Design - Example of story/ scene development

Here's an example of story/ scene development for you to see how it's done.

This picture was broken down into light displays for each scene as follows:

Design - selecting a theme

In this section we'll discuss selecting a theme and how to find some theme ideas.

 A theme is a story with a central "character or element" and supporting cast or resources revealed through "scenes" or sequences of light displays.

The basic "characters or elements" for Christmas are:

• Nativity scene of Jesus
• Santa and co
• Using music to drive a light show based on vocals and/ or instruments
• Other concepts such as using your house roof line, trees/ bushes, fences, etc.

The choice is yours!! If you're stuck for ideas, look at photos/ video clips in GOOGLE (TM) images/ YouTube. It's worthwhile looking at EL Wire product in BASICs page because it's a new, exciting product.

Here's a story I designed and made in 2002. It's made of these sequences:

1. Santa in field 
2. Santa & Reindeer in field
3. Santa hitches reindeer to sleigh
4. Santa puts a tree into sleigh
5. Santa etc fly thought the sky
6. Santa on the roof with presents in a bag
7. Santa next to tree in house
8. Santa on roof with reindeer
9. Santa etc fly thought the sky - turbo mode (Reindeer has flashing red nose)
10. Merry Christmas sign

As you noticed, I reused various parts of light displays in each sequence or scene. The controller was a special device called a PLC and it took a lot of programing. Today, it could be done in 2 hours and the controller will fit into your hand!

So get the creative juices stirred, do research, ask your family for ideas and select your theme!

  

Note: the theme can be developed over a number of years or slightly changed as new lighting products come onto the market.

NEXT >> Developing a story and scenes

Electrical theory 1.2b - how to calculate series circuits

In this section I'll explain how to calculate various electrical values for series and parallel circuits for DC (AC is different and of little interest to us).

Series circuit

Here's a series circuit made of these components. Note: there's only 1 current path and it flows through each component in the series circuit - as shown.

Components:

• batteries: connected + - + - + - + - + - + - means you add up all voltages to give 9V;
• LEDs have individual voltage drops according to the colour of light emitted - see LEDs. These LEDs are shown as conducting and emitting light;
• resistors have specific resistance as made. When current flows, it has a voltage drop. This resistor could be the resistance value of a relay coil;
• mechanical switch has closed resistance of 0 and open resistance of infinity;
• electronic switch like a transistor or MOSFET operate similar to a switch except the ON (closed) voltage drop is around 1.1V. Its OFF value of 10's volts (as rated).... so no current can flow;
• silicon diode is shown conducting so its voltage drop is 0.6 - 0.7V; if it was connected in reverse, its voltage drop is 100's volts (as rated).... so no current can flow.

Important points:

1. Light bulbs, resistors, relay coil & contact, and mechanical switches are calculated on resistance;
2. batteries, transformers, switching power supplies are calculated on voltage;
3. electronic components, eg LED are calculated primarily on voltage dropped across them;
4. the above is satisfactory for simple lighting circuits using DC.

HINT:
When doing series calculations, the supply voltage/ voltage drops are processed together and the resistance values are totalled. Then Ohm's Law is used to give the current.

Calculate Current value

Analogy: to work out the series equation, stand at the battery + terminal and write down what the likely current would see (note I'll call voltage drop =Vd ) all around its path back to negative terminal of battery:

battery voltage 9V sees (=) red LED Vd + blue LED Vd + resistor * current + switch resistance (closed) * current + closed electronic switch Vd + diode Vd

Putting in values     9V = 2V + 3.4V + 33* current + 0* current +1.5V + .6V 

so                            9V = (2+3.4+1.5+.6) V + (33 + 0)* current

so                               9V = 7.5V + 33* current

so                            33* current = 9 - 7.5 = 1.5V

using Ohm's Law, current = voltage / resistance

                                 current = 1.5/ 33 = 0.045A

It's this easy to calculate the current.

Hints for series circuits:

• add the resistance values to give a total value;
• add voltage drops and substract from supply voltage. This gives a net voltage value;
• use Ohm's Law to calculate current.

Calculate a specific resistance value

You can also work out resistance or voltage values by slightly changing the process. Let's assume the switch is faulty and it offers resistance in the closed state. We measure the current to be 0.03A ... what's the switch resistance (called X Ohms)?

