# Flasher/relay or load resistor for LED? ***Solved***

After installing a pair of HID headlights on my truck, I want to change out some of the bulbs for LED ones. The original bulbs are rated at 21w and the LED ones are 7.5W. Installing LED for the indicators/signal lights may make them hyper flash. To prevent this, I’ll need to either install some load resistors or a relay.

Here’s my questions.

1. Should I install a LED flasher/indicator relay or resistors? What are the benefits for using 1 to the other?

2. How do I know what kind to get or how strong the load resistor needs to be? What is the calculation to figure this out?

The original bulbs are rated at 21w and the LED ones are 7.5W. The ones I see on ebay are mostly 50W 6ohm resistors.

As you can see above, I know jack about this electrical stuff. Does anyone have experience with this or can tell me how to calculate the load resistence I need?

You’re probably going to need to use the Load Resistor. Unfortunately, the downside is that the excess watts will be converted to raw heat. You’ll have to mount the resistors somewhere that won’t be bothered by heat. In your situation, 21w - 7.5w = 13.5w of heat is going to be generated in the resistor any time it’s on.

To calculate the needed value, take nominal vehicle voltage (13.6V), square it (13.6 * 13.6) and divide by the number of excess watts you need to burn (13.5W)

(13.6 * 13.6) / 13.5 = 13.7 Ohm.

You’ll need a 13.7 Ohm resistor rated higher than 13.5 Watts. You probably can’t find an exact 13.7 Ohm part, so pick the closest value you can find.
http://www.mouser.com/ProductDetail/Xicon/280-CR25-13-RC/?qs=sGAEpiMZZMtbXrIkmrvidN3MKyVy9kQs9iN%2FA0g2bW4%3D

PPtk

Thanks for the formula and the link

When I put LED flashers on my motorcycle, I replaced the flasher relay with an electronic unit. Works fine and it was like \$10. Personally, I’d rather do that than add load resistors.

I broke down and ordered a \$2.50 led flasher relay.

Reason being is that I have side indicators that are a (5 letter word for female dog) to get at. I would have to install the resisters from under the dash instead of closer to the light source.

Anyways we’ll see what how these 11W LEDs will look like compared to the stock 21W incandesents I’m replacing.