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Why diode with lamp ?

Why diode with lamp ?

(OP)
The rear roof light in grandsons car has a diode connected in series with a 10watt filament lamp. As the is a 12 dc circuit I’m at a loss to see what duty this diode has. It is clearly forword biased as the lamp works, reversed bias = no light.
Can someone offer an explanation please.

RE: Why diode with lamp ?

Perhaps to drop the voltage a bit? The PO might have experienced the bulb blowing too often.

RE: Why diode with lamp ?

Does it look like an original installation, or does it look like a retrofitted hack job?

RE: Why diode with lamp ?

You say "rear roof light". Is this the rear center-mounted brake light? If so, it might have to do with not being a sneak-path to disrupt the proper operation of tail lights when the same bulbs is used for both brake and turn signals.

I have also seen diodes used on bulbs when there was a dimmer circuit used to dim dashboard indicators that were also connected to switches to activate devices and options. Again, the diode makes it possible for proper operation of the bulbs and also allows for the dimmer circuit to control brightness properly.

RE: Why diode with lamp ?

There are only so many wattage bulbs available. Could be the maker thought standard was to bright and harsh. A diode would carve off about !5% of the light output dimming the lamp to 85% of normal.

Oh and it will last 2.2 times longer.

But, I agree with Com, it's likely for some logic function preventing back feeding. I had to add diodes for an alarm system that detected door openings using the domelight switches.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Why diode with lamp ?

(OP)
Hi to all.
The car is a current VW Golf. The diode is an original item being crimped into the plated steel lamp fixings. The light is in fact the boot/ trunk light mounted in the rear roof trim and illuminates above the parcel shelf. There is a requirement to add additional leds below shelf.
The existing light would seem to be controlled by a simple micro switch actuated by 5th door/ boot/ trunk.
The existence of the diode is just a curiously point. Switched +ve connected to diode connected to filament lamp connected to solid -ve. While all comments are valid none seem to be a good reason for VW to throw in a diode ??

RE: Why diode with lamp ?

..."would seem to be" ... but is it? Especially considering that we are talking about a current model VW here.

Chances are, the hatch switch is an input to a lighting module and the lighting module controls that lamp. Among other things, this allows the electronics to turn off the interior lighting after a period of time with the hatch open in order to avoid killing the battery.

It still doesn't explain the diode, but it may have something to do with someone wanting to fine-tune the brightness of the lamp, and opted for a diode for a calibrated voltage drop.

RE: Why diode with lamp ?

The diode in this case would be used to isolate the light from another, it may have a different switch on some models to let just this light operate or multiple lights from a different switch.

RE: Why diode with lamp ?

I have a Volvo which will tell me if any single lamp in the car is burned out, and it is old enough that it does not use an electronic control module. It has diodes on many lamps, I presume for sensing the loads in nice clean steps.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Why diode with lamp ?

That's a common way of checking 4-20 mA instruments. Connect the ammeter across the series diode and the current diverts from passing through the diode to meter because it has less Voltage drop and there is no interruption of the signal.

RE: Why diode with lamp ?

(OP)
Hi Forum. Thanks for those constructive comments, all of which relavent but dont quite satisfy my curiosity. When and if I find a better reason I shall finalise this tread.

RE: Why diode with lamp ?

Short across the diode and see what happens.

RE: Why diode with lamp ?

Quote (asymptote)

The existing light would seem to be controlled by a simple micro switch actuated by 5th door/ boot/ trunk

Nothing on a current-tech VW is controlled by a simple mechanical switch turning current on and off- everything is run through ECMs of various types.

I can't speak for every other manufacturer- I own 2 modern Volkswagens and have been elbow-deep in both of them many times, but I would assume everyone else is using similar architecture.

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