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I'm new to electricity so don't judge me too much. I need to power 5 watts of led strip. Is there a difference between a power supply and a transformer? I found a transformer in my local hardware store and it says: connecting power 10-50 watts, can I use it to power my led strip or do I need to look for suitable power supply?

This:

enter image description here

Or this:

enter image description here

Edit: The led's are 12v 7.2w per meter the power supply shown in picture is 12v 12w, I'm actually building a light pad and searching for a simple way to power led's

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You haven't provided much info. First white thing is labelled as transformer for "LV Halogen Lamps". Most likely it's just a step down transformer which will fry your christmas LED strip during reverse polarity.

Second one is a mystery box without much info. I wouldn't go for that either.

Here is what you need to do:

Figure out what voltage is required by your LED strip - 3V, 5V, 12V or something else?

If the LED strip is 5 watts, then you can calculate the current requirement as follows:

Current = Power / Voltage required.

Ex- If LED strip needs 5V, then you will need 5W/5V = 1A of current.

Go to the market and ask for a 5V 1A(or higher) wall adapter. Modify the voltage and current accordingly. I am just giving an example.

Get something less intimidating like this:

wall adapter

EDIT after the question was edited:

Figure out what length of strip you want to use. I will assume 0.7 meters which will give you total power requirement:

Power = Length X Power required per unit length = 0.7 X 7.2 = 5 watts.

Voltage = 12 V

Current = Power / Voltage = 5/12 = 0.42 A

Better to go for 12V 0.5A (or higher) wall adapter or any DC power source. Make sure it's a DC power source and not just a step down transformer. If the power source is DC 12V 12W, then it will do the job without any issues.

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  • \$\begingroup\$ The led's are 12v 7.2w per meter the power supply shown in picture is 12v 12w, I'm actually building a light pad and searching for a simple way to power led's \$\endgroup\$ – Giancarlo Dec 21 '15 at 17:27
  • \$\begingroup\$ @giancarlo what's a light pad? Like a lighted drawing table? \$\endgroup\$ – Passerby Dec 21 '15 at 17:46
  • \$\begingroup\$ @Whiskeyjack What if I use a simple socket plug and connect wires to the power supply? \$\endgroup\$ – Giancarlo Dec 21 '15 at 17:47
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    \$\begingroup\$ In most of the LED strips, they have the resistors soldered on the strip itself. Resistors are important otherwise there is a good risk of burning the LEDs. See this image. You will see black resistors soldered next to LEDs. You just connect the power supply and you are good to go. \$\endgroup\$ – Whiskeyjack Dec 21 '15 at 18:23
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    \$\begingroup\$ To backlight negatives for a camera system you will need very even diffusion of lighting. LEDs vary in intensity and colour even within the same batch. You will notice hotspots and coldspots even with a diffuser. You may be able to demonstrate this by laying a sheet of office paper over the LED strip and see the differences in colour and intensity. High quality backlights used in industrial vision systems have the LEDs matched for intensity and colour. \$\endgroup\$ – Transistor Dec 21 '15 at 23:43
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LEDs require DC (direct current), not AC (alternating current). A transformer on its own will provide AC only.

If it's the standard 12 V self-adhesive LED strips which can be cut every 100 mm or so along the strip then the internal circuit is most likely similar to that shown below.

schematic

simulate this circuit – Schematic created using CircuitLab

Each LED requires 3 - 3.5 V so three of them can be powered safely from a 12 V supply. The resistors limit the current to a safe value for the LEDs. The pattern is repeated along the length of the strip.

It should be obvious that each group of three LEDs forms a circuit from the positive conductor to the negative. Cutting the strip other than at the marked positions will result in the non-illumination of the LEDs in that short section.

As you have probably figured out, you need to get the voltage right to match the LED configuration. You need to get a power supply with high enough current capability to power all the LEDs. You can use a more powerful PSU provided the voltage is right - the LEDs will only draw the power they need

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