Wiring LEDs

[rbeckey]

I have not done any electronics wiring since high school. That was waaayyy before LEDs were found in the consumer market. I understand that they need a resistor wired in front of them. How do you determine which resistor to use? The LED I wish to use is rated for 15mA 12 volts DC. If a diode protects against reverse polarity, which diode should be used?

If I wire a SPST on-off switch in the positive side of a circuit and add the LED and associated components to the on side, can I connect it directly to ground and then wire in the launch button? Apparently the LED draws very little current, so would it effect the circuit before or after the launch button is pushed? Continuity would be handled separately, this is merely a power indicator.

 

[SCE to AUX]

If the LED you are using is rated at 12VDC, then it already has the series resistor built-in.

There is no need to reverse polarity protect the LED, as it is a diode itself. It will only light when forward biased. If you connect it backwards across 12V, the LED will be reverse biased, and will not light. Unless you apply a reverse voltage sufficient to avalanche the diode junction, no damage will be done.

You need to include a series resistor if you are using a raw LED, as opposed to a premade assembly. Sizing the series resistor is done by using Ohm's law.

You need to know the following:

The forward voltage drop of the LED (Vf). Typically around 1.4V for a red LED, with other colors being somewhat higher.

The forward current rating of the LED (If). Typically 5-20 mA.

The supply voltage you want to operate from (Vs).

The formula is as follows:

R=(Vs - Vf) / If

For a typical red LED with a forward drop of 1.4 volts @ 10 mA, operated from a 12V supply:

R = (12 - 1.4) / 0.010

R = 10.6 / 0.010

R= 1060 ohms.

The closest standard values would be 1000 ohms or 1200 ohms, and either would work fine.

 

[cornyl]

Well said SCEtoAUX. My other hobby is anything to do with leds. Here is a quick schematic for rbeckey.

View attachment simple circuit.JPG

 

[phatbob02]

I have not done any electronics wiring since high school. That was waaayyy before LEDs were found in the consumer market. I understand that they need a resistor wired in front of them. How do you determine which resistor to use? The LED I wish to use is rated for 15mA 12 volts DC. If a diode protects against reverse polarity, which diode should be used?

If I wire a SPST on-off switch in the positive side of a circuit and add the LED and associated components to the on side, can I connect it directly to ground and then wire in the launch button? Apparently the LED draws very little current, so would it effect the circuit before or after the launch button is pushed? Continuity would be handled separately, this is merely a power indicator.

Here is what I did to check continuity for a 3 rocket launcher. I can launch 1,2, or 3 at once.