 1.5v, so assuming they are in series, the total for 8 would be 12v. The current should be limited to about 20ma through the string, so a wall wart transformer rated at 10-12v should work. Using a dropping resistor is problematical, since LED's may see the peak voltage, not the RMS.
 
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Old 11-25-07, 01:16 PM
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that is kind of a broad statement of the voltage. Different colors typically are rated for different voltages with oranges/reds around 2 volts and blues and whites around 3.5 volts.

the design voltage varies with intended brightness levels as well.

Without the specs of the LEDs involved, it's a crap shoot.

It would be wiser to simply replace the damaged part and return things to original.


since LED's may see the peak voltage, not the RMS.
that isn't a "may" It is what will happen. It may not cause a problem though as the time of the peak may not produce enough heat to cause a problem. Now, with lower voltages, the range of voltage would be lessened a great deal so using the lower voltage would definately be preferable.

(kind of like the original recommendation of replacing the transformer isn;t it?)





Now one problem they may face on AC voltage is in reverse bias, they are limited to around 5 volts max and the resistance is different than in forward bias, which would throw a wrench into the entire circuit.

For a variation of design, one might try alternating the bias of the leds so they would light alternately with one side of the circuit lighting with positive (forward biased) current flow and the other lighting on negative (reverse biased) flow.



Without the specs of the LEDs involved, it would be very easy to severly diminish the life or reduce the output of the LED while trying to design a proper circuit.
 
 

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