Modify - convert a 120VAC set of LED multicolour lights: Part 1

In this section we'll convert a 120VAC set of LED multicolour lights (without controller) to operate off  DC and we'll fit a controller for fuzzy lighting affects. This project is more advanced than previous one but it has more ZzingG!!. I did make the design process to have more problems to show you how to create solutions.

Note: the original light string can't have a controller wired into it! It stuffs everything up.

Assumptions
We have a light string with 50 multicolour LEDs and it has a separate bridge rectifier (AC/DC). The colour LEDs are arranged as shown. See Step 1 diagram. There is no extension socket.

I can buy a 120V/ 24VAC 2W transformer.

We can buy a red/ blue/ green controller which operates on 12 - 24Vdc and can switch 2A per channel - see photo. This controller suits the LEDs' common anode configuration - see Part 2 and Diagram 1 below.

Controller
This controller was made to control red/ green/ blue (RGB) LED strip. Basically it's a 3 channel controller and provided the electrical use is correct, it will operate spectacularly! Just turn on, use the mode button to change between RGB combinations and see what occurs.



Design Process
First do a rough calculation to determine a satisfactory series parallel configuration then do a final design.

Step 1
Rough Calculations

The LEDs have typical voltage drops of red/ yellow/ orange = 2V ea; blue/ green =3.4V ea.

So a group of 5 LEDs ( RYOBG) has a total voltage drop of 2+2+2+3.4+3.4 = 12.8V.

Since there's 50 LEDs and 5 LEDs per group, no. groups = 50/5 = 10 groups.

If all groups are connected in series, total LED voltage = 12.8 * 10 = 128V

Supply voltage is 120VAC, bridge rectified Vdc = Vac * 0.9 = 120 * .9= 108V

Since total LED voltage of 128V is greater than Vdc of 108V, we can expect all LEDs to be series/ parallel connected. A very careful inspection of wiring between LEDs reveals:

• There are 2 parallel lots of 25 series LEDs., ie 47 LEDs have only 2 wires connected;
• each end LED has 3 wires (2 supply + 1 series) as shown;
• either 25th or 26th LED has 3 wires - it's easily seen in Step 1 diagram.
• This determination can be simplified by looking at LED construction as shown below.

Transformer

I measured the transformer output to be 26V AC at no load with 120V AC input voltage.

Determine Light configuration

The LED colours have either voltage drops of 2V and 3.4V; so we could use an average LED voltage drop of 3V for these rough calculations.

Calculate no. series coloured LEDs using our transformer:
= 26 *.9 / 3 = 7 LEDs

So we could divide a series group of 25 into groups of 7. Yes, you're right 25/ 7 = 3 groups + 4 left over. This is hard to make it work!!

A simple solution is to use a variable voltage DC regulator (see photo) to decrease the rectified voltage down to the selected LED voltage. These regulators are rated 2A, are energy efficient and only cost about $2. However, the controller must operate on smooth DC. Tests showed this was correct. A great solution!!

So we could just use the 5 basic coloured LEDs as a series group then have 50 LEDs/ 5 = 10 parallel groups. (this gives good, even brightness).

So we chose:   10 parallel lots of 5 series LEDs (all the same colour mix)

Final Design Calculations

Total LED voltage per series group = 2+2+2+3.4+3.4 = 12.8V

Since we'll use a regulator, we can set the voltage. So chose a resistor value about 10% of LED voltage, = 10% * 12.8V = 1.28V.

We need to calculate a current limiting resistor.
Now we want .02A flowing through the LEDs which also flows through the resistor (it’s a series circuit). So use Ohm’s Law to calculate the resistance value:

R = voltage/ current
R = 1.28/ .020 = 64 Ohms; the closest standard value is 68 Ohms.

For this project, we'll use a controller and its electronic switches have a voltage drop of about 1.5V across its "contacts". We add this voltage because the "contacts" are in series to LEDs.

Revised series voltage = 12.8 + 68*.02 + 1.5 =  15.7V. We will set this voltage on the regulator.

Resistor power rating: P= I squared * R

= 68*.02 * 0.02 = .03W; we could use a 1/4W size

Total current= no. parallel lots* current series group

=10 * .02= .2A

Total power LED circuit= .2*15.7 = 3.14W
the regulator is about 80% efficient (ie if 1W went in, 0.8W comes out), so:

input power =  3.14/ .8 = 3.9W

BUT the transformer is rated 2W so the total LED + regulator load (3.9W) is too high!!

A solution is to buy a 110 - 240Vac to 15Vdc 1A switching power supply (SPS)for $5. This is cheaper than transformer + regulator! Note the total current was 0.2A which is less than power supply's 1A rating. So you could install a DC extension socket and plug in similar lights.



The SPS apply less volts to LEDs so recalculate the resistor's value to compensate:

V resistor = 15- 12.8 - 1.5 = 0.7V
I = .02A, so R= 0.7/ .02 = 35 Ohms.... closest standard value = 33 Ohms.

Power resistor = I squared * R = .02 * .02 * 33 = .013W, we could use a 1/4W size.

Final Design Parameters:

10 parallel lots of (5 series LEDs plus resistor 33 Ohms 1/4W)
110 - 240Vac to 15Vdc 1A switching power supply

In simple terms, the new light string will have 3 channels of flashing lights activated by a controller which has 40 combinations of display! The brown box is the 33 Ohm resistor.

NEXT >> Step 2 Cutting up the lights