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    kgolem is offline knows Jacques Detritus

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    Designing with LED's

    Designing With LED’s

    Specifically Tailored for the Marine Aquaria DIY’er

    Introduction:

    The idea here is to present the necessary information to the enthusiastic marine aquarist, so that the individual may be able to design and build a moonlighting feature for his or her reef tank.

    The design techniques in this article are intended to be used with low power, 5mm or smaller LED devices with forward current ratings of 100 mA or less. When designing with high power LED emitters that are rated at 1W or greater power consumptions, attention must be paid to the ability of the device to dissipate heat under the ambient conditions within the design environment. That is to say, that the greater the ability of the device to rid itself of excess heat, the greater the forward current values that can be safely used within circuit without damaging the LED.

    This design criterion does not apply to the lower power devices which are the topic of the following discussion.

    The reader must be aware of a few safety considerations.
    1. Household electrical supplies are capable of delivering lethal voltages and currents to the unsuspecting and ignorant user. Let’s be aware of what we are doing.
    2. Using homemade electrical projects may cause issues with insurance companies if it is deemed that the project failure is the reason that an insurance claim is filed.
    3. Electrical circuitry in the vicinity of water is dangerous where the potential exists for the circuitry to become submerged.
    4. #3 is particularly dangerous when the water in question carries a dissolved electrolyte such as artificial ocean salt mix.
    Don’t worry; following the instructions carefully is the key to safety. If in doubt, don’t do it and only do it if you are no longer in doubt.

    Be certain that any pre-assembled components, such as power supplies, are CSA - Canadian Standards Association approved if you are using the component in Canada. If you live in the USA, then your pre-assembled components must by approved by Underwriters Laboratory. For European DIYers, the pre assembled parts must carry the mark of Conformité Européenne or CE;

    When buying components, purchase from a reputable dealer, from a retail outlet or a online distributor that is responsible to their customers. There are tempting deals to be had from E-Bay stores that promise components are greatly reduced prices. One gets, what one pays for. My theory is that at the manufacturing facility, if a statistically significant number of components, in a production line or lot, fail quality control tests, rather than being disposed of properly, the components leave the factory via the back door to be auctioned off on E-Bay for pennies on the dollar.

    Prototype your project. Build a test circuit and run it for an extended period of time under supervised conditions, this means many hours. Test for extreme temperatures of the components. If you burn your finger on component, then something is wrong. Verify that voltages and currents are the values that have been calculated, do this by measuring with a multimeter.

    When you have completed your project, again, run it for an extended period of time under supervised conditions. Check, check and triple check connections. Look for exposed wires and solder. Only when you are satisified with your workmanship, can the project be installed in its place.

    LED’s, resistors, fuses, power supplies are all designed to connected together much like home theater components or computer peripherals are made to work together. It is just a matter of learning how to do it properly. Nobody would connect the output of the home theater amplifier to their DVD player. Likewise, the sensible person wouldn’t connect an LED to a 12 volt power supply without a current limiting device in place, it is common sense and the realization of right and wrong that comes from knowledge of what you are doing.

    By the time we are finished here; the reader will be able to decide if the course of action is safe or not. Our circuits are designed with fuse protection. We will understand the difference between reasonable design criteria and that which seems dangerous.

    Before proceeding the reader must be comfortable with his or her own abilities. A DIY project is a wonderful thing to finish, but we must be realistic in our expectations of ourselves and the scope of the project undertaken.

    Finally, a DIY project of any type must be fun to build and test. Certainly, there is as much satisfaction to be gained from the building as there is in the admiration of the finished product.

    Conventions Used Here:

    There is some mathematics involved; the math skill level isn’t higher than elementary school arithmetic. The reader will be expected to be able so use basic addition, subtraction, multiplication and division with a square or square root thrown in.

    The following symbol key will be used…
    + addition or anode (positive terminal)
    - subtraction or cathode (negative terminal)
    * multiplication
    / division
    = equals
    ^2 exponent of two; eg. 3^2 = 9 ; three squared equals nine
    SQRT square root eg. SQRT9 = 3 ; square root of nine equals three

    V Voltage in Volts
    I Current in Amperes or Amps
    Resistance in Ohms
    P Power in Watts

    Common SI prefixes such as milli, kilo, and mega will be used.

    VResistor or IResistor refers to the voltage across or current through a resistor.



    OK, let’s get at it.

