Electrical theory 1.10 - Light controllers

In this section we'll briefly discuss how light controllers for LED lights work - this is a technical subject so it will not be deep. We'll look at the variety and use of controllers in the design section.

Key points:

• A controller is a little box which makes the lights flash in a pattern selectable on a switch. It may also include the diode bridge rectifier with Vac input supply .

• Controllers use an electronic circuit to turn on individual strings to give the pattern. The example below shows 4 strings being switched according to the control of a pattern generator. You will see 4 switches connected to the 0V rail - these are electronic switches which complement the LED configuration which is common anode, ie all anode terminals of LEDs are connected together to +Vdc.

• TECHinfo : the actual type of electronic switch varies according to the type of DC supply: pulsating DC can use SCR (thyristor), MOSFET, Transistor; constant DC can use MOSFET, Transistor.  

• HINT: if controller is supplied with Vac don't connect it to a constant DC power supply - it will mal-function!

Sketch shows typical block diagram for a controller and RHS is a photo of one.

END

Electrical theory 1.9 - Solar powered LED lights

In this section we'll discuss solar powered LED lights.

Operation:

Obviously the sun shines onto a solar panel to charge an internal battery. In the night, the battery powers the lights and controller (if fitted).
Usually there is an inverter IC in the unit to step the battery voltage up to LED voltage.

 

Advantages:

• Lights can be used anywhere without running power wiring.

 

Disadvantages:

• More expensive than powered lights;
• Restricted in number of LEDs and variety of displays;
• Usually these lights are not as bright as mains powered lights because of limited battery power;
• Battery may fail due to long storage time of approx. 10 months;
• Units are usually poorly sealed and water caused corrosion occurs;
• More attractive and easy to steal;
• More difficult to repair.



Photo shows battery and terminal corrosion in solar unit

END

Electrical theory 1.7 - Transformers

In this section we'll discuss transformers. As stated before, low voltage lights are safer than mains supply lights (110V+).

• Transformers provide electrical safety by isolating and reducing the voltage from 110 … 240Vac to 12 or 24Vac. You don't need to know how they work - if you want to know see reference http://en.wikipedia.org/wiki/Transformer.
  
  

   

   
• Transformer device may output AC or DC voltage (it has an internal diode bridge and usually a storage capacitor) – check the nameplate as shown in photos. AC symbols: AC, ac, ~      DC symbols: DC, dc, =  .

1. To check if it's a DC device: energise device and measure its DC output with DMM set to 200V DC.
2. To check if it's a DC device with storage capacitor: energise device and measure DC output with DMM set to 200V DC then turn off device; measure output voltage again immediately. If the output voltage stays near the same value, it has a storage capacitor. (Over time this voltage will decay to 0V.)

• The transformer pack shown in LHS picture usually output 10 - 30% higher voltage than rated voltage at no load, eg 10V rating = 13V measured.  The reason is the transformer is undersize (for Watts output) and the manufacturer poorly compensates by increasing the output voltage. This factor must be considered with LED lights which can be greatly affected by supply voltage changes.

• Transformers have a power rating which must exceed the light’s load otherwise it gets hot and burns out.

Here’s the equations:

• Transformer nameplate: 10VA or 24V * 0.42A (approx 10VA)
• LED loading: no. LEDs * power per LED (V*I) eg 60*3.0*.02= 3.6VA  OR
• Bulb loading: no. bulbs * power per bulb (V*I) eg 12*5*.15= 9VA

Result: Either the LED or bulb strings will be suitable for the transformer.

This is all the important info. required.

Note: for lights, the VA value equals the same value in Watts (W).

NEXT >> Switching power supplies

Electrical theory 1.8 - Switching power supplies.

In this section we'll discuss switching power supplies (SPS) which are a computer derived product.  As stated before, low voltage lights are safer than mains supply lights (110V+).

