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Power supply considerations

A project log for RGB seven-segment display [RSSD]

My journey to create a big but thin, evenly illuminated seven-segment display.

JanJan 11/14/2018 at 08:120 Comments

A quick primer on powering WS281X LED's

There's some important things to keep in mind when designing with Worldsemis WS281X LED's. I'll try to keep it short and concise. Here are the main points to consider:

We will discuss these in the following paragraphs

LED types

Worldsemi (that's what WS stands for) has those LED's in many different styles.

The famous WS2812 found on LED strips is most likely the WS2812B type, which has a current consumption of 20mA max per color (60mA at full brightness white). They offer some dimmer variants as well, the WS2812C for example. It has only 5mA per color at much less brightness.

It's the same with the WS2813. There is a 12V variant too, the WS2815B.

If you're interested in a smaller case, the WS2812C-2020 or the WS2813B-MINI are your choice, which come in a 2020 respectively 3535 case.

I ordered my LED's at lcsc.com. I guess Aliexpress etc. has them too (wouldn't be surprised if there are fakes out there).

Active LED's at once

This is the most important factor for choosing a power supply! You should always consider the worst case scenario of your application.

In my case this will be the time displayed with most segments on at maximum brightness in white color (*3 because of 3 LEDs per segment):

Time23:
58
LED's lit
(5 + 5)*32
(5 + 7)*3
Current consumpt. [mA]
1800
1202160

This is 4.08A at 5V = 20.4W. At an estimated 85% efficiency of a wall plug this is 23.5W of total mains power usage.

Another way to ship around this is to use only 1 or 2 LED's per segment. With 2 LED's the current would be 2.76A, with only one LED 1.44A.

Power supply considerations

4.08A is too much for the standard USB 1.0 / 2.0 output (500mA max) and even for all those USB-phone chargers which deliver up to 2.1A.

We also need to take into consideration that when programming the clock, it might only connected be to the PCs USB port (which might even be USB 1.0, who knows). So there needs to be a low-power mode for programming which is preferably activated automatically.

So, for this clock I will use a 5V, 25W standard power supply. Expecting all segments active would draw 5.4A from the supply which is totally unrealistic...

Final considerations

The clock won't run this brightly for any longer period of time, so the above values are very conservative. I expect it to use a maximum of 1.8 - 2A at a "good to read in daylight brightness". But in normal operation it'll draw something around 1A or less.

The FastLed library has options to restrict power consumption, we'll look into that later on.

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