To add a load resistor to an indicator (turn signal) it has to be wired into each LED light bulb in parallel. I.e. The load resistor goes across the light bulb connections, between the supply and ground.
A load resistor is required for each LED light bulb in the turn signal circuit, (you may only want to upgrade the rear of the vehicle? In which case you only need 2 x load resistors).
Don’t forget, load resistors can generate lots of heat, so position carefully.
A typical load resistor for a 21 watt turn signal light bulb would have a rating of 50 watts, 6 ohms. Though please check with your supplier.
To someone familiar with electronics, calculating the size of a load resistor may be easy. But to anyone else, it could be a bit confusing…
Therefore I’m not going to try and explain in too much detail, but just give an understanding of how the size is calculated.
The load resistor replaces the load lost when the filament light bulb was replaced by an LED light bulb.
In other words:
You can use calculations to obtain the size of the load resistor.
First we need to calculate the difference between the filament light bulb and the LED light bulb in watts:
A = The size of the filament light bulb (in watts).
B = The size of the LED light bulb (in watts).
The difference = A – B
Using Watt’s law and difference between the filament light bulb and the LED light bulb (in watts) calculated above, we can find the current (amps) needed to mimic the filament light bulb.
We will then use the current (amps) calculated above to find the resistance (Ω or Ohms) of the load resistor.
The resistance and the number of watts are the two parameters needed for obtaining the size of the load resistor.
The wattage of the load resistor is the value used in the calculation above…
The filament light bulb (watts) – The LED light bulb (watts).
This will give you the lowest wattage figure, but the load resistor will get very hot (just like a filament light bulb). So the heat needs to be dissipated using a much larger wattage load resistor (at least double).
One of the problems with this calculation, is often the LED light bulb specification isn’t readily available, or is a bit vague. Which then makes calculations difficult (guesswork).
Researching on the internet, it seems a typical load resistor to replace a 21 watt turn signal light bulb would use a 50 watt 6 Ω (ohm) load resistor. Note the 50w rating to dissipate any heat.
A good supplier of load resistors should also be able to advise and assist with your selection, if you are unsure what to purchase.