Modify - Convert 120VAC set of LED lights to 24VDC PART 1

In this section we'll convert a 120VAC set of LED lights to operate off 24VDC which could be a transformer and rectifier, or a 12V car battery and inverter/ booster (like a LM25977 IC), or a 120VAC /24VDC switching power supply.

Note: this process can be applied to any voltage light set - just substitute the new voltage into the equations, eg Vdc = 12V instead of 24Vdc. Note: the Vac is only used for rough calculation and then the transformer is bought to suit the AC supply voltage.

If you have any questions, please email me with all the details so I can help you.

Assumptions

We have a light string with 30 blue LEDs and it has a separate bridge rectifier. Also it has an extension socket. See Step 1 diagram.

I've got a 120V/ 24VAC 10W transformer from old light bulb set.
 

Design Process

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

Step 1

Rough Calculations

A blue LED has a typical voltage drop of 3.4V.

So 30 blue LEDs have a total voltage drop of 30*3.4= 102V.

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

Since total LED voltage of 102V is less than Vdc of 108V, we can expect all LEDs to be series connected as shown in Step 1 diagram. A careful inspection of wiring between LEDs confirms this is correct, ie 28 LEDs have only 2 wires connected; each end LED has 3 wires (2 supply + 1 series) as shown. (Note if the total LED voltage was greater than Vdc, we can expect LEDs to be series parallel connected.)

Determine Transformer size

The total wattage for lights = no. lights * V/ LED * current

We'll chose 20mA (0.02A) as current value.

So wattage = 30 * 3.4 * .02 = 2.04W .... transformer is big enough.

I measured the transformer output to be 27.6V AC at no load with 120V AC input voltage.   The light's wattage is 2.04W and it's about 1/5 of transformer's 10W rating. So you could connect an additional 3 identical light sets to the transformer through the extension socket.

Determine Light configuration

Calculate maximum no. series blue LEDs using my transformer:

= 27.6 *.9 / 3.4 = 7.3 LEDs

(this gives good brightness). This number shows we need parallel lots of series groups.

It's best to have the same number of LEDs (and colour mixture) in a series group so the brightness is even across the string.

So we could chose:             5 parallel lots of 6 series blue LEDs, or

                                            6 parallel lots of 5 series blue LEDs, etc.

 I'll chose 6 parallel lots of 5 series blue LEDs to suit my diagrams.

Final Design Calculations

Total LED voltage per series group = 5 * 3.4 = 17.0V

The supply voltage can vary between 105% to 90% of nominal. We'll ignore this factor because its affect is small.

The Vdc output from bridge rectifier = 27.6 * .9 = 24.8V

We need to calculate a current limiting resistor. To calculate the voltage across the resistor, subtract the LED series voltage from the supply voltage:

V resistor = Vdc output – LED series voltage = 24.8 -17.0= 7.8V

Now we want .02A flowing through the LEDs which also flows through the resistor (it’s a series circuit - see Diagram 4). So use Ohm’s Law to calculate the resistance value:

R = voltage/ current or V/I

R = 7.8/ .020 = 390 Ohms; this is a standard value!

Resistor power rating: P = V*I = 7.8 * 0.02 = .156W; we could use a 1/4W size but it will get too hot. So use a resistor about 3 times the wattage to keep it cool, say a 1/2W size. This will only cost an extra 10c!

Final Design Parameters:

6 parallel lots of (5 series blue LEDs plus resistor 390 Ohms 1/2W). This is shown below as an electrical diagram.

 

NEXT >> Step 2 Cutting up your lights!