Electric question...

[jackb_ms]

Hello All

Could someone check my math?

The high beam LED signal on the dash is too bright.

It uses 40mA.

If I'm correct, fitting a 320 Ohms resistor in series should drop the brightness by approx 50%?

Jack

[jbcollier]

Electronics is not my specialty but LEDs do not work like incandescent bolts.  They do not just dim with lower voltage.  Below a certain threshold point, they simply won't light.  You will have to experiment or just fit a lower output LED.

[Jonathan Kay]

We once tracked down the part numbers and alternatives for the instrument lamps: should be in the archives.

Tape is also available.

Jonathan

PS: My mother's Stiletto had little eyelids that rolled over the lights to dim them.

[K7 VCT]

Just put a small pice of black tape over it.

I used a hole punch form paper to stamp.pne out of so tape .... it perfect now 

[revilla]

40mA is a heck of a lot for a modern LED, no wonder it's very bright.

With the engine running the supply voltage will be about 14.4V. A typical blue LED has a forward voltage of about 3.4V (blue and white LEDs have much higher forward  voltage than other colours due to the chemistry) leaving 11V dropped across the series resistor (which may be built into the LED if its a "12V" type must be there somewhere). That means the existing series resistor must be about 11/0.040=275 Ohms.

So to halve the current fed to the LED you need to double this, or add another 275 Ohms.

You suggest adding 320 Ohms. This will give a current of 11/(275+320)=0.0185 or 18.5mA, slightly less than half the original but close.

One thing I will add though; subjective perceived brightness is quite different to current drive. Modern blue LEDs can still appear quite bright to the eye, especially in the dark, with much lower currents, say 5mA. That would require an additional resistor of (11/0.005)-275=1925 Ohms. So you may need to play around with resistance values to find what works for you.

I've shown the way I've calculated the values above so you can plug other numbers into the formulae if you need to.

[Andrew 21 Edney]

Just to say that the a small circle of black insulating tape was the solution I used for the same problem (not sure if Jack remembers that...)

Andrew