tjet325 asked in Science & MathematicsEngineering · 9 months ago

# How to safely power large LED project?

I am working on a project that will require about 1150, 5volt LEDs. This will require about 70 amps that I will split between two 30 amp and one 20 amp power supply. The total power requirement will come out to 350 watts. Even though the power requirement is fairly low, due to the low volts, a hefty amount of amps are needed.

My question is this:

If I were to plug all three power supplies into an extension cable or power block, do I need to worry about the cables burning up? Any advice on powering this project safely?

Relevance
• 8 months ago

This is slightly more complex than it appears to be.

Are all of the LED's the same color?

You can greatly reduce the cost of the LED's by using 1.5v LED's rather than the 5V RLED's. LED's run about 1.5V@ ~10ma.

You can use a 110V isolation transformer instead of the power supplies.

Connect 16 strings of 72 LED's across it, Half forward and Half backward. This way 8 strings will be lit at a time and the current will be 80 to 90 mA. each way. The flicker will not be visible except to a camera. The input should be protected by a .12 or .15A fuse. The power will be 120*.09= 11W.

I am an Electronic engineer with 30 years experience working with LED's at HP.

I can assist you with this.

Different color LED's  Have different Vf's at 10mA. It is important to balance them out.

If one in a string dies the whole string dies and the rest of the strings get more current.

• 8 months ago

Let's start from the beginning.  LEDs are *current* driven devices, not voltage driven.  There is no such thing as a "5 volt LED".  There may be an LED that is specified for 100 mA max forward current, and at that current may nominally drop 5 volts.  But the voltage is not absolutely defined.

Once you understand the above, the continue with your project.

(hint, you will probably want to use series strings of LED rather than trying to put them all in parallel.)

• Vaman
Lv 7
8 months ago

Each LED draw less than milli amps. Total current will be less than 2 amps. Therefore. 5 volt. 2 amp transformer will be sufficient. You can plug it anywhere. No LED draws more than a milli amp current.

• 9 months ago

How can you handle a project of this magnitude and yet be unaware of how to calculate wire size. I have many years of design experience and I would be wary of a project of this scope.

I'm guessing, you think you can take a 70 amp 5v DC supply and connect hundreds of LEDs across its output. That will not work. LEDs are current devices and cannot be driven directly by a voltage source. And they cannot be paralleled directly. You need something to control the current into each one, either resistors or current regulators. One common method is to put lots of LEDs in series with one current regulator and drive them from, for example, a 200 VDC supply. That way you can have, say, a current regulator with a 10 volt drop and still have 190 volts for 38 LEDs in series.

Here is how to calculate wire size.

A. For DC wires

1. Determine maximum voltage drop allowed in the wire. For example, 1 volt.

2. Determine current in the wire, for example 70 amps

3. Determine length of the wire, eg, 10 feet

4. R = E/I = 1/70 = 0.014 Ω or 14 mΩ

5. R per foot is 14/10 = 1.4 mΩ per foot

6. look up wire sizes and resistances in wire table and pick wire size. In this case USAWG copper #10 is 1.02 Ω per 1000 ft or 1.02 mΩ per foot and would be fine.

Note that if you speced 0.1 volt drop max, that would be 0.14 mΩ per foot, which is \$0 wire, a very thick wire. Look into distribution systems.

B. for AC wires

If this is 120 or 240 volts AC, you also have to meet wire codes for the country you live in.

350 amps at 100 volts (picking the lowest normal line voltage, worse case) is 3.5 amps. In the US, #14 wire is OK for this as long as there is a 15 amp breaker or fuse in the line.

repeat for each wire,