Wobblycogs Workshop

Workshop Lighting – Power

—

by

Doozer
in

Powering your LED lights is as simple as attaching a 12V power source and basking in their cold white glow. Powering your LED lights and getting the most out of them is a little harder though and will require some calculations and experimentation.

I’ll start off by showing you a simple calculation for the theoretical power draw of a strip of LED ribbon. Note that I said theoretical, the actual power draw will probably be somewhat less than this due to a number of factors.

Example Power Calculation

To properly understand this calculation you have to know about Kirchhoff’s Current Law as well as Ohm’s Law. This example is for a short run of easily available LED ribbon. If you are going to be following along with this project you need to be able to do your own calculations to make sure the system is safe.

Ribbon: A 1m length of 3528 ribbon running at 12V will typically consumes 4.8W and has 60 chips per meter. Assume a forward voltage of 3.1V for the LEDs.

Total current per meter : P / V = I --> 4.8 / 12 = 0.4A or 400mA

The ribbon is broken up into repeated segments which are wired in parallel. Each segment consists of three LEDs and a current limiting resistor. Each segment (and therefore each LED) draws: 

Current divided by the number of segments
0.4 / (60 / 3) = 0.02 or 20mA (by KCL)

White LED’s typically have a forward voltage of around 3.1V so subtract the voltage drops across the LED’s from the supply voltage to find out what power needs to be dissipated by the resistor.

V / I = R --> (12 - (3.1 * 3)) / 0.02 = 135Ω.

A 130 ohm resistor is typically used for this type of ribbon.

Power dissipation by the resistor

I^2 * R = P --> 0.02^2 * 130 = 0.052W

Each segment consumes

4.8 / ( 60 / 3 ) = 0.24W

Each LED consumes

( 0.24 - 0.052 ) / 3 = 0.063W or 63mW

Workshop Circuit Design

For my workshop I’m going to be using 5630 ribbon with 60 LEDs per meter, the seller states that this requires 75W per 5 meters which is the length it’s sold in, therefore 15W per meter. The longest runs I will have will be 2.5m and powered from a single end.

Measured Power Draw

I connected a 2.5m strip of the ribbon I’ll be using to my lab power supply and set it to 12v and 1.8A. Switching it on the LEDs drew 1.65A and the power supply voltage limited itself.

That gives a power draw of 

1.65 / 2.5 = 0.66A/m = 7.92W/m @ 12V.

This is significantly lower than the power draw stated by the seller but let’s not jump to conclusions.

I noted that the current draw rose slightly as the LEDs warmed up. This is probably because the forward voltage of an LED drops as the temperature rises causing more current draw. Power draw increased from approximately 7.5W/m cold to 7.9W/m warm.

As part of the measurement I also checked the voltage at the end of the 2.5m run, it was only 10.73V, this is important later.

The ribbon I’m using has 39Ω resistors but if I calculate the expected resistor for the given current draw it’s significantly different:

Divide the current drawn by a meter of ribbon by the number of LED segments (an LED segment is 3 LEDs and a resistor)
0.66A / 20 = 0.033A - 33mA

Take the supply voltage and subtract the voltage dropped across the LED's (assumed to be 3.1V per LED). The rest has to be dropped by the resistor.
(12V - ( 3.1V * 3 ) )/0.033A = 81Ω

Why does the it calculate as 81Ω when the ribbon appears to have 39Ω resistors? It’s due to the voltage drop along the length of the ribbon, the ribbon itself has non-trivial resistance, at least 16Ω over the 2.5m.

If the calculation is repeated in reverse using 39Ω you can see that the power requirement is much closer to the manufacturer stated 15W/m coming out at 16.6W/m.

(12V - ( 3.1V * 3 ) ) / 39Ω = 0.0692A/segment
0.0692 * 20 = 1.384A/m       20 segments per meter
1.384A * 12V = 16.6W/m
1.384 / 60 = 23mA/LED

Initial Layout

The 5630 LED is smaller than the more common 5050 but higher power, typically 150mA. The 5630 typically run hot, this recommends sticking with 5050 as the most powerful you should use.

Lighting consists of:
6 * 1.25m strips = 7.5m
8 * 2.5m strips = 20m
Total Strip Length = 27.5m

Total power requirements:
15W/m * 27.5m = 412.5W = 34.375A

While I probably could find a power supply that could handle that amount of power it wouldn’t be a good solution. Getting any amount of power over more that a short distance at 12V means using thick cables. It’s much better to install multiple power supplies close to where the 12V power is needed.

My initial installation plan looked like this:

---- 1.25m (PS1) ---- PS1 ---- 1.25m (PS1) ----
---- 1.25m (PS1) ---- PS1 ---- 1.25m (PS1) ----
                      |
---- 1.25m (PS2) ---- DB1 ---- 2.50m (PS2) ----
                      |
---- 1.25m (PS2) ---- PS2 ---- 2.50m (PS2) ----
                      |
---- 2.50m (PS2) ---- DB2 ---- 2.50m (PS2) ----
                      |
---- 2.50m (PS3) ---- PS3 ---- 2.50m (PS3) ----
                      |
---- 2.50m (PS3) ---- DB3 ---- 2.50m (PS3) ----

This diagram shows three power supplies each in their own box. The box containing PS1 has four 1.25m strips attached (the PS1 is repeated because that’s the only way I could think to show it). There is then run to an distribution box and then a run to another power box where PS2 is housed. Another run though a distribution box takes you to PS3. With this setup the power requirements for the three power supplies are:

PS1 => 1.25m * 4 = 5m = 75W = 6.25A
PS2 => ( 1.25m * 2 ) + ( 2.5m * 4 ) = 12.5m = 187.5W = 15.625A
PS3 => 2.5m * 4 = 10m = 150W = 12.5A

Cable

E157914 PRIMAX RU AWM 1185 80 degC 300V VW-1 CSA LL110529 AWM I A/B 80 degC 300V 18AWG FT1 LM