Switching power supplies provide electrical safety by isolating and reducing the voltage from 110 … 240Vac to regulated 12 or 24Vdc (other voltages are available). Regulated means:

• the input voltage can vary between 110 to 240V and the output voltage stays constant;
• the output current can vary between zero to full load and the output voltage stays constant. This makes it easier to design light displays;
• usually you can adjust the output voltage by a small potentiometer - refer to manual;
• the DC voltage is constant so the correct type of electronic switch must be used. We'll cover this later.

SPS have a power rating which must exceed the lights' diversity load (see next paragraph) otherwise it overheats and burns out. Usually high power units are fan cooled so it's important that cool air surrounds the unit and expelled hot air can escape the enclosure. Failure to do this can cause the unit to fail or cause internal overload protection to operate >> LIGHTS OUT!!

 

Usually you'll buy a SPS unit to power up all your lights operating on a common voltage say 12Vdc. An important factor is whether light strings are flashing or light strings are being switched on/ off as required. So to use the case of all lights on in a load calculation can be an expensive mistake. A reasonable load diversity factor (ratio actual load/ theoretical maximum load) is 80%. Here’s the equations:

• LED loading: no. LEDs * power per LED (V*I) * diversity
•  
• eg 660*3.0*.02= 39.6W * .8 = 31.7W
• So buy a 36W power supply.

END

Electrical theory 1.6b - AC/ DC Rectification

In this section we'll discuss converting AC to DC power to power your lights. This is the most difficult topic to understand so there's plenty of pictures for you to visualise what's happening. I'll give you the 'bare bones' needed for light shows and some detail for interested people.

Bare bones:



A bridge rectifier converts AC to pulsating DC of value = Vac * 0.9 (approx). Connect the diode so that current flows in direction of arrowhead (same as a LED).

Cathode end    Anode end

More detail:

Super critical point to understand:

A diode is a polarised electronic device which conducts current one way when the voltage between anode TO cathode is positive. This is called 'forward biased' and is shown in diagrams for AC half cycle cases. The diode is connected in series in the circuit.

Now when the  voltage between anode TO cathode is negative, the diode becomes practically an open circuit - this is called 'reverse biased'.

 

Diagram for each half cycle of AC supply showing a forward biased condition (ie current will flow)

 Case 1                                                 Case 2

A bridge rectifier consists of 4 diodes arranged so that:

• for positive cycle AC, 2 diodes conduct voltage/ current ( case 1) to output as shown below;
• for negative cycle AC (this is half a cycle later), the other 2 diodes conduct voltage/current (case 2) to output with same polarity as shown below. Note their direction agrees with above;
• in respective half cycle, the grey diodes do not conduct because they are 'reverse biased'.



In the diagram, I've used an analogous battery to show the voltage according to the half cycle shown in the graph. This battery makes it very easy to  see what's happening and when. The output which is shown with a meter, the 2 half cycle outputs are consecutive in time, so the pulsating DC waveform in bottom graph is the resultant output.

 

If you don't believe my analogy, take the TEST!! Get a 9V battery and a bridge rectifier. Connect the battery to the AC input terminals of a bridge rectifier. Measure the polarity of DC voltage against the rectifier's + - terminal markings. Now reverse the battery connections to AC terminals and measure the polarity again. It will not change and it will agree to + - markings!

  

If you don't understand above, please email me and I'll give more explanation or demo an experiment.

For more info. go to http://en.wikipedia.org/wiki/Bridge_rectifier

 

Approximate equation for ‘full wave’ rectification:

Vdc= Vac * 0.9

eg 27Vac = 24.3Vdc

eg 110Vac= 99Vdc

Here's a photo of a bridge rectifier (4 separate diodes) located in a damaged light controller.

Note: for simplicity, filtering capacitors are not discussed or usually needed in Christmas lights.

END

Electrical theory 1.6a - AC/ DC Power

In this section we'll discuss alternating current (AC) and direct current (DC) power. This is the most difficult topic to understand so there's pictures and an analogy for you to visualise what's happening. I'll give you the 'bare bones' needed for light shows and some detail for people wanting it.

Bare bones:

AC: AC voltage and current periodically change their amplitudes from positive to negative in a cyclic order. This is shown in the graph.

