Auto dimmer for LEDs

Asking here in the hope someone has come across this and may have an answer.

Car interior lights have an auto 'dimmer'. They do a graceful fade in and fade out when switched on and off. On a Merc. Owner wanted some slightly brighter lights so changed from tungsten to LED. The auto system still does a graceful fade out - but sort of leaps to on, rather than fading up. Later Mercs have got LED lighting which does fade in and out OK - but it's not just a case of changing the module which on the later cars is part of other things too.

On this model the module is stand alone, so would be possible to modify it.

Reply to
Dave Plowman (News)
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Why don't you replace the traic ;-)

Reply to
whisky-dave

Probably. I can't imagine it being worth doing. Tungstens have zero output at near half power, LEDs have zero output near zero power, hence the behaviour you see.

NT

Reply to
tabbypurr

This sounds like a simple circuit charging up a capacitor to bias on a transistor. The problem probably is that the settings used assume a lower resistance load than the led. It depends if its using someform of feedback to achieve the more smooth gradual illumination. Brian

Reply to
Brian Gaff

'cos car electrics are dc

Reply to
charles

Except for Ford model T.

NT

Reply to
tabbypurr

Is there any fundamental reason why early pre-electronic car electrics were DC and not AC?

Reply to
NY

Yes.

Reply to
The Natural Philosopher

not very relevant in the contexct of a Merc's interior lights.

Reply to
charles

to go further, the car electrics rely on a battery - this inherantly a DC device.

Reply to
charles

Quite so. The Ford T had nothing powered by the battery bar the trembler co il, and its ac system was unable to charge the battery. In fairness with no electric starter, and no load bar briefly the trembler coils at start time , it didn't need much charging, so plugging it in on occasion was workable.

NT

Reply to
tabbypurr

True, although early cars had no starter motor (and therefore needed no battery). Were there any other pre-starter-motor cars that had AC electrics?

Reply to
NY

They need not rely on a battery. If, for example, you have a starting handle and carbide lamps.

Reply to
Roger Hayter

Nowadays such things are more often done with a microprocessor. It is probably actually cheaper than the high value stable capacitor you would need tor the analogue timer. My bathroom fan does its timer this way, making modifying its behaviour a work in progress.

Reply to
Roger Hayter

You will however need a magneto

Reply to
The Natural Philosopher

On cars that used a magneto and which therefore generated alternating current at HT voltage, was the phase of the magneto "hard-wired" to that of the crankshaft (eg by gear/chain/toothed belt drive), so each pulse of the magneto corresponded to the instant when a spark was required in one of the cylinders, or was it totally asynchronous, with it being a matter of luck whether the magneto was outputting maximum voltage at the instant when a spark was required? Or did they rectify the magneto output and smooth it slightly with a capacitor to keep the HT at a high voltage irrespective of the phase of the magneto?

Reply to
NY

AC ignition could easily have been devised. Even if it actually wasn't.

Reply to
Roger Hayter

The T was handle start, carbide lights, magneto ignition & no electrical accessories. But it still needed a battery, the magneto couldn't generate a spark at crankhandle speed.

Start sequence: Switch to trembler coil ignition Crank Switch to magneto ignition

- Or shout for assistance if it just ran you over.

NT

Reply to
tabbypurr

No, actually it couldn't.

How you gonna time a spark if your AC happens to be zero at that time?

Remember even rectifiers were hard to come by then.

The solution was the magneto. Whether you call that AC or DC is debatable.

Reply to
The Natural Philosopher

Magnetos have the magnet in the edge of the engine flywheel, producing one timed spark per rev.

Trembler coil ignition ran all the time, resulting in very ropey spark timing & rough running. Hence it was just used for starting.

NT

Reply to
tabbypurr

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