# calculating resistance for LED strips to use as parking lamps and turn signals

I'm installing LED strips in my car's tail lamps to convert it from conventional bulbs to LEDs. I have a understanding of what I'm doing but not the complete knowledge to make the circuit work correctly without doing some guessing so I figured I would ask here.

This is the product I'm planning on working with and I need a length of 16", 15" and 14" to make then look correct in my housings. When I did the math the 19.7" strips have about 1.5 LEDs per inch and need to be segmented into 3 LED groups so I may have to get creative with the placement.

Ffom what I understand I need 2 diodes and a resistor. One diode on the parking lamp power, and one on the turn signal/brake power and then a resistor on the parking lamp wire to step down the voltage enough to make them dim enough to use as a parking lamp and distinguish it from the brighter turn/brake lamps.

Does anyone know what size resistor and what size diode to use in this project? The voltage, with the car running, is around 13.8V to the rear lamps. I figure it should be running around 7V to the LEDs to light them as parking lamps.

Also will leaving out a cluster of 3 LEDs change the strips intensity to where I'll need different resistors for each strip?

LED Strip Basics

As you might be aware, these LED strips come as parallel groups or 3 series LEDs with one series resistor. Connecting 12V to the main connectors is all it takes to light them up. They can be cut apart, but only in groups of three at the appropriate markings on the strips. The embedded resistor value varies for different types of strips (LED color, manufacturer, etc).

simulate this circuit – Schematic created using CircuitLab

Replacing the bulbs in your car with DIY LEDs strips may not be legal. Car indication lights have to be within a specific brightness range. You to need to ensure the LEDs have the correct candella rating to be used as tail or indicator lights.

Practical Considerations

Dimming an entire bank of LEDs by placing a resistor in series with the power line is not a good idea. A lot of power will be dissipated in this resistor, as in the dropped voltage multiplied by the entire LED bank current.

Some car turn signals operate a bit strange... the ON resistance of the indicator bulb actually determines the speed of the blinker. This is part of what causes a blinker to double in speed when one of the bulbs is out. Replacing the bulb with LEDs may change the speed of the blinker if the resistance is not matched. There is also the factor of heat. LEDs don't produce very much heat, meaning your light housings could frost over in cold weather - something prevented by the heat from standard bulbs.

Also, powering the strip with 7V will probably not produce any light at all. Bright white LEDs typically drop about 3V apiece just to barely turn on. That means you need at least 9V for the LEDs plus a bit more for the embedded resistors. The extra source voltage is dropped by the embedded resistors, and this is also what determines the LED current: I_LED = [V_source - (3 * V_LED)] / R. LED brightness is determined by forward current; however, the forward voltage also changes with the forward current. A curve relating the two should be available in the LED datasheet.

How to Do It (Your Way)

If you really want to move forward with this idea, a standard rectifying diode is a good bet, but the actual part is determined by how much current will be used by the LEDs - the diode will need to be rated for at least that of the entire LED array. Since the signal won't be switching quickly ( turn signals are usually 1 - 2 Hz) that is not a factor.

Finding the necessary series resistance is a bit trickier, but doable. You will need to know how much current to pass through the LEDs to get the dimmer output you desire, then add up the LED voltages at that forward current plus the dropped voltage across the embedded resistor (V = IR). How ever much voltage is left will need to be dropped by the additional series resistor. However, keep in mind, that this resistor will have the entire LED bank current going through it...

• thanks kurt- after doing more searching on the site where they have the LED strips they sell mini dimmers for led strips that uses pulse width modulation to make it adjustable from 10% brightness to 100% brightness. do you think this is the best way to go to do it by vision? also since I will have 2 strips with the same amount of LEDs and a third strip with 3 less- should i install 2 dimmers- one controlling the number of LEDs that are alike, and one to controller the smaller bank so i can match the brightness to all 3? May 13, 2013 at 18:40
• One Dimmer should work for all of them, just hook the strips in parallel. That would be no different then how they were connected before you cut them apart. It will vary the brightness of each individual string of 3 LEDs, how many of these strings are in parallel is only limited by the power output of the dimmer. May 13, 2013 at 19:12