What power supply do I need for my LED light strips?

Question

My set up is this: Power supply > RGB LED WiFi controller > 1 to 3 cable splitter > 2M cable (x3) > 132.5cm 120leds/m 5050 RGB strip (x3)

I contacted the eBay seller about what power supply I would need to power the 5M cable they provided. They told me 12V 5A. After a little research it seems that this is wrong.

I read that 5050 LEDs draw 60mA per segment (3 LEDs per segment). There are 200 segments in this 5M strip, so 60mA x 200 = 12,000mA. Which would mean I would actually need a 12V 12A power supply. Unless the current draw from 5050 LEDs differ from different manufacturers?

I won't need this much current since I'm cutting off 3 individual 132.5cm strips from mine. Each strip has 159 LEDs, which equates to 53 segments (159 / 3). 60mA x 53 gives a current of 3180mA for one strip. The total current of all three is 9,540mA (3180mA x 3). So I guess this means I need a 12V 10A adapter? This is expensive though, so I am wandering, if I'm not powering them to white all the time, I could manage with less current?

I've already tested a 12V 5A adapter on the strips and it seems to work fine. It doesn't get hot or anything. I even tried a smaller section of strip to see if it was any brighter and it's seems to be the same brightness. Surely it should be brighter than using a longer strip??

Answer 1

Stick with what the seller has told you. The one thing you don't know is the rated voltage (forward) of each LED. If you cut out LEDs and don't see a change one of two things is going on.

1. There is a current limiter inline to the diodes.

2. You have reached maximum intensity, and going with any fewer LEDs would cause a short life span. After you get to maximum intensity, more current does not show a greater intensity, it just damages the LEDs.

As always, it would help if you had the data sheet for the LEDs, so could know the maximum POWER (V x I =P) that the diodes could handle. Then you could figure out no matter how many LEDs you were using what the voltage should be.

By removing that section of strip, you may have shortened their life. See if the seller can fill in that info gap.

Answer 2

You have the general idea right but there are a few caveats.

1. These led strips arent actually designed for 20 mA at 12V (per segment per color). The LEDs tends to be underdriven at that voltage. They tend to have resistors targeted for 20mA at 14.2V. At 12V they will be lower, likely 17 mA. You can test this with a ammeter or voltage meter.

2. These led strips are on relatively high resistance FPC. As the strip gets longer and the combined current across the copper increases, the voltage across the strip starts dropping. Which leads to dimmer LEDs at the far end from power and a lower overall current as the system balances itself out. For you this means that the total current needed is lower than stated.

3. The sellers tend to be resellers with no real understanding of how the product works so they regurgitate whatever info they think they were told about it.

Your best bet is to simply power one section and measure the actual requirements.

As to one small section being brighter than a large section, it wouldnt be, as long as the supply can provide the current as set by the current limiting resistors on the segment.

Answer 3

Your LED strips have LEDs grouped with three in series, and the groups are all in parallel. Each group has its own current limiting resistor.

Because of the arrangment of series and parallel for the groups (and because each group does its own current limiting,) the current drawn by each group is pretty much independent of the other groups.

If the power supply cannot deliver enough current for all the groups in parallel, then you would see your expected result when removing groups - removing a group would make the others brighter.

What you have read about LED strips in general doesn't have to apply to the strips you have at hand. The manufacturer can decide how much current each group should get. They pick an operating voltage (12V) then subtract from that the forward voltage of the LEDs in series. Then, they pick a desired current for the LEDs. From this data, they calculate the needed resistance to limit the current.

You could try to work backwards from known data (supply voltage, resistance, estimated forward voltage of the LEDs) to get the current needed for each group.

Or, you do it the easy way.

1. Use a multimeter in current measuring mode.

2. Put it in series with your LED strip.

3. Power up your strip.

4. Measure current.

5. Divide measured current by the number of groups in the strip.

You now know how much current each group needs. From that, you can calculate the total needed current from your power supply.

The basic unit for your strips is the the 3LED group. When calculating the current, you need to figure out how many of those you will be using.

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Remember to over rate your power supply somewhat. If you calculate, say, 800mA then a 1A power supply would be OK. If you calculate 1500mA, then you'd probably want a 2A or maybe a 2.5A power supply.

The idea being not to push the power supply to its limits. It will run cooler and last longer that way.

Also, if it can supply more current than needed then you won't have to worry about it wimping out - the LEDs will always be at (about) the same brightness.