LED trailer lights
QI have recently purchased a trailer fitted with LED lights. These work fine but do not activate the dash warning light when the indicators are on. I have other trailers with a lighting board (filament bulbs) and it works OK with these. I have fitting 6 ohm/50 watt load resistors to the trailer indicator lights. This makes the dash warning lamp work but the indicators on the trailer do not work. The towing vehicle is a 2002 Hyundai Santa Fe with normal filament bulbs.
I have two questions:
▶ Is it a legal requirement to have a dash warning (visual or audible)?
▶ What size load resistor should I fit? Ohms law works out at 6.8 ohms for a 21 watt bulb. Do I need a 25 watt 6 ohm resistor? Nick Lett
AThe construction and use regulations specify that there should be some form of alert to let the driver know trailer indicators are working correctly such as a buzzer or light. Either is acceptable and many bypass systems rely on a buzzer in the rear of the vehicle which allows for easier wiring in of the trailer socket. The formula to work out the resistor required will need to consider the LED voltage and LED forward current. The forward current is the voltage required to make the LED light and this varies according to the colour of the LED. White and blue are typically 3.4V, whilst the red, green and orange LEDS have a forward voltage of 2V.
It will also need to include the number of LEDS connected – if the trailer board has multiple LEDS in the circuit this will affect the calculation. The formula is Resistor = (Battery Voltage – LED voltage)/desired LED current.
Using the 12V power supply and the desired LED current of 10ma, the
formula for a red LED would be: Resistor = (12-2)/0.010 – this makes the resistor value 1000 ohms. The wattage rating would also need to be calculated and the formula for this is to multiply the voltage drop through the resistor by the current flowing into it. So the above would be: (12-2) x 010 = 0.1 using the next available resistance wattage of a quarter.
These calculations are for ensuring the LED operates when connected to a 12V supply, not to ensure it gives the correct load to allow the vehicle on-board systems to sense the presence of the trailer board. This calculation can be tricky without knowing all the specifications but the calculations have been done by various companies and a unit is available which is a 12V 6 ohm load resistor with a maximum power consumption of 50W.
Owners have found that when wiring a resistor directly into the circuit of the trailer board the recommended resistor is 120 ohm 1W (5% tolerance) – this is colour-coded brown, red, brown, gold.