If a LED is connected to AC voltage, it will emit light for first positive amplitude then blow up on first negative amplitude because it doesn't like voltage applied in wrong direction (see section on LEDs).

AC is converted to pulsating DC using a diode bridge (see next section - link is below).

DC: DC voltage and current have constant or varying amplitudes over time as positive amplitudes ONLY (this is called uni-directional).

LEDs love DC of either type ( ie constant or varying).

Detailed explanation:

What is AC:

Over time, AC voltage and current vary their amplitudes which cyclically change from positive to negative. This is shown on the graph. For 50Hz/ 60Hz supply, the change occurs ever 10ms/ 8.33ms.

Analogy: If you could sit on an AC voltage, you would be going up and down like sitting on a horse going around on a carousel (time passes ie Time >>) and the horse going up above (positive height) and going down below (negative height) the mid point!

This is all you need to know about AC because you'll prefer to use a DC supply for LED lights.

What is DC:
DC voltage and current has an amplitude offset from 0 value in one direction only which could be positive or negative. We'll only use positive DC as shown in the graph opposite.

There are 2 basic types of DC - constant or varying - see graph.

• Constant DC has a fixed amplitude, eg a battery.
• Varying DC has a amplitude varying over time. An example is DC obtained from rectifying AC which we'll cover next.

NEXT >> AC/DC Rectification

Electrical theory 1.5 - EL wire

In this section we'll discuss EL Wire (Electroluminescent wire ) which is a modern lighting technology.

What is EL wire:

• EL wire is a thin copper wire coated in a phosphor which glows when a voltage is applied to it (110Vac 100’sHz). It makes a 360o unbroken line of visible light and comes in many colours.

• Its thin diameter makes it flexible & ideal for use in Christmas lights, on costumes, etc.
• EL wire’s colour will eventually fade from sun exposure. EL wire used outdoors in direct sunlight, has a life span of 1 - 2 years and when used indoors, it will last as long as 3 years.
• The voltage applied to EL wire affects its life span. An oversized inverter delivers higher voltage and makes EL wire brighter but this causes the EL wire to lose its brightness over time. So buy the wire and inverter together.
• EL wire has a minimum bending radius which must not be exceeded or it will fail. This radius is proportional to wire thickness.
• EL wire or inverter can’t be repaired.

USE:

Simply wrap EL wire around something and turn on!

 

If you want to learn more:  http://en.wikipedia.org/wiki/Electroluminescent_wire
 

 

 

 

 

 

 

 

 

 

 

 

 

 

END

Electrical theory 1.4 - Light bulb strings

In this section we'll discuss light bulb strings so you can easily repair them.

A light bulb glows incandescently and the filament operates around 600oC. So keep the bulb away from flammable material.

Light bulb strings can be series or parallel connected.

 

Series:

• As shown below, only one wire connects between lights. The second wire runs to the last light for the return path for current.
• Light bulbs are low voltage (2 – 10V) and must be replaced with same voltage and power ratings. Otherwise, it’s a possible fire risk or some lights will be dull or overbright.
• A flasher bulb may be used to turn lights on/off/on… Replace the flasher with the same type otherwise it may burn up or flash too slowly. Flashing reduces the light bulbs’ lives.
• Light bulb has no polarity connections and can be tested with DMM on resistance (range 200Ω); its usually between 2 to 10Ω.
• If a bulb fails, all the bulbs (in the series group) do not light.... except when,
• Some 110V light bulb strings contain a 'shunt in each light bulb. When the bulb's filament goes open, the parallel shunt shorts the light bulb, it stays dark but the remaining lights turn on brighter - refer to the instructions supplied with the light. The shorted light bulb should be replaced ASAP because the remaining light bulbs will fail sooner. Note: sometimes this shunt fails to short and all lights are out.

 

 

 

 

 

 

 

 

 

 

 

Parallel:

• Two wires connect between each light as shown below.
• Bulbs are usually larger and their resistance is around 200Ω each for 120V.
• If a bulb fails, only it does not light. The replacement bulb should have the same voltage and power rating otherwise it will have a different brightness.

