Boxerworks.com

Just been to maplins and have got 3mm LEDs , resistors and some very fine wire.
cost was peanuts.
I'm sure that it all can be fitted into the OEM trim for the 90/s screen.

Perhaps because I am electrically impaired I don't understand why the resistors are necessary.
If a small LED is added, in parallel, to the circuit for a regular 1156 turn signal bulb, isn't that the same as if the turn signal bulb was drawing a very tiny bit more electricity? The increased amps required are miniscule and I would think the relay and wires wouldn't notice.

I am also impaired, the LEDs are specced at 1.8v so the resistor is neede in the circuit Oslo the LED does not blow.

Charles

The whole issue with LED's is that they are just diodes that happen to emit light. Diodes are not very good at limiting current as they have only a small voltage drop. The more voltage you put across them the more current flows until they are damaged. And they are quite sensitive to damage by even small excess currents which initially results in a very large reduction in light output and then eventually they blow. Automotive use is quite challenging for an LED because the supply voltage is not stable being anything from 12V to maybe 14.5V. That makes the simple series resistor as a current limiter less than ideal. One of the reasons why the better LED lamps are expensive is because they have built in active current limiting circuits. By active I mean proper feedback current limiting circuits with components like transistors. This is far more efficient than a single LED with a series resistor that is wasting 5 or 6 times more energy than the LED consumes. In this senario they are potentially less efficient than a normal filament lamp. When using a series resistor to control LED's in a 12 volt circuit it's far more efficient to have a number of LED's in series so that the voltage drops add up to something closer to 12 volts. That way less energy is wasted across the resistor. The great attraction of LED's is the basic efficiency in terms of light output and that they can last practically for ever but only if the current flow through them is very carefully managed.

Barry,
Thanks for the info, I'm mainly using an led as it the small size of the bulb enables me to mount them into the screen trim, light efficiency is not really an issue.
Charles

Charles, you may already have done this but to calculate the resistor value the formula is

Resistance = (supply voltage - LED forward voltage drop) / LED forward current in amps

Typically in your case = (14 - 1.8)/0.020

= 610 Ohms

Nearest preferred resistor value is 620 Ohms

I used 20 milliamps as a typical current for a small low power LED but substitute the correct value for your LED and pick the nearest preferred value resistor. In this example the power that the resistor dissipates is 0.244 watts
(12.2 x 0.020 = 0.244) so you need a 1/4 watt rated resistor. If your LED current rating was higher say 40mA then as well as the resistor being half the value in ohms it will also need to be rated at twice the power so a 1/2 watt resistor is needed. Conversely a 10Ma LED could use a 1/8 watt resistor.

The guy from maplins did the calcs for the resistors, the ones I have seem to be in the right range!

I din use any resistors.
All has been well for nearly two years
I don't see what all the fuss is about.
Sorry, couldn't resist.

http://led.linear1.org/led.wiz

A little LED calculator, pretty handy.

Interesting calculator. Instead of doing a straight forward ohms law calculation it's adding roughly 10% to the calculated resistor value as an added safety factor.