    Part 1 – “Currently” LED’s are the State of the Art

    The first consideration is the difference between conventional lighting devices such as fluorescent and incandescent lamps, and LEDs. An LED is not a bulb; it is a P-N junction like a transistor or a diode. A bulb has a filament or a gas which glows incandescent or fluorescent when excited with electricity. A LED emits a photon whenever an electron passes across the junction of silicon or galium arsenide. By the way, LED is an acronym for Light Emitting Diode.

    In conventional lighting systems the brightness of the lamp is controlled by the voltage or potential applied to the lamp. Increasing the voltage causes the filament to heat resulting in a brighter glow, or the gas to become more excited on an atomic level resulting in greater light output. An LED's brightness is proportional to the amount of electric current flowing through the P-N junction.

    If an LED works by electrons flowing past a P-N junction emitting photons, then it stands to reason that more electrons flowing will result in more photons. Since electron flow in a conductor is defined as current and photons are light, then it follows that more current means brighter light.

    So designing with LEDs is all about controlling the current through the LED, this is easy. And here is why.

    A P-N junction has a tendency to have a specific voltage drop across it based on the chemical composition of the silicon or gallium arsenide that the junction is made of. What does this mean? In a circuit where there is voltage to be distributed according to Ohm’s law, The voltage across the P-N junction will remain constant regardless of the value of the resistances in series with it.

    See the following example



    Figure 1: Two Resistors in Series with One LED

    In the above circuit the diode properties are such that 0.7 volts will appear across the junction. This leaves 5 - 0.7 = 4.3 Volts to be distributed between the two resistors. Regardless of the resistor sizes, 0.7 volts will be measured across the diode junction.

    This is why controlling the current is easy. With a single series resistor, it is always known what voltage is across the resistor (Vpower supply – Vdiode = VResistor).

    Since we know the voltage across the resister, we can control the current flow through the resistor by changing the value of the resistor. Practical examples of this technique will appear later.


    Part 2 – Getting a “Leg” up on series and parallel circuits

    Some examples showing series and parallel circuits.






    Figure 2: Series Circuit Connection





    Figure 3: Parallel Circuit Connection

    Figure 2 shows resistors connected in a series circuit; figure 3 shows resistors connected in parallel circuit type.

    The easy explanation is that in a series configuration, the devices are connected “end to end”. In a parallel configuration the devices share a common connection at each terminal.

    The reference to a parallel connection is to say that the devices are connected “across” the device. The corresponding reference to a series connection is to say that a device is connected “in line” with the device. This nomenclature will present itself later in this explanation.

    Of course, the two types of circuit configurations can be combined as shown in figure 4.



    Figure 4: Combined Series / Parallel Circuit Connection



    Part 3 – The Light Emitting Diode Data Sheet.

    Before the design can start, the designer needs to know some electrical characteristics of the devices that are to be used. The best source for this information, the data sheet, is provided by the manufacturer of the device. A typical data sheet is a wealth of information about the specific LED used in the design. Physical, electrical and optical information is provided here.

    The case size and dimensions between the leads is documented to assist in building mounting fixture. Light dispersion characteristics are charted and measured for the designer.

    But what we really care about, is how to connect the LED to a power source and light it up safely. Also important is the longevity of the LED while it is installed in our setup.

    The two important numbers to note on the data sheet are the values for forward voltage V(f) and forward current I(f).

    Vf is the voltage that is measured across the LED when it is operating.

    If is rated current flow for safe operation of the LED.

    Sometimes, LEDs are obtained from the surplus supply store and the electrical characteristics are unknown at time or purchase. In this case empirical test methods are employed to determine the electrical operating parameters. That being said, it is always a good move to purchase extra parts when in doubt of the electrical operating parameters.


    Part 4 – Proper Use of an LED In a Circuit

    Ok, so lets look closely at an LED, paying attention to the shape of the device and the wires where the electrical connections are made.


    Here are a couple diagrams.



    Figure 5: LED Outline diagrams.

    The diagram shows two views of an LED case, the bottom view on the left and the side view on the right.

    If we look at the diagram, there are two things that are immediately noticeable. A flat side on the case and a difference in electrical lead length are apparent.

    An LED is a device that has polarity, that means that in order to function, electrons must flow in one direction only. In fact an LED will behave like any other diode, a one-way valve for electron flow.

    The flattened side on the case corresponds with the shorter lead. This is the cathode or negative terminal of the LED and is connected to the circuit ground or the negative terminal of the power supply. In the schematic diagram of the LED it is the same as the bar on the symbol. I remember this by thinking that the flat side is more like the minus (-) or negative sign than it is the plus (+) or positive sign.

    Now that we know how to properly connect an LED, we can actually begin to design and build a working circuit.