 

 

Fuses:
Some light strings may have 1 or 2 fuses in the AC supply. The fuse(s) which are located inside the plug, are intended to "blow" when an overload occurs (eg too many extension light strings plugged in) or there's a major insulation failure such as a person accidentally stapling through the wire. Most strings supply spare fuses which can be replaced in the plug.

SAFETY: only use the same type and rated fuse otherwise you can create a fire or explosion hazard!

 

 

 

 

 

 

 

 

 

 

 

 

END

YOUR Safety

Here's some good suggestions for you to think about....

Electric Shock – It’s a lot safer to buy lights which use a transformer. Lights supplied directly from a power point (ie 110VAC … 240VAC) can leak electricity when wet to metal like guttering or posts…. DANGerous!! The IP rating states the immunity from hazards and is a good safety guide to use.

 

Fire risk – light bulbs must be kept away from curtains, plastic, etc due to their very high temperature. LEDs are cool, low voltage and not a fire risk.

 

Fuses – some plugs have fuses to blow for overloads which can start a fire. Obviously, only replace a fuse with the same type and current rating – see label on light string for details.  

 

Transformers and controllers are not made for outdoor use unless IP rated 57 or 67. If necessary, use a weatherproof box to house the transformer and controller.

 

 Repairing/ testing – make electrical connections with the power off and then turn on to test. Then if smoke appears, it’s easier & quicker to turn off! Also don’t breathe this toxic smoke – open windows and leave the room … now!!

END of Safety

Photos of tools

DMM on diode test with white LED lit.


Soldering station (variable temperature)

Resistor leads - top shows 3 resistors soldered in parallel and tube slides over to insulate them. Crocodile clips soldered to end of wires.

Examples of wire strippers

END of Tools

 

Useful Tools

Here's a list of useful tools which will greatly help working on lights:

•Digital multimeter ($10-$200) – buy one with (LED) diode test and AC current functions (photo).

•Soldering iron & rosin cored solder [1.6mm diameter, 60/40 grade] ($40) It's best to buy a variable temperature type because the temperature can be set to suit size or type of job.

•Wire strippers – buy one as shown: it clamps wire then strips. It’s safer than using a knife.($20)

•Retractable knife ($2), Side cutting pliers ($14), crocodile leads ($6).

•The above items can be bought on eBay™ or similar supplier.

Fault finding items:

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

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

–  24V LEDs – 1 of 820 Ω 1W

–  120V LEDs – 1 of 5,000 Ω 10W

–  120V bulbs – 3 of 220 Ω 10W connected in series (= 660Ω)

–You will have to make these – see next slide for an example.

Note: resistor wattage was made oversize to reduce its temperature. Ensure live parts are insulated as shown.

NEXT >> Photos of tools

Electrical theory 1.3a - LEDs General

Here I'll explain in simple terms what a LED (Light Emitting Diode) is.

You only need to know that particular chemicals are used to produce each light colour plus basic info. presented in following sections. We'll also cover various types of LEDs and how they're used for festive lighting. Refer to Wikipedia for more info. http://en.wikipedia.org/wiki/Light-emitting_diode.

There are many LED types, sizes ( round 3, 5, 10mm diameter), colours, shapes, self flashing, brightness, angle of view, etc. - too many to cover! The LED's electronic symbol is a arrowhead with light beams as shown below. Note a plain arrowhead is used to show a LED in my diagrams.

Brightness
The brightness range of LEDs goes from candle power to a CIA interrogation light! So when buying your LED check the spec sheet with it. To use a LED's brightness, we have to go techy here with some basic units of light measurement which are: 

• lumen is a measure of the total "amount" of visible light emitted by a source. (reference http://en.wikipedia.org/wiki/Lumen_(unit))
• candela is a  power emitted by a light source in a particular direction, weighted by the luminosity function (reference http://en.wikipedia.org/wiki/Candela).

Since the light output from a LED is restricted to +- 20 degrees from centre, you could say lumens roughly equal candelas. Also 1000 milli candela (mcd) = 1 candela (cd); ditto lumens.

