Frank Warner's Guide on Automotive Instrument Lamp replacement with LEDs.

Why use LED replacements?

The 3 watt instrument lamps are hard to replace on some BMW motorcycles, and other vehicles. When inside sealed instrument pods, surrounded by flarings they can be quite a job to get too. Replacement of these lamps with longer life LEDs (Light Emitting Diodes) would be a good idea. The LEDs are about 4 times the price (lamp $1.74; 4 LEDs $7.40 [in lots >10]), but worth it in terms of reliability. Some LED manufactures state that the life of the LED is 100,000 hours (lamps 10,000 hours?), and that LEDs don't fail like lamps, they just get dimmer over time. Operating LEDs at a lower than the maximum current (and light output) will lengthen their life. Operation at high temperatures (over 35C) does mean that the LED current should be reduced to keep the reliability. So designing the operation for a temperature of 45C (about my maximum temperature) should lead to a increases life at lower temperatures (where the temperature is most of the time). I use 4 LEDs in series, this reduces the reliability due to any one of the 4 failing making the whole 4 fail.

Where not to use LED replacements

The LEDs use a lot less current than the lamps, so use as the alternator / battery lamp is not recommended by me unless you place a resistor in parallel with the lamp. Some people on the airhead list have already done this to ensure charging does take place even if the lamp blows. A 470 ohm 1/2 watt resistor should be OK. This circuit needs a fair amount of current to function correctly, the addition of the parallel resistor current should make the circuit work all the time. The LEDs will not be as usefull as the lamp in showing faults such as diode falure. My stats on these lamps is one failure every 10 years. This is about the failure time of the leds used as night instrument lighting. Your call on replacing this lamp.
The LEDs may not have as much light output as the original lamp. So for critical applications (eg the oil lamp) it may be best to leave the lamp!

Power comparison.

I used high intensity LEDs Jacar part No 1758 yellow and No 1759 red, 3 mm diameter 2 volt 20 mA 5000 milli candle. These were a reasonable cost for the light output and size. This is a power of 0.04 watts - no where near as much as 3 watts! However the LEDs are very efficient at converting the power (watts) into light (milli candle), where as the lamps convert at least half their power to heat - not light! The LEDs also are directional in their light output, most of their light goes forward, where as the lamp sends it every where. So the 3 watt lamps - 1.5 watt (heat loss) - 1 watts (light loss) becomes 0.5 watts about the same as 12 LEDs.

LED and diode lead identification

(figure a) identification - LEDS, diode, resistor, lamp, vero board,
The LEDs have two leads (a lead is one piece of metal wire); one long and one short. The short lead should be connected to the negative side of the supply. If incorrectly connected the LED will be blown up! (in this application) The LEDs convert electrical energy to light.
The diode is used to prevent damage to the LEDs from incorrect connection. The diode will not be damaged by incorrect polarity. One end of the diode body (the plastic bit in the middle of the 2 leads) will be marked all the way around the body in a colour contrasting with the body colour, this signifies the end that should be connected to the negative side of the supply.
The resistor limits the current through the LEDs, and the resistor leads can be connected in either way around. I used 350 ohm resistors to limit the current to 5.4/350 = 0.015 Amps = 15 milli amps.

A dircect plug in replacement

Take 4 LEDs and a piece of vero board. Cut the vero board to the dimension shown in figure 1. Solder the first 2 LEDs together the long lead of one connected to the short lead of the other, dress (cut and bend) the leads so that the LEDs are adjacent to one another. Repeat for the other two LEDs.
Mount one set of LEDs by soldering the long lead of the unsoldered leads to the board. The short lead is soldered to the diode anode (see figure 2 for identification) that is dressed to lay under the set of LEDs as shown in diagram 2.
Mount the other set of LEDs by soldering the short lead of the unsoldered leads to the board. The long lead is soldered to the resistor (not important as to which end of the resistor) that is dressed to lay under the set of LEDs as shown in figure 3.
Dress the remaining soldered leads of the resistor and diode so they connect and solder these together. Finished replacement shown in figure 4.
Once constructed, place them into the original holder, if the lamp does not light when it should then remove the LEDs and rotate by 180 degrees and re insert. It should now light - if it does not then check the the original lamp still lights (ie the bikes wiring is still alright?) if that is ok then check your connections of the LEDs.
If all is OK then use some 5 minute araldite to secure the leads and components to the vero board.
The colours of LEDs to use in warning lamps should match. However blue and green LEDs can be expensive so experiment!

Replacement night lighting mounted directly onto the instrument board.

I did this inside a BMW K model instrument pod. It leaves the original lamp mounts intact just incase you need them. You can spread the LEDs out so the light is more distributed around the instrument.
Construct a "chain" of LEDs by soldering the LEDs together the long lead of one connected to the short lead of the another, dress (cut and bend) the leads so that the LEDs are adjacent to one another. See figure 5.
Add the resistor to the chain of LEDs then solder the completed chain to the instrument panel. Ensuer the you connect the LEDs the right way around or you will blow them up. Mount the chain of LEDs by soldering the long lead of the unsoldered leads to the boards negative side and the other end to the positive.
Glue the other leads to the circuit board to secure the components using 5 minute araldite.

You can e-mail me at Yahoo. Dated 20 May 2001

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