Hi. As I try to learn electronics, through reading plus trial and error, I thought building a desklamp would be nice small project for learning. Lamps body is ready, I've brought a 5050 led strip and that's when the trouble started. Before all of this, I've tried to consume many info on electronics as I can for a beachhead.

What I used is 5050 led strip of 15 cm (9 leds with 150 ohm resistors)
Label and the owners site only says 12VDC and 14.4W/meter max.

I've tried using 4 1.2V batteries for making the lamp mobile but didnt even lighted up. Found an 12V 1A adapter, plugged it in, overheated quickly, plugged it out. Although I would liked to wait for it to create a fire hazard.

12 V Lights up whole 5 meters, so why using batteries for 15 cm is not enough?
12 V for 500 cm, also 12V for 15 cm, why is that?
I also need to find out how much amp each cell needs for assembling power supply. After finding it, should I multiply that amp with the number of leds? Because that doesn't gives me reasonable results, what am I going to do with amps?

I've searched for an answer but didn't managed to find one and started to experiment. Used batteries to light up individual parts of one led so looks like there are no 12V minimum to pass electricity on them to produce light. I've burned and dissected many leds and wasted so much on searching and still didnt found the answer. How the hell am I going to light up this simple led strip effectively, what am I doing wrong?

I'm feeling really stupid for not being able to create, even understand the right power supply for a basic led strip.

 

Welcome to AAC!

It seems that the LEDs are designed to operate from 12V. If it's 14.4W/meter, 5 meters would require 72W. That translates to 12V @ 6A.

Your 1A power supply was being overloaded.

15cm should run fine from your 12V 1A supply. If you want it to operate from batteries, you need to supply 12V.

 

If the LEDs (and their series resistors) are wired in parallel, then you will need the same voltage regardless of how many LEDs you use.

 

Welcome to AAC!

It seems that the LEDs are designed to operate from 12V. If it's 14.4W/meter, 5 meters would require 72W. That translates to 12V @ 6A.

Your 1A power supply was being overloaded.

15cm should run fine from your 12V 1A supply. If you want it to operate from batteries, you need to supply 12V.

The missing math:
72W/12V=6A

 

Hello,

The ledstrip will likely have markings where you can cut the strip into pieces.
When you do not respect the markings, you can get unwanted effects.

Bertus

 

Looks like a nice community.
Thanks for the answers.

Dl324: 12V 1A adapter overheats the 15 cm strip quickly so I had to unplug it. Doesn't that means too much amp is running through it?
Also, it was not an adapter for led. Although I'm not sure how much difference it makes.

MrSoftware: But how about voltage drop or running too much power through it? Resistor limits it but I don't feel that's enough. It's not going to be significant for 15 cm, but even we wire it parallel running 12V through the strip and getting the same results for 15cm and 5 meters is a thing that looks odd to me.

Shteii01: Thanks for the formula.

Bertus: I've respected the markings, probably getting odd results because lack of info.

The one of the things that boggles my mind is, should I design custom adapters for each strip length or is it unpractical?
Btw, if you know a good source that can explain these concepts to a begginner, that can be good too.
I'm probably asking stupid questions and not explaining myself well, but there is much info to digest.

 

The battery is no mystery. 4 x 1.2V = 4.8V which is not going to light up 12V LED strips.

The 12V 1A supply should work okay with the 15 cm strip, but it might not depending on the type of adapter.

Some adapters marked 12V 1A mean that it will provide 12V when 1A is drawn, but could be significantly higher, like maybe 15V if less than 1A is drawn, in other words, it is relying on the voltage dropping at full current. This is normally the older type of supply that uses a heavy transformer. The newer tiny lightweight ones are normally regulated switching power supplies which will provide 12V no matter what current is drawn.

Based on your 14.4W / m and 15 cm, that shortened strip should use 2.16 W or 180 ma. So I think you have an old unregulated adapter and it is putting out a voltage higher than 12V which is why they are getting hot.

But also, be aware, "hot" is a relative term. The will warm up when used at the correct voltage, the question is whether the heating is excessive.

Bob

 

When engineering a supply, always build it to deliver at least 1.5 times the power you will need. If you need 1 amp then design for 1 and 1/2 amps. The safety margin is what keeps your electronics running efficiently and cool. To assume a 1 amp supply will deliver 1 amp - well, yes, it will. But it may be at the upper limit of its ability, and can warm up.

Also, your supply may state it's capable of 1 amp, but who knows the truth? It might not be able to. I've seen supplies rated for service that could not handle what was stated on the labeling. That's the nature of buying things from outside of the country. (won't name names).

I typically use the 150% rule (1 1/2 times) but others like two times. That's fine if you want to build that way. My opinion is that it's overkill. But overkill means your project will work. Under-kill - well, you're likely going to kill something like your 1 amp supply.

 

BobTHP in post #7 pretty well nailed it for you--your so-called 12 volt adapter really puts out much more than 12 volts if it is lightly loaded. Your load of 180 ma is only about 20% of its full load.

If you have a multimeter, measure the voltage with nothing attached to it. Suppose it is 14.4 volts. You need to drop 14.4 to 12 so you want to drop 2.4 volts at 180 ma. R = E/I (I think we are supposed to write it R=E/A nowadays). That is 2.4/.18 = 13.3 (the 180 is mA so you divide by 100 to get amps.) Round up to the next easily available resistor which is 15 ohms. Calculate the wattage you will need by watts = volts x amps which works out to 2.4 x .18 = .42 so buy a half watt, 15 ohm resistor to put between in series with your 15 cm LED strip and your transformer will work. Of course, my figures are just a guess--you will have to measure and calculate the proper resistor for your circuit.

If you don't have a meter get one. In the US Harbor Freight sells one for about $7 and they sometimes give them away with any other purchase.

 

Great answers.

BobTPH: It would melt the thin plastic on the strip and render the strip unsafe pretty fast, but i would like to see how much heat the parts can take. I'm going to measure and dissect the adapter to see what more I can learn from your advice.

Tonyr1084: 1.5 rule looks pretty solid, I will do it that way. Although I would like to kill them to see the potential of jury rigging.

whitehairednovice: I guess it's time to buy a multimeter and measure everything.

Thanks again for the enlightening answers everyone!

 

Keep this in mind: If you have a circuit that draws 1 amp and you put a 1 amp supply on it - you're going to run hot. But if you put a 2 amp supply on it - the circuit will still continue to draw only 1 amp. Even if you put a 1 million amp supply on it - the circuit, by ohms law, is still going to draw only 1 amp. Putting a larger supply doesn't mean everything else will run hotter. It's like the difference between you taking a drink from a glass and taking a drink from a 5 gallon bucket. You can still swallow only what you can. You won't drown just because you're sipping from a 5 gallon bucket.

A power supply doesn't force current through a circuit, it provides the current the circuit wants. Provided the supply is large enough to handle the need. So having a larger supply won't harm your circuit in the least. As to your comment about melting plastic - I'm not sure I understand what you were speaking of. If you can, please clarify.