    Part 5 – Basic Circuit Design

    Lets review a concept that was covered in Part 2. In particular, remember the series circuit in figure 1. That is very close to a basic connection diagram for a working LED circuit.


    Consider this circuit…



    Figure 6: A working LED circuit

    A resistor in series with an LED connected to a 12 V power supply. Please note the polarity of connection of the LED, that is, the negative lead of the LED is connected to the negative terminal of the power supply. In this configuration, current flows in the proper direction across the P-N junction so that photons are emitted.

    The hypothetical LED in the circuit has a hypothetical data sheet that describes the following electrical characteristics;

    Vf = 1.8 Volts (V)

    If = 20 milliamps (mA)

    The schematic shows a power supply voltage of 12 volts.

    About the only thing in the drawing that we know not too much about is the resistor.

    From Part 1; (Vpower supply – Vdiode = VResistor),

    VResistor = 12 V – 1.8 V

    V[size=1]Resistor[/size]
    = 10.2 V this is the voltage that appears across the resistor

    If we want maximum safe brightness from the LED, then If =
    20 mA. This current flows through both the LED and resistor.

    The LED is designed to conduct 20 mA of current while having 1.8 volts appear across it. That means that the LED is designed to dissipate the power that is the product of that 1.8 volts and 20 mA.

    Calculating the power dissipated by the LED.

    P = V * I This is the common definition of power where P is expressed as Watts derived from the product of voltage multiplied by current.

    PLED = 1.8 V * 20 mA 1 mA is equal to 1*10-3 Amps
    PLED = 1.8 V * (20 * 10-3)A
    PLED = 1.8 V * (20 * 10-3)A
    PLED = 0.036 W

    The LED is designed by the manufacturer to safely dissipate a fixed power level when inserted into a properly designed circuit. It is up to us to properly design the circuit. All we must do is select a resistor that will limit the current flow through the LED to 20 mA, and at the same time be able to conduct that same current while 10.2 Volts appears across that resistor. Easy !!

    We know a few things about the resistor, namely…

    VResistor = 10.2 V
    IResistor = 20 mA

    This is enough information for us to be able to calculate the value of the resistor and the power dissipation capability of the resistor.

    First, the value of the resistor…

    Ohm’s Law states that resistance, voltage and current in a single device or circuit are related in the following manner…

    Resistance is equal to voltage divided by current.

    Or
    = V / I

    Substituting what we already know…

    = 10.2 / (20 * 10-3)
    = 10.2 / 0.02
    = 510 Ohms

    The calculated value of the resistor is 510 (ohms). The practical value of the resistor will be what is available from our friendly electronic component retailer. The standard value that is closest to our requirements is 511 - I declare this to be close enough, we will use a 511 resistor in our circuit and use that value for the remainder of our calculations.

    We now know the resistance value required, now we need to know the power handling capability of the resistor.

    P = V * I
    P = 10.2 * (20 * 10-3)
    P = 10.2 * 0.02
    P = 0.204 Watts ....... Remember this number.

    Remember, we stated that we should use the practical value of the resistor for the remainder of the calculations. Let’s see if we can do that.

    P = V * I

    From Ohms Law; = V / I

    Re-arranging the terms of Ohms Law; I = V /

    Substituting into the original power equation;

    P = V * (V / )
    P = (V * V) /
    P = V^2 /

    We now have a means of using our practical value of resistance in our design.

    P = 10.2^2 / 511
    P = 10.2^2 / 511
    P = 104.04 / 511
    P = 0.2036 Watts

    Amazingly, this power value is very close to the power value that we calculated earlier. We can use a resistor that is capable of dissipating at least 0.0204 Watts. Once again we are forced to accept what is practically available from the retailer. In this case we will use a ¼ watt resistor with a value of 511 ohms.

    Our circuit now looks like this…






    Figure 7: A working LED circuit with component values

    We have successfully designed a circuit that will function and, as a bonus, not destroy itself. Are we finished? Well we could be, but there is more yet to consider.


    Part 6 – Circuit Design Considerations

    What do we need to think about further? Going back part 5 for a moment, we were forced to use practical values of components that were readily available to us. Thinking on that a little further, there are ways to improve the practical aspects of our simple circuit.

    First, we should consider the physical size of the current limiting resistors. The difference in this regard between 1/8 watt, ¼ watt and ½ watt resistors is considerable. A small resistor will fit nicely into a compact fixture, or can be less visible when wired inline with the LED and covered with heat shrink tubing for a nice, attractive installation.

    But how can we control the size of the resistor? Knowing that the current flow is fixed by the characteristic of the LED, a parameter that is, in fact, a design target and cannot be changed, we are left with voltage that appears across the resistor.

