What happens when a transformer is below the power consumption of an LED?

8 Jun 2017
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United Kingdom
I have installed a run of 5m LED strip lighting, rated at 24w (300 x SMD 3528 leds for those interested).

However, I realise I made a mistake and connected it to a 12V DC transformer rated at 12W. The lights are in the kitchen and have worked fine without any noticeable problems.

What risks are involved with this? I am surprised the lights turned on at all but they seem ok. What am I missing?

I have obviously disconnected the LEDs until a replacement transformer can be found. Just interested!

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It will probably run for an amount of time, but it will soon overheat and cut out. This could be a fairly long time, or a matter of seconds. You'll find the driver would become very warm, very quickly which is obviously not idea.

You might also get some flickering/flashing as the driver struggles to cope.

I've done that myself in the past (by adding lengths of tape to the same driver over time and not keeping track of the wattage installed) and when that driver overheated they would flash on for a second and then off again. Better quality drivers will cut out and stay cut out until they cool down
There are 3 likely results :
1) The driver will run the excess load, and as stated above, will eventually fail through overheating.
2) It will self-limit to it's design current, and with a load like LEDs the result will be that they'll not be at full brightness. I have a power supply designed for float battery operations which will supply up to 11.something amps. If you run the battery down (eg during a power cut) then it will reduce it's output voltage to limit output power to it's design limit - restricting recharge rate but allowing the system to keep functioning.
3) It will simply "trip" and turn off to protect itself. Some will trip and stay off until the supply is disconnected, others will keep trying (sometimes giving repeated brief flashes).

In your case, 3 is ruled out as the lights came on. You can only differentiate between 1 and 2 by checking the manufacturer's specs or by measurement. Sadly, while it is trivially easy to build in protection to a supply, there are lots of poorly designed devices without such overload protection.

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