Why would I need a resistor - I am just changing a normal bulb for LED / SMD?
Load or Ballast resistors are used when upgrading normal bulbs to LED. Because LEDs use much less power the indicator relay thinks that a bulb has blown and flashes rapidly as a warning. The relay is preset with a value of all the indicator bulbs on the vehicle and anything below this such as a blown bulb or replacing a bulb with a lower power consumption such as LED results in this warning of fast flashing. A resistor is required for EACH bulb changed to LED .
How do resistors work?
The resistor takes the power and converts it into heat which then creates the required load
Why do I need a 50W when 25W is cheaper?
For indicators you need 50W (25W can be used for side lights which we will cover in a seperate review) the resistors are rated at 50% of their capacity for continuous use. This means a 25W resistor is only rated at 12.5W for continuous use where the indicator you are replacing is 21W meaning the resistor will become dangeroulsy overloaded. The seller of the underrated product will argue that the indicator flashes on and off so the resistor is not used constantly which is true, but this also means the resistor has between 0.3 and 0.5 seconds to cool in between flashes. So over a few seconds that resistor will become very hot. If you wait for your central heating to turn off and then touch the radiators TEN MINUTES later they will still be red hot! To be fair to the seller of the 25W resistor if they included a bespoke heatsink with the size calculated for each application and location, mounted the resistors using a thermally conductive paste then they would actually do the job. The last time we checked there was no such listing so 25W are not suitable for use in indicator circuits.
Why do they all look different?
The short answer is they should not! The correct resistor should look like this
The metal body is the heat sink, they may be different colours but the design should be the same.
What not to buy when replacing a 21W bulb with LED!
Ceramic resistors - the largest only has a rating of 10W which equates to 5W constant load which is four times less than what you need
Still a ceramic resistor just hidden in heatshrink that will probably melt if used in an indicator circuit application.