12v Desk Lamp LED Bulb problem

[spike47]

Have a desk lamp that will not work with a LED cob Bulb which was in it ( The LED bulb just lights and goes straight out ) , but have put a halogen bulb in and it works fine ! .
anything I can do to make it work with a LED cob bulb! .
Spike

 

[Externet]

A LED bulb must have a resistor as current limiter built-in or external to work in the same socket an halogen bulb works. Even when polarity was respected, there is a good chance your LED is very blown milliseconds after powering it.

 

[augustinetez]

Is the 12V supplied by a transformer or switch mode supply?

If it is a switch mode supply, most likely is the LED cob doesn't draw enough current to keep the supply happy and it is shutting down.

 

[spike47]

A LED bulb must have a resistor as current limiter built-in or external to work in the same socket an halogen bulb works. Even when polarity was respected, there is a good chance your LED is very blown milliseconds after powering it.

Hi
No it is still working ok on another 12v supply.
cheers
Spike

 

[spike47]

Is the 12V supplied by a transformer or switch mode supply?

If it is a switch mode supply, most likely is the LED cob doesn't draw enough current to keep the supply happy and it is shutting down.

Yes thought that, not sure what I could do to get the LED to work
, other than to get a seperate LED driver.

cheers
Spike

 

[spike47]

A LED bulb must have a resistor as current limiter built-in or external to work in the same socket an halogen bulb works. Even when polarity was respected, there is a good chance your LED is very blown milliseconds after powering it.

Yeh it has got quite a few resistors built in on the COB LED Bulb

 

[spike47]

Is the 12V supplied by a transformer or switch mode supply?

If it is a switch mode supply, most likely is the LED cob doesn't draw enough current to keep the supply happy and it is shutting down.

Switch mode I believe

 

[Visitor]

Was the lamp built for an LED bulb or for a halogen bulb? If it was built to take a halogen bulb, it may have an AC output. A 12V LED COB will be expecting 12VDC.

 

[augustinetez]

OK, you'll need to get the multimeter out and check if it is AC or DC 12 V as per Visitor's comment.

If it is DC, as a test, you can wire a resistor in parallel with the LED cob to increase the current draw - start with a 120 Ohm 5 watt resistor - it should draw an extra 100mA in addition to the LED current.

If the LED stays on, then you have the solution, although some of the advantage of moving to an LED is lost because of the increased current.

On the other hand, if the supply is AC, I would expect the LED to run at ~half brightness, rather than come up bright and then go out. The fix for that is to add a bridge rectifier to the supply to convert it to DC.

A picture or two of the actual desk lamp would help too.