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I installed the LEDs and made some measurements last weekend. The LEDs are a huge improvement over the old bulbs but they make the dirt on the inside of the gauges' lenses obvious so I'll have to take them apart and clean them!
The rheostat puts out 0 V when turned fully counter-clockwise. Turning it clockwise, a click can be heard and the voltage jumps to 9.3 V. At fully clockwise, the voltage is 12 V. This is with the engine off so it'll be higher when the car is running. The LEDs come on at approx 7.5 V so they're already pretty bright when the dimmer switches on at 9.3 V. There is some dimming through the range of the rheostat but not much. Still, it's good enough for my needs so I'll leave it alone. If one wanted to increase the dimming range, a PWM circuit would work but I would personally use a power op-amp that scales the output of the dimmer to suit the LEDs.

Thanks for posting you results. I think that confirms that the dimmer in the light switch is not consistent, and depending on the condition it dims different. As an example, the LED's in all my 3 cars (71, 82, and 87) don't turn off complete as yours.

Thanks again
Engelbert

I'm afraid I wasn't very clear - the LEDs start to come on at 7.5v when tested with a variable power supply. In my 1970 911T, the dimmer's voltage range is 9.3 to 12v, so they're pretty bright at the lowest setting. They get a bit brighter as the dimmer is turned clockwise, reaching 12v at the end of its range. This means that they never go off, that they vary between bright and very bright, which is fine for me.

OK, clear now. I also tested the voltage at the dimmer in the 87, that the car which dims the least. With Battery power only, I got 11V at the lowest and 12.1V with the dimmer full open. On this car is not much difference between low and high.

I got a 250 Ohm, 5W dimmer and connected it between the switch and the cable to the instrument lights. With the light switch dimmer full open I can now dim the LED's from complete off to bright, with a nice range.

Here is a video with the additional dimmer.

<iframe width="560" height="315" src="//www.youtube.com/embed/70znMNb0U0M" frameborder="0" allowfullscreen></iframe>

Cheers
Engelbert

So is the 5w dimmer a separate component with it's own dial/rheostat?

Looks like this dimmer has its own rheostat separate from the Porsche light switch rheostat, right?

Yes, it's a separate dimmer. Like I described in the post above, I connected it between the light switch and the cable that goes to the instrument lights.

Cheers
Engelbert

Correct !

Cheers
Engelbert

Do you sell this too? Can you post a pic of how it looks please?

I think your LEDs dim enough on there own, complete black out does not seem necessary :eek: at least for me.

There was a lot discussed about dimming, so I gave it a try and it worked great. I did not install the dimmer since I don't need to dim them down that much at this time. I just want to proof that it can be done, in case it's required.

I got the dimmer 250 Ohm, 5W from Didgikey. I can post the detail if needed.

Cheers
Engelbert

Perhaps a short wire extension with a fuse and the dimmer in line would be a nice upgrade for the instruments and also protect the instrument light circuit.

Please post the dimmer part number if you could.

LOL, funny how we are complaining about dimming the gauge lights down :rolleyes:

ain't that the damn truth.

Is this the one you got from Digikey?

http://www.jkllamps.com/pdfs/ZDM-01.pdf

Hi Porwolf, from the sounds of it Rothaus has just used a 250 Ohm 5W Potentiometer (variable resistor) to drop the volts fed to the illumination line that gangs all the LEDs together. I think I may try this alongside the in-line fuse.

Cheers, Neil

This is the dimmer I used from Didgikey p/n 026TB32R251B1B1 is $8

http://airboxer.com/images/dimmer.jpg


I was thinking making it in a package with pre-wired and mounting plate, but I think that there would be enough demand. So if someone want to dimm the LED's down like the standard bulbs, this dimmer can be used. You need also a knob, that's another $8 or so.

Hope this helps SmileWavy

Cheers
Engelbert

I did the same using a similar pot of 500 Ohm value. I also installed a 5A fuse in line with the pot. The result is bright to extremely dim from one end to the other of the adjustment. The variation isn't linear with the turning of the knob but it does work really well. There was already a hole in the dash that now has a nice black adjuster - very happy with the LEDs and being able to dim them.

Cheers, Neil

Make it stealthy non-intrusive bluetooth adjustable and I am sold :-).

I am just not diggin the idea of a secondary dimmer hardware. I have enough electrical components to worry about. My 2 cents.

There should be a rather simple electronic solution besides a PWM, to mimic the 500 ohm potentiometer regulating the LED current with the factory 5 ohm rheostat. Any ideas? I will think about some solution also.