finding the correct resistor

@Gregcz1 posted:

I have searched but am not coming up with an answer for my question.

I would like to find the correct resistor to use for 3v LEDs (each one receiving a resistor) with an 18v power source.  I have tried some that work, but one began to give off a burning smell, the other gets quite hot.  I am not well-versed in the "ohm" verbiage and was never a strong math student LOL.  Can someone just tell me what resistor I need... the color code etc.

Also, when a resistor is added to the wiring, should it be covered in shrink wrap tubing?

Here's one video guide: https://www.digikey.com/en/blo...-resistor-for-an-led

Short answer: it depends in part on the characteristics of the LED you're trying to protect, but I think you'd probably be in the vicinity with something like a 1K ohm, 1/4 watt resistor, but that's just my seat-of-the-pants guesstimate.

As to using shrink wrap, most pre-wired LEDs I've seen use something like that, but you want to be sure the resistor is robust enough in wattage that it will not overheat inside the wrap. OTOH, you'll need to insulate the wires in some fashion, so I'm not sure what a better alternative would be . . .

You can use this LED Calculator.  Enter the parameters and it will calculate the proper size resistor and draw the circuit.  In your case:

Power supply Voltage=18v, LED Voltage drop 3v, LED current rating 20ma (usual choice for a good balance in light brightness and longevity):   Resistor size will be 750 Ohm, 1/2 watt.

If you want it a little dimmer (Current=15ma), the resistor will be 1K 1/4 watt.

Other considerations:  
If you are using multiple LEDs, wire them in series, and you can use lower wattage resistors and less power would be wasted as heat.  The LED Calculator allows you to enter the number of LEDs up to the maximum the supply voltage can light.

LEDs can be powered with DC voltage with only the LED and a resistor in series.  If you are using AC voltage, you should use a diode (1N4001, 1N4002, 1N4003, etc) in series with the LED and resistor to protect the LED from reverse voltage. Alternatively, you can use 2 LEDs in parallel (opposite polarity) in series with one resistor (only enter one LED in the LED calculator in this instance). Multiply the AC voltage (RMS voltage) by 1.4 to get the AC peak voltage, and use that figure in the LED Calculator.

Bob

Note:  I edited the comments about using AC to drive LEDs.  When using the LED calculator, the supply voltage should be the AC voltage x 1.4.

Bob

@Gregcz1 posted:

I have searched but am not coming up with an answer for my question.

I would like to find the correct resistor to use for 3v LEDs (each one receiving a resistor) with an 18v power source.  I have tried some that work, but one began to give off a burning smell, the other gets quite hot.  I am not well-versed in the "ohm" verbiage and was never a strong math student LOL.  Can someone just tell me what resistor I need... the color code etc.

Also, when a resistor is added to the wiring, should it be covered in shrink wrap tubing?

Thanks,

Greg

There's a super easy rule of thumb for a single LED and any voltage source.

I subtract the operating voltage of the LED from the supply voltage and multiply that by 50.  That's the value of resistor to use for 20 milliamps, typically the operating current for a vast majority of LED's.

Here's a link to a resistor color code chart that you can use to determine the value of 4-band resistors: Resistor Color Codes (4-Band Resistors)

Any bare connection to a resistor should be insulated.  I typically just cover the resistor, but if I want to be able to read the value, I use clear heatshrink.

Greg.....I do use the method GRJ wrote. Sometimes I will use a Elenco Model RS-400 resistance Substitution Box to fine tune light output. I use this $20 ish gadget more than I ever thought I would. Easy to find on the web. As an example, recently on a passenger car drumhead and side warning lights I tapped in at the end of a led strip connected to GRJ's light board and after I had my overhead leds adjusted right, Drumhead and warning lights were too bright with a 470 ohm resistor. I used the box to dial down he three leds,read the box setting and used that value for a soldered in resistor. Some where I have a color chart for resistors. I just grab a multimeter and check to be sure I have chosen the right resistor. BTW The box is a kit that is a good basic soldering course.

Richard