# Value of the resistor in Radio Shack Led holders?

I got this LED from radio shack and I want to recreate it with smaller LEDs.
Does anyone know the specs of the resistor?
I think it's 680 Ohms, but is it 1/4 or 1/2 watt?
Thanks

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Quercus austrina5 years ago
Blue, Gray, Brown, Gold = 680 ohm +- 5%. Looks to be 1/8 watt size. If in doubt about the wattage, check aganist the physical sizes available at Radio Shack. I wouldn't use anything over 1/8 watt for a single LED like that as it would be a waste.

For future reference: LED Calculator.

Qa
RoboRay (author)  Quercus austrina5 years ago
Thanks, so putting it into a 12v source should be ok?
5 years ago
I'm sorry, I called the resistor calculator a LED calculator. With that out of the way, let's proceed.

With a 12 V source and a red LED forward voltage of around 2.0-2.2 V, the LED will be limited to 15-17 mA. According to the LED wizard above, the resistor should be in the 1/2 watt range if you intend to run it for protracted amounts of time. Short runs may warm the 1/8 watt resistor, but it should last in that scenario. Depends on your application.

Qa
RoboRay (author)  Quercus austrina5 years ago
Thanks,

I don't have another 680 ohm resistor, so I'm going to wire 5 150 Ohms 1/8 watt resistors in a series.

Is it better to have a resistor attached to each bulb, or have a line with resistors before the it split off to each bulb? So for example, If i had 3 LEDs, they would share that 1 resistor. One of the calculator you linked shows that.
5 years ago
The preferred way to run LEDs is 1 LED/resistor combo, because you only risk 1 LED at a time if something goes wrong. Plus, it helps to balance out LEDs perceived brightness in a string because each individual LED is current limited.

If you chain them together in series, you need to make sure that you use the correct forward voltage and correct current limiting resistor as calculated using all parameters. You run the risk of blowing a string of LEDs if something goes wrong this way. All LEDs in the string share the overall current and if 1 LED draws more current than the others, it can look brighter  and/or make the others appear slightly dimmer, plus the brighter one usually will have a shorter life.

However, running the LEDs in a series string with 1 resistor has the attractive advantage of dropping more voltage across the chain and you can use a lower value resistor that has to dissipate less energy. In this way, you use less parts.

It all depends on what is most important to you. Go to the LED calculator link above, then click the SERIES/PARALLEL link there, plug in 12 V, 2.2 FV, 20 mA, the number of LEDs you want to use, click off Wiring Diagram and Help with resistor codes. You will see how they put it all together. The wattage of the resistor can be determined by how much power it dissipates, as described in the text below the diagram.

If you had 3 LEDs, you would need a 270 ohm resistor. 2-150 ohm resistors in series is fine because it will make the LEDs draw slightly less current. Play around with the mA numbers to see where you can get close to the resistor value(s) you have. And try to find out the correct info for your LEDs. Most manufacturers have datasheets online.

Good luck,
Qa
RoboRay (author)  Quercus austrina5 years ago
thanks for your help, but the links doesn't tell me how much voltage I dropped with my resistors. maybe you can help?

I don't know much about the LEDs since I got them on ebay.
They are 3mm LEDS.

I currently have a 12v source. Since the radio shack led uses 1/8w 680ohm resistor.
I decided to link 5 1/8w 150ohms resistors. 150 ohms is the only resistors I have on hand. With the combined resistance of 750 ohms, how much voltage is going to the LED? Will I be fine with just using 4 150 ohms resistors?

You mentioned 1/8 resistors will heat up. Do you know if the heat will be dangerous? I don't want a fire.

Will having more resistors also help spread out the heat among the resistors?

Is using 1/2w a safer option? Does it help with LED life?

The LED will only be on at night, for about 4 hours each day.
The resistors are for each LED, and they LEDs are in a small confined area, with not a lot of cooling.
Thanks