# need help with voltage



## stew22 (Jun 3, 2005)

i'm now trying to set up lights on my track and i'm not having much success with the voltage. i'm using a 3.5 power source to run led lights and low volt bulbs and i keep blowing the bulbs/leds. i have purchased a few led lights with built in resistors from radio shack and they work great. so the question is, what type of resistor should i get to make all the lights i bought work? i asked the guy at radio shack and he couldnt tell me s**t. if anyone can help, please keep it simple without going into technical physics of mcd,ohms...whatever. :freak: its like talkin chinese to me. hopefully someone can help me here. :wave: or if anyone has any new suggestions of how to run the lights. thanks.


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## hankster (Jan 1, 1998)

What is the voltage of the power supply? Or you can figure it out yourself using:

The resistor value, R is given by:

R = (VS - VL) / I

VS = supply voltage
VL = LED voltage (usually 2V, but 4V for blue and white LEDs)
I = LED current (e.g. 20mA), this must be less than the maximum permitted

If the calculated value is not available choose the nearest standard resistor value which is greater, so that the current will be a little less than you chose. In fact you may wish to choose a greater resistor value to reduce the current (to increase battery life for example) but this will make the LED less bright.

For example
If the supply voltage VS = 9V, and you have a red LED (VL = 2V), requiring a current I = 20mA = 0.020A,
R = (9V - 2V) / 0.02A = 350ohm, so choose 390ohm (the nearest standard value which is greater).


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## stew22 (Jun 3, 2005)

the power supply is 3.7v 0.35A exactly
the leds are 2v and led blinkers are 5v and 6v
clear bulbs are 3v

do i need a resistor for each and every light? can i have one resistor for the whole set? i want to daisy chain the whole lite set if i can. i can buy a 1.5v power adapter but i'm afraid that the bulbs would be dimmer than i want them to be but confident that the leds would run ok.
i've been doing test sets first to make sure my ideas are going to work. i did a 3 light set test connecting the + to the - on each. (-/+/-/+/-/+) when i connect the first - and last +, the first led was brighter than the 2 others but didnt blow. tried connecting all + together and all - together and they were nice and bright but they did blow out. remember this was on a test on just 3 leds.


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## Franko (Mar 16, 2005)

Simply put, LEDs generally need a resistor in the series. Check out http://www.casemodgod.com/led_calculator.htm to determine what resistance needed.


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## Franko (Mar 16, 2005)

You can have the LEDs in series with one resistor.


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## Franko (Mar 16, 2005)

Mixing the LEDs with different V might be problematic though. I would have one series circuit for each type of LED.


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## hankster (Jan 1, 1998)

That page has good info and you can use that to figure it out.

BTW, what type of power source is it? If it is a cheap "wall wart" type supply you should check the voltage coming out of it. Many of them are not very well regulated and can easily have double (or more) the voltage coming out of them then on the label. The voltage on the label is normally measured when under full load. Personally, I would suggest using a common regulated 12 volt power supply... you can get them well under $10.00. Some available at http://allelectronics.com ... they also have a wide selection of LEDs and resistors.

http://www.allelectronics.com/cgi-bin/category.cgi?category=480&item=PS-1250 looks like it would fit the bill.


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## AfxToo (Aug 29, 2003)

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## Slott V (Feb 3, 2005)

You can run the bulbs in *series* (= positive to negative). The amount of output voltage and the bulb voltage will determine how many bulbs in series you can run before they all dim. This is how Christmas tree lights work. Each additional bulb added to this circuit acts like a resistor.

If you run bulbs circuits in *parallel*, the formula relies on wattage output to determine bulb count per transformer. This is how, for instance, your Malibu lights are wired around your patio. Most model train layouts will run *parallel * circuits or "busses" for lighting.

And like the rest have said, you have to install the appropriate resisitor inline for LED's to limit your input voltage. The resistor OHM rating needed is determined by your power pack output voltage. Most all LED's operate within 2-3 volts and very low amperage. The WATT rating of resistors should be considered when dealing with higher amperage output, but that's really not an issue with LED's. LED's are best run in a separate *parallel* circuit. LED's can only be wired into a DC circuit one way, with the short leg of the LED being the ground. Combining bulbs and LED's in the same circuit is not a good idea, but they can be run from the same power source.

Radio Shack has some cheap beginner books for electronics projects. :thumbsup: 

Hope this helps.
Scott


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## AfxToo (Aug 29, 2003)

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