12V 2A plugpack burnt out while powering small led strip

[dashy1981]

HI,

I had ordered a LED strip : http://www.ebay.in/itm/5M-LED-Strip...31645865522?ssPageName=ADME:X:PPPSBRE:IN:1123

I cut it into two parts around 3.5m and 1.5 m in length. 3.5 m strip I connected with 12V adapter which came along and its working fine.

When I connected 1.5 m strip with the another similiar adapter, after 5 mins it burnt out and fumes were all over. Luckily I was around and saw it happening otherwise it could have been a mishap.

So the question is, Why it could have happened. IS it due to the fact that I reduced the effective resistance(by shortening the strip) while keeping the same rated power source. With lower resistance it could have drawn higher current and it might have caused the burn out ?

If I am correct then should I have a power resistor in series to avoid this ?

Any suggestions or analysis are welcome.

Cheers
Dashy

 

[BobK]

The strip has cutting points at every 3 LEDs. Hopefully you cut it in the right place. A 1.5m strip will require 600mA at 12V. It should not have destroyed a 2A power supply if connected correctly. I suspect you had a short.

Bob

 

[dashy1981]

I guess the point I cut was correct as it worked for sometime... around 5-7 mins with those connections.

You may be right about short, as at some places I had strip overlapping(it was put in a groove cut at the periphery of a rectangular tea table).

But I need to get one basic fact clear. Please help me understand, and correct me where am I wrong in below understanding:

Power supply 12v, 2A
Working Current/Meter: 0.4A
5M LED strip : Needs 2A(5*0.4)
1.5M needs: 600mA

- Now If I use 12V,2A supply then we are supplying more current then needed. Would it not burn out the strip ?
- Other doubt it regarding ohm's law: LED strip is a load for me which is equilant to a resistor. So if I cut short the strip I effectively cut short the resistance. Now with V being same(12V,2A) if R decreases then I increases(I=V/R). So I should limit current using a series resistor ?

These points always confused me while I was having basic electronics course. Now I am trying to put use to the principles

 

[Harald Kapp]

Now If I use 12V,2A supply then we are supplying more current then needed.

Aahh, again that common misunderstanding:
A voltage source provides a voltage as rated on the label. The load (LED strip here) will draw as much current as is needed (600 mA here).
The rated current of a voltage source (power supply) is the max. current this supply can deliver without being damaged or shut down or reducing the voltage or whatever protection mechanism there is. It does not mean that this power supply will force a current of e.g. 2 A into the load.

According to Ohm's law (many, but not all loads cane be considered to obey this law - your LED strip is strictly speaking not an ohmic load, but can be approximated by one for this purpose): I=V/R (current = voltage/resistance). You can set two of the three values (e.g. voltage and load resistance), the third will follow according to the equation.

So I should limit current using a series resistor ?

The current limiting resistors are an integral part of the LED strip. There's no need for additional resistors. Otherwise it were impossible to cut the strip to length as it is advised in the instruction manual.

 

[Harald Kapp]

When I connected 1.5 m strip with the another similiar adapter,

Check that other "similar" adapter. Not every 12 V adapter puts out 12 V. Some have a much higher voltage when the current drawn is small compared to the ratings. Also make sure that pther adapter has a DC output. An AC output will destroy the LEDs due to high reverse voltage every second half wave. Can you upload a photo of the "similar" adapter's type label?