Modify - Convert 120VAC set of LED lights to 24VDC PART 2

We'll continue the conversion of a 120VAC set of LED lights to operate off 24VDC.

Step 2

Now we have to divide the 30 series LEDs into 6 parallel lots of 5 series blue LEDs AND get the polarity right! Also we must find the existing resistors (for 120V) and remove them. This is easy to do if you carefully follow these instructions.

The letters show ends of series LED group; X shows the cutting spots.

Note: if the light string does not have an extension socket:

• wire in a new A supply wire and connect to first LED at A (red wire);
• wire in a new C supply wire and connect to last LED at C (black wire). See Step 2 diagram;
• if wanted, continue these 2 wires to a new DC extension socket, as shown.

How to divide the 30 series LEDs

Get some masking tape and attach a label at LEDs: 1, 5, 10, 15, 20, 25, 30.

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 - see Step 2 diagram.

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
                                    25th LED label C, 26th LED label A

Now carefully trace by hand down the supply wire from LED 1 [red wire in diagram] (not the series wire) down to the extension socket. When you arrive at LEDs 5, 10, 15, 20, 25, attach a label A at each spot.

Now carefully trace by hand down the supply wire from LED 30 [bold black wire in diagram] (not the series wire) down to the 120V plug. When you arrive at LEDs 25, 20, 15, 10, 5, attach a label C at each spot.

What you have done is divided the series lights into groups of 5 series LEDs and prepared to make 6 parallel lots - see Diagram Step 3.

Photo shows labelling before joining

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 (one with insulation; one after insulation was removed).

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 C supply wire and install resistors. See diagram and below for details.

How to do this:

It's less confusing to only cut one series wire and join it to supply wires one set at a time. To do this:
• Cut series wire between the 2 labels, eg LEDs 5 - 6. Strip each wire 12mm.
• Cut one supply wire, say 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 the supply wire label C and strip each wire 12mm. Slide on 6mm insulation tubing onto 1 supply wire.
• Solder a 330 Ohm resistor to C wire. Slide insulation tubing over the resistor and wire.
• Twist the 2 C wires and resistor lead together and solder. Slide tubing over the join.

Repeat above for each pair of LEDs, ie 10-11, 15-16, 20-21, 25-26. See photos for more info.

Photo shows correct joining of wires

For 120VAC supply
Also connect the transformer to the bridge rectifier's AC wires. This connection must suit the type of terminals on the transformer. You may want to:
• extend the wires between transformer and bridge rectifier to keep the transformer within shelter;
• if necessary, cut off the extension socket and change to a DC socket which suits your needs.

For 24V battery supply
Connect a 2A fuse holder to positive terminal of battery then connect it to one AC wire of bridge rectifier; connect other AC wire of bridge rectifier to negative terminal of battery. The bridge rectifier ensures the correct polarity DC is connected to LEDs.

Testing

Check the tubing covers each joint then energise the lights! Since it works, heat shrink the tubing and use cable ties to stress relieve the soldered joints.

HEelpp!!

1. one series group is dull: check again if an original resistor was not removed. It's resistance is too high and severely limiting current;
2. One series group does not light: check quality of joins and if the LED polarity is the same as other groups. If different, reverse the first and last connections to supply wires.

Your lights are finished and can be used!

NEXT >> Summary page for design selection and calculations   or Example 2

Repair - Fault finding method 1 - 120V Light bulb fitted with shunt

There are many fault finding methods presented on the internet but in these sections I'll show you the best for each particular application. What you NEED from a fault finding method is to be simple, quick, reliable and requiring the lowest technical knowledge and equipment to do.

Method 1: Suits 120V light string fitted with a SHUNT in each bulb (see photo)

If the string doesn't light:
Check power is at power point and fuses in the plug are not blown.

When you remove a light bulb, check the leads are aligned correctly, the bulb is OK with a DMM on resistance function, and the contacts in the light's base are not corroded.

The method is shown on video, http://www.youtube.com/watch?v=WuVtMvMNFE4 and the Lightkeeper Pro (TM) tool is shown below. As shown in video, this tool injects a high voltage spike into the light string when the trigger is (repeatedly) pressed. The spike breaks down the insulation on the shunt inside the failed bulb; it becomes a good short. Then the good lights in the string light up - replace the bad bulb ASAP.

This tool should not be used on LED lights because it will blow them up.

 

NEXT >> Method 2 For light bulb strings (without shunts) 120 .. 240VAC

Repair - Fault finding method 2 - light bulb strings (without shunts) 120 .. 240VAC

Method 2 For light bulb strings (without shunts) 120 .. 240VAC

This method is for plain light bulb strings. Here's some good hints to make fault finding easier:

• If no light bulb glow check power is at power point and fuses in the plug are not blown.
• If lights randomly flash, press each light bulb into its base because a bulb could be loose.
• Check if any bulbs are broken before starting fault finding.
• When you remove a light bulb, check the leads are aligned correctly, the bulb is OK with a DMM on resistance function, and the contacts in the light's base are not corroded.