Use the analogy again:

battery voltage 9V sees (=) red LED Vd + blue LED Vd + resistor * current + switch resistance (closed) * current + closed electronic switch Vd + diode Vd

Putting in values 9V = 2V + 3.4V + 33* current + X* current +1.5V + .6V

so                                   9V = (2+3.4+1.5+.6) V + (33 + X)* current

so                                   9V = 7.5V + (33+X)* current

so                                   (33+X)* current = 9 - 7.5 = 1.5V

using Ohm's Law, current = voltage / resistance, OR resistance = voltage/ current

current = 0.03A   so                   resistance = 1.5V/ 0.03A = 50 Ohms

so           33+X = 50, therefore X, switch closed resistance = 50-33 =17 Ohms

Calculate supply voltage value

You can also work out supply voltage value by slightly changing the process. Let's assume the switch is good and it offers zero resistance in the closed state. The current of 0.045A will burn out the LEDs and it must be reduced to 0.02A ... what's the new supply voltage (called Vx)?

Use the analogy again:

battery voltage Vx sees (=) red LED Vd + blue LED Vd + resistor * current + switch resistance (closed) * current + closed electronic switch Vd + diode Vd

Putting in values     Vx = 2V + 3.4V + 33* .02 + 0* .02 +1.5V + .6V

so                            Vx= (2+3.4+1.5+.6) V + (33 + 0)* .02

so                            Vx = 7.5V + (33)* .02 = 8.16V 

So reducing the supply voltage from 9V to 8.16V reduces the current from 0.045A to 0.02A.

We'll need the above processes when designing LED circuits. I'll fully explain each design step so you can follow the calculations.

NEXT >> Parallel circuits

Electrical theory 1.2c - how to calculate parallel circuits

In this section I'll explain how to calculate various electrical values for parallel circuits for DC (AC is different and of little interest to us).

Parallel circuit

Here's a parallel circuit made of resistors and a DMM supplied from a bridge rectifier/ transformer. (For this topic we'll ignore the bridge rectifier/ transformer.)

Note: In a parallel circuit, more than one current flows in the whole circuit. The very important point is the voltage across each parallel path is the SAME!

Important points:

1. Light bulbs, resistors, relay coil & contact, and mechanical switches are calculated on resistance;
2. batteries, transformer/ bridge rectifier, switching power supplies are calculated on voltage;
3. electronic components, eg LED are calculated primarily on voltage dropped across them;
4. the above is satisfactory for simple lighting circuits using DC.

HINT:
When doing parallel calculations, the supply voltage is applied to each current path/ branch and Ohm's Law is repeatedly used to give each current.

Calculate Current values

Let's assume the DMM voltmeter (V) measures 10.0V dc which is applied to each branch.

DMM current

The resistance of DMM is 1MΩ which is short hand for 1,000,000 Ohms. Using Ohm's Law,

current (IDMM) = voltage/ Rm = 10/ 1,000,000 = 0.000010A

330Ω resistor current

again,                            I330= V/ R1 = 10/ 330 = 0.030303A

1kΩ resistor current
The resistance is 1kΩ which is short hand for 1,000 Ohms. Using Ohm's Law,

again,                             I1k = V/ R2= 10/ 1,000 = 0.010A

Total current

The individual branch currents are shown on the diagram above. At the rectifier terminal, the total current equals the sum of all branch currents.

So              total current =  IDMM  + I330 +  I1k
                                      =   0.000010A + 0.030303A  + 0.010A = 0.040313A

Alternative calculation method for resistor branches only

In practical terms we can ignore the DMM because its resistance is very much greater than other resistors.

So we can use the parallel resistor equation to get a total resistance value which must be less than either resistor values because there's 2 branches.

R total =    R1 * R2
                (R1 + R2)

so putting in values              R total =    330 * 1000          = 248Ω
                                                             (330 + 1000)

so total current                        I = V/ R total  = 10/ 248    = 0.040303A

This total current approximates the exacting total current value calculated before.

For lighting, this is all you need to know about calculations for parallel circuits.

