Faulty LED

Is there anybody in the forum who has knowledge about 12v LED lights I recently purchased lights from eBay only to find that the taillight did not work ,it did work when connected to the wire for the brake lights hence the bulbs are ok it just doesn't work in the dimmer taillight mode all the other functions i.e.indicators,reverse lights work ok I have checked the wire which connects to the circuit board and it seems ok ,as a mechanical engineer my knowledge of electronics is limited any help would be appreciated ,I hate to discard an item which may be easily repaired

Some LED bulb replacements are dimmable and some aren't. Look for "dimmable" in the description. Has to do with their built in power management system.

With a DC supply they are all dimmable - simply by reducing the current with a series resistor. Clearly the resistor is duff or the connections are poor - dry joint?. That resistor is bypassed for the brighter illumination.

LEDs start to pass current at about 0.6V, so all need a current limiting resistor on a 12V supply - they would simply thermally run away without a current-limiting resistor.

More info would help and I fail to see why a circuit board is involved unless you just mean the tracks in the light cluster. You appear to have purchased replacement LED lights for a car, or at least a vehicle that is not a motorbike as you seem to need more than one of each. A tail light is no different to a brake light so far as putting 12V across it. The brake light takes more amps/power and is brighter.

Does the suspect LED work in the other tail light socket? Is the other tail light illuminating in both sockets. With limited info it is only possible to say the following. A conventional filament bulb takes a lot more power than your LED. If the other LED works in both tail lights and the suspect LED works in neither then you have some form of duff LED. If the one socket refuses to light the LED then that socket has a fault in the form of a bad wiring connection.

A bad wiring connection may well be overcome by the demand from a filament bulb. It may well arc across the connection and seem OK for some time, even if it gets hot/warm locally.

Do some swapping about of bulbs in sockets where possible and you should solve it.

Dennis

Are you sure that the socket for the tail light and the stop light are the same ? Some car bulbs are single centre contact. On these the connections are the single centre contact and the outer metal. The other type are small bayonet type and they have two have two contacts. The outer metal is not used as a connection on that type. There also combined stop and tail lights that use two contacts and the metal casing but you do not seem to have any of that type.

Les.

I'm not sure what a dimmable brake light is but I can supply some information about LED brake lights.

Filament bulbs, when they get hotter, increase their resistance and this reduces the increase in current that comes from an increase in voltage. This is quite useful when the 12V supply of a car can vary from <12V to >15V.

LEDs in contrast have an exponential increase in current with increasing voltage once the threshold voltage is exceeded. Because of this most LED brakelights include a constant current regulator to keep the LED at its maximum safe current no matter how the 12V supply varies.

If you have a variable dimmable voltage supply fighting with a regulated constant current supply there is a good chance that the constant current supply will lose.

LED lights designed to dim with a variable voltage will either have very complicated electronics or very crude and inefficient resistor regulation where ~90% of the power to the LED is dissipated in a resistor.

But I suspect you may just have the wrong pins for the socket (as well as a dimmable problem).

Good Luck. Colin

A bit more information would be useful:

1/ Make & model of vehicle

2/ Make & part number of lights and/or link to ebay listing.

3/ Why are you changing them, did the old ones work OK?

Note that unless the lights are from a reputable supplier with "E" markings they are almost certainly illegal for use on the road.

Robert G8RPI.

Side lights, brake lights are all driven by the supply voltage of the car, in your case 12volts. You have a connection problem, not a voltage problem. As others say, check the socket / pins on your bulb and especially, check the earting of the socket to the body of the car. If its an old car, this would be my first thing to investigate. On my 6 volt cars, all electrical equipment has its own earth wire.

BobH (Austin 7 and 2CV)

Have you got the polarity correct. Leds are diodes, so must be connected correct. Try using two piece of wire and your 12 volt battery together with your Leds, first one way then the other, if you see what I mean. John

As David said at the start 'it did work when connected to the wire for the brake lights hence the bulbs are ok it just doesn't work in the dimmer taillight mode', I'd agree with Jeff's reply as amplified by Colin. It sounds very much as if David has a non-dimmable LED plugged into a dimmable circuit. Solution for that is to use an ordinary filament bulb in the tail-light or to find a dimmable LED equivalent.

The fault could be due to some of the other suggestions but I'd start by swapping the bulb because that's easier to do than chasing down earth faults, dud electronics, polarity issues etc.

Why is nothing is ever easy?

Dave

Perhaps the new lamp is faulty.

This is not always true for modern cars. Typically the lights are driven via electronic switches that measure the current and continuity of the filament for failure detection. LEDs mess this up. Some even use PWM (switching on and off very rapidly) to change the brightness. I've only seen this used on DRLs and originally fitted LED light though.

Robert G8RPI.

Edited By Robert Atkinson 2 on 18/07/2019 12:18:43

In the 1970s the family had a Triumph 1300 (?) with these. When you turned on the headlghts, a relay switched a resistor in series with the brake lights. Reduced dazzle to the driver behind. I'm sure the technology has moved on a bit.