Instrument lights

[simpo one]

The posting about the recently-discovered fan switch led me to this: my Griff has a round flat black button near the steering column. It's a locking push-to-make switch and when you press it - voila - all the instrument lights (speedo etc) go out.

I cannot for the life of me imagine why anyone would want to do this. Even racing after dark you'd still need to see revs, oil pressure etc. It would be far more useful to make it a rheostat so I can adjust the brightness of the illumination. Anyone tried this, or know why the switch was fitted?

[shpub]

It is a dimmer... just it has two settings. Stick a high wattage variable resister in series or a multiway switch with different high wattage resistors or create a PWM box to act as a dimmer, or a vraiable constant current source... or treat it as another little TVR eccentricity.

Switches are more reliable and don't get very hot compared to rheostats.

Steve

>> Edited by shpub on Wednesday 17th July 12:13

[MajorClanger]

quote:I cannot for the life of me imagine why anyone would want to do this.

As far as I know, and it's how I use mine, it's for when you wish to leave your side lights on when parked and don't want the instrument lights left on. Apart from not using up the battery (and I'm not sure how much all the instrument lights would actually use up any way) it doesn't draw quite so much attention to the car from sticky little fingers!

MC

[simpo one]

'Stick a high wattage variable resister in series or a multiway switch with different high wattage resistors or create a PWM box to act as a dimmer, or a vraiable constant current source... or treat it as another little TVR eccentricity.'

Blimey, given that lot I'll think I'll take the eccentricity. I might try that sort of tinkering on loudspeakers but not when it's attached to a TVR!

On the rheostat/switch reliability matter, the BMW has a perfectly good little thumbwheel - a rheostat I presume - that does the job and has done for 9 years. But of course put it in a TVR and it would instantly break....

What's the difference between a rheostat and a potentiometer anyway? I think of one as wirewound and one as carbon track, but they are both variable resitors.

[shpub]

quote:
What's the difference between a rheostat and a potentiometer anyway?

1. About £2 at Maplins.
2. About 40 points depending if you can get the P on a triple word score...

Rheostats imply big heavy heating element things that can dim mains without flinching. Potentiometer is a variable resister that is usually designed for lower power uses...

AS for the BMW thingy... bet it's hooked to a electronic box that does the current limiting.

Steve

[simpo one]

Always amusing to read your replies! Remember those big slidy rheostats in school physics lessons? They were wirewound and about 10" long. I'll fit one of those bastards...

[Guillotine]

pot - potential devider, is a a variable resistor with the wiper taking a sliding value of the whole across its length.ie it is a two pole device.

rheostat is a three pole device where the min / max AND a division of the whole is required.
usually used in high current,greater accuracy applications as superior construction is.....zzzzzzzzzz. zzzzzzzzzzzzz

[simpo one]

So if I stick some Germanium in a rheostat I've made a big transistor?

[shpub]

quote:
So if I stick some Germanium in a rheostat I've made a big transistor?

Who do you suggest? Schumacher? Boris Becker?

[simpo one]

'Who do you suggest? Schumacher? Boris Becker?'

Hadn't thought of that. You could use one of their legs for Emitter, the other leg for Base, and as for Collector, well, it makes your eyes water....

(and I gave up electronics at 14, nothing I made ever worked!)

[ianwayne]

You may not believe it but I still use those 'huge' rheostats at work!
When testing aircraft power supply systems in a workshop, you need them 'cos of the high currents and / or 3 phase voltage requirements. Its a bit "throw the switch, Igor!" sometimes