However, here's some important points on paralleling components:

• batteries: connected in parallel means the output voltage equals one battery voltage. However, the maximum current output capability is summed by all batteries;

• LEDs have individual voltage drops according to the colour of light emitted - see LEDs. You can't connect LEDs of different colours directly in parallel - the lowest voltage drop LED will stop other LED(s) turning on;

• You can't connect diodes in parallel to increase current capability - use a higher rated diode.

NEXT >> Series Parallel circuits

Electrical theory 1.2d - how to calculate series-parallel circuits

In this section I'll explain how to calculate various electrical values for series - parallel circuits for DC. This will use the knowledge you gained from their individual topics. The complexity of the total circuit will be limited to lighting applications.

The electronic switch turns the 3 parallel light groups on and off together. SPS is a switching power supply with DC output of 24V.

Step 1

It's best to learn the process by starting at the end and work backwards to the total circuit. Then you can see the stages of reducing the circuit. This is like planning a route between points A to B. You don't start selecting streets at point A because you need overall direction to point B. Don't worry, it will become clear as we work through this.


First we need to know the voltage across the parallel circuits - call this Vp as shown. This is easy to calculate using series equation:

Vdc = Vswitch + Vp

re-arranging and putting in values :

Vp = Vdc - Vswitch = 24 - 1.1 = 23.9V

Step 2

We  briefly focus upon the parallel branches to recognise they all have the same voltage, 23.9V across them. This is an intermediate step. Note as we progress, we ignore the circuit parts we've already processed.

 Step 3

Because the 3 branches are in parallel and have the same voltage across each, we can focus on each branch and process them separately. This process is basically the same for any series circuit: group and process voltage source and voltage drops; total resistances; use Ohm's Law.

I'll show you how to calculate the hardest branch of mixed colour LEDs. For the remaining branches I'll state the current value flowing in each.

You remember for a series circuit, the voltage drops are added together.

So the voltage equation is:      23.9V = 2 + 3.3 + 1.9 + 3.4  + 2.1 + R* current

 

We know the resistor is 1K, so   23.9 = 12.7 + 1000 * current

Solving this                                current = (23.9 - 12.7)/ 1000 = 0.011A (or 11mA)
  

Red LED branch  current              = (23.9 - 5  * 1.9)/ 680 = 0.021A

Dark green LED branch current    = (23.9 - 5 * 3.3)/ 390 =  0.019A

Step 4

 The output of switching power supply is connected in series to electronic switch and parallel groups of LEDs/ resistors. So therefore, the TOTAL of 3 branch currents must equal the current from switching power supply.

To add all branch currents:   series SPS current = .011 + .021 + .019 = 0.051A


That's how to calculate electrical values for the light string's series parallel circuit shown in Step 1.

Summary: Just break the big circuit into series then parallel subcircuits then series branch circuits (individually) and process each according to their electrical laws.

END

Basics - LED Characteristics

Light Colour		Forward voltage at If= 20mA
		Min V	Typical V
Red		1.7	2.0
Yellow		1.8	2.1
Orange/ Amber		1.8	2.1
Green (low)		2.0	2.2
Green (high)		3.0	3.1 to 4.0
Pink		3.0	3.2
Purple		3.0	3.2
White		3.0	3.3
Blue		3.2	3.4

 

Notes:

1 LEDs can have low brightness (< 1000 mCd) and angle of light directionality of 85 degrees;

2 LEDs can have high brightness (> 5000 mCd) and angle of light directionality of 40 degrees;

3 Green LEDs can be either low or high brightness and have a range of forward voltages as shown.  

4 Reverse breakdown voltage is about 5V. Exceeding this usually damages the LED;

5 LEDs with internal flashing capability may have higher forward voltages than above;

6 Always use manufacturer's data if available.

7 below is for 2 Pin/Leg LED connections

 

Modify - convert a 120VAC set of LED multicolour lights: Part 2

In this section we'll continue the instructions to convert a 120VAC set of LED multicolour lights (without controller) to operate off VDC and we'll fit a controller for fuzzy lighting affects.

Step 2

Now we have to divide the 50 LEDs into 10 parallel lots of 5 series LEDs AND get the polarity right! Also find the existing resistors (for 120V) and remove them. This is easy to do if you carefully follow these instructions.

