LED BULBS AND LIGHTS

This is some simple information to help remove some of the myths surrounding the use of LED bulbs and lights.

A typical 5 watt tail light/ side light bulb draws a current of 0.42 Amps. (Current = Watts/Volts) where as a single bright white led draws about 0.04 Amps.  That’s 10 times less current.

In this diagram the FOUR white led’s are equivalent to one bulb in terms of voltage. BUT draw 10 times less current.

 

Many modern cars are equipped with bulb failure warning lights. These generally work by measuring the current draw in each individual circuit.

If you replace a 5 watt bulb with an equivalent led “bulb”, the current draw will be so low, it will be detected as a bulb failure.

The only way to prevent this is to replace the missing current draw.  So the led circuit needs to contain something that acts like a 5 watt bulb. The standard method is to add a resistor in parallel with the led light.

The added resistor causes the same current draw as the single bulb so no bulb failure is flagged.

This resistor can be built into the LED light or as an added extra.

Remember….This resistor is replacing a bulb that got hot...so this resistor is going to get hot.

A suitable replacement for a 5 watt bulb would be a 30 Ohm 5 watt or more resistor.
The nearest size you can find may be 47 Ohm at 5 watts

(You must use a high power (watts) resistor for this, or the resistor will over heat and catch fire)

If you fit a LED light to your car that does not have this extra resistor, you will get a bulb failure warning.

Some people describe this problem as a “can-bus problem” which It is not.  It has resulted in people selling “can-bus safe” LED lights. 

So called “can-bus safe” LED lights should contain a resistor and not put the bulb failure light on, so these are the only ones worth buying.

Remember...One  reason for using led’s is to cut down on power consumption. If you fit LED’s that don’t bring on a bulb failure warning light, you are using the same power consumption as the original bulb, you replaced !!!!!

 

LED’s used in cars often have a resistor attached. This is not for bulb failure warning prevention.

This is because LED’s cannot work on 12 volts. A white LED typically works on 3 volts.

If you connected an LED to 12 volts it will be destroyed instantly.

To use LED’s on 12 volts you must either arrange them in groups of FOUR in SERIES or in ONE’s, TWO’s or THREE’s with a suitable balancing resistor in SERIES.
In this diagram the FOUR white LED’s are equivalent to one 12v bulb as far as voltage is concerned.

In this diagram the TWO white LED’s plus the resistor are equivalent to one 12v bulb as far as voltage is concerned.

How do you know what size resistor to use ?

This depends on the make and type of LED so don’t guess. Check with the supplier to get the working Voltage and Current for your LED’s.

E.g.

Voltage: 3.0 - 3.4 v

Current: 30 mA - 40 mA.  (40 mA is 0.040 Amps)

So for this example the maximum allowed voltage is 3.4v and maximum current draw is 0.040 Amps.

So the with a 12v supply we want the resistor to take 8.6v and leave 3.4v for the LED. (12 – 3.4 = 8.6)

The current drawn by the LED is 0.040 amps so using Ohms law.

RESISTANCE = VOLTAGE/CURRENT

The resistor we need will be 8.6 volts / 0.040 amps which is 215 ohms. You can’t buy 215 ohm resistors so you use the nearest HIGHER resistor available which is 220 ohms.

If you want to run TWO LED’s in series using 12 volts you will need a different resistor.

E.g. Two LED’s in series need 6.8 volts (3.4v + 3.4v). So with a 12v supply we want the resistor to take 5.2v. (12 - 6.8 = 5.2)

The current drawn is still 0.040 amps so using Ohms law.

The resistor will need to be 5.2 volts / 0.040 amps which is 130 ohms.

A reasonable rule of thumb for LED’s on 12 volts is to assume each LED missing from a group of four should be replaced by 75 ohms.

 

To save space and money, most manufactures arrange 12 volt powered LED’s in SERIES groups of FOUR.

Then they don’t need the extra balancing resistor. This does mean that if one LED fails, FOUR will stop working.  This is allowed for in vehicle lighting regulations. More than FOUR LED’s not working is considered to be a “bulb out offence”.

REMEMBER.

Unlike bulbs, LED’s only work one way round. So if you fit a LED lamp and it doesn’t work. Try turning it round the other way.

 

FINALLY.

Fitting LED “bulbs” or light units to a car is a change from the manufacturers specification. Which means insurers may not like it.