I'm looking to replace burned out instrument/indicator lights with LEDs as a project to learn more about electronics. I'm working on an indicator light in a switch now. The LED I have in mind is a 2V/60mA "5730 (5630) Red SMD LED Diode Light Surface Mount Chip 5.7mm x 3.0mm DC 2V 60mA 0.2W, Super Bright Lighting Bulb Lamp".

From what I've learned so far, I should plan on 12-14 volts from the car. My math tells me 14 V / 0.06 A = 233 Ω required, and 14 V * 0.06 A = 0.84 W minimum power rating on that resistor. Am I thinking about this correctly? What power rating can I safely use? Would putting this LED / resistor in place of an incandescent bulb affect the other bulbs on the circuit?

  • \$\begingroup\$ You may find that the LED is much too bright with 60 mA. You can increase the resistor value to reduce the LED current, and hence its brightness. LEDs work fine with less than the maximum specified current. \$\endgroup\$ – Peter Bennett Sep 6 '17 at 23:28

That will be fine.

A minor correction on the calculations: since the LED will drop 2 V that only leaves 12 V across the resistor. So \$ R = \frac {V}{I} = \frac {12}{0.06} = 200 \; \Omega \$. 220 Ω will be fine.

Would putting this LED / resistor in place of an incandescent bulb affect the other bulbs on the circuit?

There are two circumstances where this might matter:

  1. Series connected lamps. This doesn't apply in your case.
  2. Indicator (turn-signal) lamps. Older cars used a bi-metallic strip contact for the flasher. The current ran through the contact and when it warmed up (in about half a second) the contact opened turning the lamp off. It cooled down in about the same time and turned back on again. They had the nice feature that if one bulb failed the flash rate would slow down to about half and alert the driver to the fault. Some electronic flashers mimic this behaviour. Given that you've chosen red LEDs it is unlikely to be a turn signal indicator so it shouldn't be a problem.

In general, parallel lamps don't affect each other. (All your house lamps of different power ratings are wired in parallel.)

  • \$\begingroup\$ Thanks for the note about house lamps wired in parallel! I was tripping up because I assumed they would be wired in series \$\endgroup\$ – sam Sep 6 '17 at 23:21

The real math is (Vs - Vf) / I = R. You forgot to subtract the forward voltage of the led. But that's probably better, as the fluctuating voltage of the car can bring it over 14V. Mine idles at 14.5V. Your led will live longer this way.

As to the resistor, the next size up is 1 watt.

As for if using that led instead of a bulb. That depends on your car but it is unlikely. Each bulb in a car instrument panel is fairly independent.

  • \$\begingroup\$ Thanks for the answer! You think a resistor rated to 1W would work alright? Not sure how close to the rating is safe for continuous use \$\endgroup\$ – sam Sep 6 '17 at 23:22

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