How to divide the 50 LEDs

Get some masking tape and attach a label at LEDs with 3 wires connected. For our example, LEDs 1, 25, 50. Write the polarity onto the label by looking at LED block size - see diagram below.

Hint: look for change in orientation of LEDs at 3 wire connections. See diagram.

Energise lights and counting from first LED, divide string into groups of 5 LEDs and attach a label to first LED and to last LED in series group, eg the 1st LED will be red; 5th LED will be green.

Now on the wire between 5th and 6th LED, attach a label marked C next to 5th LED; attach a label marked A next to 6th LED (red) - see Step 2 diagram and photo below.

Repeat this between:
10th LED label C, 11th LED label A      15th LED label C, 16th LED label A 
20th LED label C, 21st LED label A       30th LED label C, 31th LED label A
35th LED label C, 36th LED label A       40th LED label C, 41st LED label A
45th LED label C, 46th LED label A

Get supply wires to series groups

Turn off lights. Look at first LED to see if supply wire connects to positive leg of LED (see Diagram above). If not, look at 25th LED to see if it's positive. We'll assume LED 1 has positive wire connected; LED 25 has negative connected as shown above.

Now carefully trace by hand along the positive supply wire from LED 1 (not the series wire) down to the end. When you arrive at LEDs 5, 10, 15, 20, 25.... 45, attach a label A at each spot.

Cathode wires

You'll need new wires to run along the string back to the controller. Since the controller has 3 channels, we'll wire the cathodes (at green LEDs) into 3 channels as follows:

Channel 1 Leds: 1-5; 16-20; 31-35;46-50
Channel 2 LEDs: 6-10; 21-25; 36-40
Channel 3 LEDs: 11-15; 26-30; 41-45

To do this:

• Run a wire from bridge rectifier to LED 50 (along the light string). Attach a label C1 on this wire at rectifier. We need this info for final connections and groupings;

• At LEDs 5, 20, 35, 50 attach a label C1 to this wire;

• Run a wire from bridge rectifier to LED 40 (along the light string). Attach a label C2 on this wire at rectifier;

• At LEDs 10, 25, 40 attach a label C2 to this wire;

• Run a wire from bridge rectifier to LED 45 (along the light string). Attach a label C3 on this wire at rectifier;

• At LEDs 15, 30, 45 attach a label C3 to this wire.

What you have done is divided the series lights into groups of 5 series LEDs and prepared to make 10 parallel lots - see Diagrams Step 2 and Step 3.

 Each LED shown in Step 3 consists of 5 series LEDs. The diagram would have become too confusing when showing 50 LEDs!

Note: in above diagram you'll see all the LEDs' anodes are connected to positive voltage. This is called "Common Anode" configuration. The chosen controller is complementary to this configuration.

Before cutting, I suggest checking the above labelling is absolutely correct against the Step 2 diagram. The above process will give correct polarity connections.

Find the existing resistors

Carefully look at each LED to see if a resistor was soldered to the leg of a LED. The photo shows a typical connection.

For each resistor: You will have to cut off insulation, unsolder resistor, slide on insulation tubing, solder a wire extension onto LED, position tubing over join.

Step 3

Now it's a simple process to cut the series wires at X, and connect label A wires to adjacent A supply wire at LEDs 5, 10, 15, 20, 25. Repeat for label C wires to adjacent C1/ C2/C3 control wire as appropriate. See photo - the 33 Ohm resistor is located between the 2 green insulation tubings.

How to do this:

It's less confusing to only cut one series wire and join to supply wires one set at a time. To do this:

• Cut series wire between the 2 labels. Strip each wire 12mm.

• Cut the supply wire, A label and strip each wire 12mm. Slide on 6mm insulation tubing onto 1 supply wire.

• Twist the 3 A wires together and solder. Slide tubing over the join.

• Cut and strip the correct control wire 12mm. Slide on 3mm insulation tubing onto control wire.

• Solder a 33 Ohm resistor to C wire. Slide 3mm insulation tubing over the resistor and wire.

• Twist the control wire and resistor lead together and solder. Slide tubing over the join. See photo.

When you're finished, check the wiring is correct for positive DC supply to red LED's anode; channel control wire to green LED's cathode.

