Dim instrument lights

I know there have been a number of threads here about the dim instrument lights, and my Boxer instruments were completely unreadable at night, so yesterday I took a look and found a remedy.

The dimmer comes out very easily having removed the Tach. (no need to remove the steering wheel, although I couldn't get the speedo out without removing it) amd is certainly the culprit. It's resistance varies from, theoretically, 0 to about 10ohms, however mine wouldn't go below about 1.5 ohms.

There are 10 instrument lamps, each 3W, so a total of 30W @ 12V = 2.5A, thats a 3.75V drop at max brilliance...not good. A lamps brightness is not linear, but some kind of square or exp. law (sorry, can't be botherd to check) so every volt is worth fighting for.

The dimmer is just a wirewound rheostat, and the contact slider on mine was not contacting where it should; there is a small 'dimple' right on the end of the slider, this should contact the element. Mine was contacting half way down the arm. The reason the dimmer won't go below 1.5 ohms is a) the slider doesn't go all the way to the end, b) the end wire seems to be made of the same resistance wire as the element. So I soldered a small piece of copper wire to short out the first three element windings, and took the wire back to the input screw and tightened it down. The windings are not easy to solder to...had to scrape them well and get them hot.

The result is a dimmer that now goes down to 0 ohms, and up to about 8 ohms...plenty high enough! (2.5A x8 ohms = 20V...it really shouldn't be necesary to dim the bulbs past about 4V or they will basically 'go out', so 12V-4V=8V, @2.5A ...around 3 ohms would be more than enough: I might put a resistor across the dimmer to reduce the resistance to this, but it would need a high power rating....2.5A @ 8ohms is 50W!)

So put it back in the car...and I can now see the instruments, all beautiful and red! Maybe still not as bright as modern cars, but perfect for me at night (I don't like them too bright), and the dimmer still works.

It should be possible to replace the bulbs with a lower wattage, which would make them brighter (sounds like a contridiction!) or use LED's. Incidentally, you can dim LED's just the same as bulbs, but the current dimmer would not be suitable, though it's a very easy fix with a cheap Pot. and a single transistor.

Just thought I'd share.

Robert

 

Unfortunately I've put it all back in the car, but its a very simple job, 20 mins start to finesh.

LED's are current bassed devices (mind you; so are the bulbs with a series pot!) hence my comment about using the transistor with the pot to a) take the power load off the pot, and b) to build a variable current source.

Probably worth doing considering the power dissipated under the dash at the moment: 30 W is a lot of heat. However, does help out the poor heater in the Boxer!

 

an internet search on "automotive LED's' will bring up pages of them.

However, some look a little suspect (very high wattage?) and it's important to select the best viewing angle and colour....quiet a bit different to bulbs.

I use very high intensity LED's in my amps, and 3V @ 20mA is all they need.

The Boxer lights are red, so a red LED would probably be best, ie most efficient.