So LED brightness goes from 200mcd up to 290,000 mcd (multiple LED chips in one package) at rated current. You can briefly look directly at a 5000mcd LED - so this is optimum brightness for light displays in the night. However, if you use a brighter LED, you can reduce its brightness to about 5000mcd by reducing the current (ie use a higher value resistor) - the advantage is gained when you have 100's of LEDs which can use a smaller power supply! Another cool reason is the ability to do shimmering -  the LED is switched at varying fast frequencies to make its brightness flicker very quickly.

Polarity

All LEDs are polarised devices just like a battery, ie correct connection gives light; incorrect can blow it up and there’s no light. It’s best to replace a LED which was energised in reverse polarity – it will usually work when correctly connected but it may fail quicker than a good LED. I'll show you the polarity markers for each type of LED in the relevant post and how to check polarity - see below. 

SMD type strip

Basic types

LED basic types for Christmas lights: surface mount (SMD) or with pins/ legs as shown.

Connection
DC Current must flow from the LED's anode to cathode (see sketch) to get light! This simply means connect positive to anode, negative to cathode (ignoring switches, resistors, etc) as shown in my circuits (link). Now for circuits using the LED symbol, current flow is in the same direction as the arrowhead.

Note do not connect a LED directly to AC - it goes banGG! Use a rectifier (see AC/DC section).

NEXT >> SMD LEDs

Electrical theory 1.3b - LEDs SMD type

Here I'll explain in simple terms what SMD (surface mount device type of component) is. SMD was developed for robotic placement on a printed circuit board. 

A SMD LED is a very small, flat profile LED which can be single colour or multicolour RGB type that can produce a palette of colour combinations. SMD components can be bought but it's impractical to use them for home projects - so we'll look at strip lighting.

Usually 3 SMD LEDs and SMD resistors are connected as a section operating on 12VDC. Then sections are connected together to make a strip - see photos. HINT: When buying a strip, carefully examine the number of terminals on the LED: model 5050 LED has 6 terminals so it has RGB capability; model 3528 has 2 terminals so it has single colour capability per LED, but there could be multiple colours distributed on the strip. So it doesn't look as good as 5050!

Polarity marks are shown on the strip. Connecting the strip wrongly can blow it up and no light is emitted.

 

The actual SMD can’t be repaired but the damaged section can be cut out– see repair section.

                        Single colour strip                                   Multicolour SMD strip

NEXT >> Legful LEDs

Electrical theory 1.3c - LEDs leg type

Here I'll explain in simple terms about LEDs with legs or terminal pins.

This type of LED can be:

• single colour = 2 legs; These are connected in series;
• multi colour flashing = 2 legs. These are new on the market;
• multicolour = 3 or 4 legs. These emit different colours depending on which legs are energised and LEDs are connected in parallel. They are made as either common anode or common cathode to suit type of switch. See following section for details.

LED operation:

·    For 3mm, 2 leg LEDs, 15mA current will give long life. For 3mm, 3 or 4 leg LEDs, 10-15mA each primary colour will give long life.

·    For 5mm, 2 leg LEDs, 20 to 25mA current will give long life. For 5mm, 3 or 4 leg LEDs, 15-20mA each primary colour will give long life.

·    Refer to manufacturer’s data for 10mm LEDs and high power LEDs which may have multiple light emitters and current maxima.

·    Higher currents will quickly reduce life and give minor increase in brightness.
.    The reverse (connection) breakdown voltage for a LED is typically 5V - exceed this and bye bye LED!

The table below shows typical voltage drop across each LED colour and brightness (MCD) for 5mm size. This voltage drop is used in calculations. However, the supply voltage must exceed (total LED 'typ' voltage +1.1V) to give good brightness. A resistor is used to limit current and protect the LED. Some cheap lights don’t have correctly sized resistors and LEDs continually fail. We'll look at correct sizing of LEDs and resistors in the advanced theory section.

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 at 20mA) and angle of light directionality of 85 degrees;

2 LEDs can have high brightness (> 5000 mCd at 20mA) 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.

NEXT >> LED Connections