Trace the supply wire from LED 1 up to rectifier and label it A. This is the positive supply wire.
Cut the bridge rectifier's DC wires and discard bridge rectifier and old negative wire.

Testing

We'll test the light string before connecting the controller because it's easier.

Connect the positive light wire to a 68 Ohm 1/4W resistor then to the positive terminal of 15Vdc power supply. This resistor will limit the current during testing.

Check the tubing is over all joints then energise the lights!

Connect one at a time, a channel control wire to the negative terminal of the power supply. The corresponding section of lights should come on. Repeat for each channel. Turn off power.

Since it works, shrink the light's tubing and use cable ties to stress relieve the soldered joints.

Disconnect the wire and resistor from power supply.

Wire in control unit

Look at supply unit and how it can connect to control unit. For our example, the plug/ socket will fit nicely but check the polarity of plug's dc voltage matches the controller socket - see below. Otherwise BOoomm!! Where did the black smoke come from?

The connection of the light string's positive and control wires requires a 4 pin adapter. Slide 3mm tubing over the wires, solder to pins in correct sequence : red arrow mark = positive wire, R = C1 wire, G = C2 wire, B = C3 wire. Slide tubing over joins and shrink tubing. See photo.

Hint: if you are unsure which terminal of controller connection is positive, use your DMM set to Vdc and measure polarity from arrowhead terminal (DMM: V red lead) to one of the other terminals (DMM: com black lead). The voltage reading should be positive. See Basics DMM.

Plug the 4 pin adapter into the controller and use gaffer tape to fix into position. Turn on controller and see your lights dance! Use the controller to get different light sequences.

NEXT >>  Summary of design selection and calculations

Modify - summary of conversion process for a LED string

This is a summary of how to convert the supply voltages on a LED string without an original controller. The supply voltage can be either AC or DC. The conversion can fit a controller of your chosing.

If you have any questions, please ask by email.

Step 1 - design selection and calculations

The 2 examples showed how to do the conversion and the design selection and calculations. If this is too complex, send ALL the info on the lights (listed below) to me by email. I have a spreadsheet program which will give all the answers and division data.

Step 2 - dividing light string into series/ parallel config.

The 2 examples showed how to do the division. The key point is to draw the whole light string including polarity and connections..... you can lump the chosen number of series LEDs into 1 LED group as I did in Example 2.

Then mark the drawing with "C X A" labels and transfer onto wires. Check it's correct by looking at LED construction as shown above and in Examples.

Step 3 - cutting and soldering

This is simply carrying out the outcome from Step 2.... then having the enjoyment of seeing your new creation!

Info required

Original supply voltage:          V           AC or DC
Is a separate bridge rectifier fitted: yes/ no
Is a separate controller fitted:         no (only good answer)
Total No. LEDs:

Select either a)  b)   c)  :

a) All LEDs are either one of or a combination of red/ orange/ yellow light colours;
b) All LEDs are either one of or a combination of blue/ white/ pink/ purple/ green;
c) LEDs are a mixture of a) and b) such as in Example 2.

For case c), what is the order of the LED colours in the recurring pattern, eg RYGBWRYGBW:

...................................

Give the numbers of the LEDs which have 3 wires connected to it and polarity?

eg 1st LED:      +      or      -- ;  last LED:           +   or    --  ;

each intermediate LED: No. ......      polarity   + or  --

New design

Required LED supply voltage:         ...... DC       ..... AC ( < 40V)

Transformer or switching power supply or battery

Controller's voltage and current rating  ..... V   ..... A per channel      No. channels:
(if wanted)
Note controller must be suitable for common anode configuration.

END

Functions and specification for controller

Functions and specification for controller

Operations:
ON/OFF: Turn on/ off
Mode: Switch the modes. Short press for modes changes and long press for change between static modes and dynamic modes.
BRIGHTNESS+/-: Increase/ decrease the brightness of static modes
SPEED+/-: Increase/ decrease the speed of dynamic modes.

Specifications:
Working voltage: DC 12V/ DC 24V
Working temperature: -20-60℃
Connection mode: Common Anode(+)
Output power: 72W(12V) 144W(24V)
Remote powered by: CR 2025
Length: 19.5cm/ 7.7in

 

 

RETURN >>

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

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.

 

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