LHS bulb has misaligned (red) wire; RHS has good alignment

Method 2 uses a special device called a 'non contact voltage detector' (NCVD) which picks up the AC electric field (50 or 60 Hz) off an energised wire. It needs some practice to use this device. If you buy one, the KLEIN NCVT-2 device (about $20) is recommended because its minimum voltage pick up is 50VAC.

Look at these videos to gauge its effectiveness and how to use it:

1   http://www.youtube.com/watch?v=3okLMo7a6x0
2   http://www.youtube.com/watch?v=ybePn7Sy4Y4

Note this method does not work on DC lights or LEDs.

NEXT >>  Fault finding method for any form of light bulbs or LEDs

Repair - Multi-purpose Fault finding method

Here I'll show you a fault finding method which will cover all types of light bulb and LED strings. It uses low cost test equipment and the normal power supply for the lights. It is based upon sound electrical theory and many examples are given.

Method 3: Divide & Conquer

The principle is to connect a resistor to bypass about 50% of lights in the suspect group and see if the remaining group of lights glow when light string is energised.

How does it work: the resistor makes a parallel path to the bad light bulb/ LED and electricity can flow in remaining 50% of lights which obviously light up (the red line in the string below shows the current path.). This process is repeated until the bad light is found.

This is a very simple, quick method which works for all type of light bulb or LED strings... whether the bulbs/ LEDs are open or shorted; mains power or low voltage lights, AC or DC, 10 or 200 lights. For a 50 light series string, after 4 tests you will narrow it down to 3 possible bad bulbs!

Here's a detailed example of how to use this method. The photo shows the red resistor lead is plugged into back of plug.

  

 

 

Yes it's this simple to find a bad bulb or light fitting.

 

Resistors fitted with 3 metre lead + crocodile clip each end (to suit):

–12V LEDs – 2 of 1200 Ω 1/2W connected in parallel (= 600Ω)

–24V LEDs – 1 of 820 Ω 1W

–120V LEDs – 1 of 5,000 Ω 10W

–120V bulbs – 3 of 2,000 Ω 10W connected in parallel (= 666Ω)

(You will have to make the ones you want. Cost about $5 each for materials)

 

•Hint: it’s good to practise these techniques upon good strings where you can easily remove a bulb/ LED to make a fault. Then find it!

 

NEXT >> Second example

Repair - Multi-purpose Fault finding method for LV LEDs

Here I'll show you a second example of fault finding method for LED strings supplied from a transformer and rectifier, ie low voltage. This shows the method's veratility and effectiveness.

General info:

• LEDs only work on DC so there must be a rectifier to convert AC to DC;
• Usually the supply voltage is 24V, so each series section will contain between 7 to 11 LEDs depending upon LED colours (remember red, orange, yellow LEDs about 2.0V each; remaining colours about 3.3V each). So if there's 70 LEDs in total, there could be 7 parallel groups of 10 LEDs in series;
• To determine the number of series LEDs, energise the LED string and mark the unlit LEDs with tape and count them. This will help in fault finding.

Method 3: Example 2

Summary of Divide & Conquer method:

The principle is to connect a resistor to bypass about 50% of lights in the suspect group and see if the remaining group of lights glow when light string is energised.

How does it work: the resistor makes a parallel path to the bad LED and electricity can flow in remaining 50% of lights which obviously light up (the red line in the string below shows the current path.). This process is repeated until the bad light is found.

Here's a detailed example of how to use this method. For simplicity, the good, parallel sections are not shown. The diagram also shows how to test a LED with a DMM. Note don't test a LED with a battery - it needs a current limiting resistor to stop it blowing up!

Note these diagrams show a LED fitted into a base fitting. Before removing LED, make an alignment mark on the LED holder and its base (see photo). When inserting the LED holder into base, ensure the 2 marks align. Otherwise the LED and its section WILL NOT work because the LED is reversed connected.

Now if the LEDs are directly wired, you have the choice of cutting wires (and resoldering) or inserting a sewing pin into the wire to get a connection (afterwards apply silicone to hole to seal it up). The choice is yours.

Red line is direction of current flow.

Resistor test lead fitted with 3 metre lead + crocodile clip each end (to suit):

–     12V LEDs – 2 of 1200 Ω 1/2W connected in parallel (= 600Ω)

–     24V LEDs – 1 of 820 Ω 1W

Refer to Replace LEDs section for info on replacing LEDs. You will do it right and not have more blown LEDs.

Electrical theory 1.3h - Advanced LED theory.

In this section we’ll look at a LED's constant voltage behaviour in more detail and see the results of an experiment to demo the key points. This will help you understand the use of a current limiting resistor and answer many questions.

Key points:

A LED has roughly a constant voltage behaviour which means:

• they have an absolute turn on voltage (across its terminals) of about 80% typical voltage; the amount of light emitted is bugger all!
• they must be operated at their typical voltage to get plenty of light;
• voltages exceeding 7% of typical voltage causes too much current to flow - this causes LEDs to overheat and prematurely fail! The role of the limiting resistor is to limit the current to safe values for the expected variations in supply voltage.

We’ll look at a typical characteristic for a RED LED again. The graph below shows that the amount of voltage applied to the LED will allow current to flow; hence its brightness. The graph shows there is a definite turn on point (1.65V), current increases linearly above 1.75V and quickly becomes excessive.

Data points:

1. No current at 1.55V, ie LED is unlit;
2. Turn on at 1.65V;
3. 20mA flows at 1.85V
4. 50mA at 1.98V (excessive current)

I’ve created an experiment to illustrate the graph with 4 RED LEDs with various voltages applied. The photo shows the resulting current flowing through the LED and its brightness (located above each DMM) – the camera failed to distinguish the brightness so I created a colour spot grading above the photo. Note each LED had 1 Ohm resistor series connected to it and it was used to measure current using Ohms Law. The current DMM photo was appended as shown.

Experiment results:

• No current at 1.55V

1.  0.73mA at 1.659V
2.  7.8mA at 1.766V
3.  14.0mA at 1.867V
4.  25.9mA at 2.053V (maximum safe current!)

These results for these LED have good similarity to the graph and significant brightness occurs just below 2.0V. This is why the typical voltage should be used to get good brightness. Note: there is a variation between LEDs of same colour because the chemical doping process to make a LED is moderately variable.

END >> RETURN to easy stuff!

Electrical theory 1.3f - Series LED connections

We’ll design a rainbow light emitting LED string. Note a resistor, relay contact or switch can be connected either side of the LED.

Case 1

The circuit is shown below. The LED voltages and the physical connections for the LEDs and resistor are also shown. The last LED is a white (light emitting) LED.

 

Step 1

Initially we'll set the voltage across the current limiting resistor at 1.0V.

 

 

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

So the total of voltages is 2.0 + 2.1 + 3.4 + 3.2  + 3.3 + 1.0 = 15V 

Note: this is the minimum acceptable supply voltage for good light output! The manufacturers' data was used for LED voltages.

 

Step 2

We’ll assume we have a 15Vdc supply and will operate the LEDs at .020A current (20mA).

To calculate the value of limiting resistance (R ), use Ohm’s Law as follows:

 

R = voltage/ current or V/I

R = 1.0/ .020 = 50 Ohms; the closest standard value is 47 Ohms

Step 3 (the resistance value changed slightly, so recalculate current - optional step)

Using Ohm's Law in different format: I= (V supply - VLED total) /R

Revised current = (15 – (2.0 + 2.1 + 3.4 + 3.2  + 3.3) )/ 47 = .021A  This current is within reliable tolerance limits.

Step 4
Resistor power rating: P = V*I = 1.0 * 0.021 = .021W; we can use a 1/4W size which is easy to buy.

Alternate equation: P = I squared * R = .021 * .021 * 47 = .021W

Note: some LED videos show a LED connected directly to a small battery without a resistor. The video circuit uses the resistance of the small battery to limit current. It has the disadvantage of discharging the battery quicker. You can't do this with a car battery because the LED will melt!

Case 2

Here’s what to do if you only have a 24Vdc power supply. (If this circuit was connected to 24V it would quickly melt because a current around 0.2A will flow!) There are 2 options as solutions:

 

Option 1: put more LEDs into the circuit and repeat above process (Case 1);

Option 2: we only want 5 LEDs so we increase the size of the resistor as calculated below:

 

Total LED voltage = 2.0 + 2.1 + 3.4 + 3.2  + 3.3  = 14.0V
To calculate the voltage across the resistor, subtract the total LED voltage from the supply voltage:

V resistor = V supply – V total LEDs = 24.0 -14.0= 10.0V

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 or V/I

R = 10.0/ .020 = 500 Ohms; the closest standard value is 470 Ohms

Since 470 is close to 500, there's no real need to recalculate the current - it's within safe limits.

Resistor power rating: P = V*I = 10 * 0.02 = .20W;

We could use a 1/4W size but it will get too hot. So use a resistor about 4 times the wattage to keep it cool, say a 1W size. This will only cost an extra 10c! (Note resistor wattage sizes: 1/4W, 1/2W, 1W, 2W, 5W, 10W)

Did you find designing a series LED circuit relatively easy?

KEY EQUATIONS:

Resistance in Ohms = (V supply - total LED typical voltages)/ required current

Power rating of resistor = resistance * current * current * resistor cooling factor (approx =4)

Here's a website LED calculator to use: http://led.linear1.org/led.wiz
Note: this program only doubles the resistor wattage; I recommend doubling this wattage again to ensure the resistor runs cool and does not become a fire hazard.

NEXT >> Parallel connected LEDs