    Using the derived equation for resistor power capability.

    P = V^2 /

    It becomes obvious that a smaller voltage across the resistor will result in a smaller calculated value of resistor power capability.

    The obvious means to decrease the voltage drop across the resistor is to decrease the power supply voltage. One might think that this is an attractive option, as a smaller supply might mean a smaller cost for the component. This is not likely true, as the cost determining factor is typically the total output power capability, a combination of voltage and current output.

    A more elegant design would be to use a single resistor to limit the current to more than one LED.

    Like this…






    Figure 8: A circuit that reduces the power rating of the resistor.


    Let’s suppose that the LED’s are identical to the previous example that we worked through in part 5. Each LED has a 1.8 voltage drop appearing across it, leaving.

    12 – (4 * 1.8) = 4.8 volts across the resistor

    With 20 mA LED current through the resistor (the LEDs in series all share the same current), the value of the resistor is.

    4.8 / .02 = 240 Which coincidently is a standard resistor value.

    And the power rating of the resistor will be.

    4.8 ^2 / 240 = .096 Watts

    We can use a 1/8 Watt resistor, which, from a physical size point of view is a much smaller package to fit into a tight space.

    A question arises with the current that flows through the resistors. In the series example in figure 8 the same 20 mA flows through each LED. Therefore the total circuit current is 20 mA.

    Consider the parallel equivalent circuit.






    Figure 9: Four LED’s in a parallel circuit.

    Each LED draws 20 mA of current, at the same time. Therefore the total circuit current is.

    4 * 20mA = 80mA

    The resistor value should be calculated using 80 mA as a current value. And the voltage drop that appears across the LED network would be the same regardless how many LED’s are wired in parallel. So the voltage drop across the resistor would be the power supply voltage minus the voltage drop across a single LED.

    I will let the avid reader work out the power handling capability of the resistor in this case. Hint; the resistor needs to have a much greater power handling capability.

    This would be a convenient time to discuss the pros and cons of both types (series and parallel) circuits. The differences are most prevalent in the case of a failure of one of the LED components.

    Let’s face it, things fail, and if one of the LED’s in either of our circuits fail, the results are different for each case.

    When an LED fails, it will fail to either an open or closed state, meaning, that it will become like a switch that is either on or off.

    Let’s first suppose that an LED fails to an open state, like a switch that is off.

    In the series circuit case, it would be like breaking a wire in the circuit, all four LED’s would be off, this is completely safe failure mode, however, it might be tricky to identify the failed LED.

    In the case of the parallel LED circuit, the current that normally would flow through the failed LED would be distributed through the remaining LED’s. This could possibly overdrive the LED’s causing damage or failure of the remaining LED’s. This is a potentially damaging situation.

    Now, what if the LED were to fail to a closed state, like a switch that is on?

    If this were to occur in the series circuit, the LED would simply fail to illuminate. Current would still flow, so the remaining LED’s would remain in an on state. A situation that is safe and easy to troubleshoot.

    In the parallel circuit however, the failed LED would effectively “short” out the other LED’s and none of the LED’s would be illuminated. On top of that, the current limiting resistor would be subject to the entire power supply voltage since there would be no voltage drop across the LED network. This would cause the power dissipated by the resistor to increase, possibly causing the resistor to fail.

    So there is much to think about when considering circuit design and layout.


    Part 7 – If your house has “toads”, does your wall have “warts”?

    Power supply considerations are very important. The most common type of power supply that will be used for a project such as this is sometimes called a “wall wart”. I am referring to a box, containing a transformer and a few electronic components inside, with a wall plug and a cord on the outside. The wall wart simply plugs into the wall, and at the end of the cord is a low voltage source that is quite suitable for our application.

    I performed a simple test with a wall wart that was rated at 6 volts and 900 milliamps. One might think that the output voltage would be approximately 6 volts while drawing a range of current from zero to 80 % of the rated current as stated on the case. This is not so true.

    Using a variable transistorized load, a current probe and a voltmeter, I was able to record the output voltage as current draw was increased from 0 to 1100 milliamps (1.1 amps). The results were hammered into a spreadsheet program and a chart derived from the data.

    Here are the results.



    Figure 10: A simple power supply output voltage verses current chart.

    As you can see the output voltage is not by any means constant at 6 volts as you might expect. In fact the voltage ranges from just greater than 9 volts at no load conditions, to just less than 5 volts at rated current + 20%.

    So how are you supposed to design around that?

    A large part of the design process is the empirical testing that is part of the prototype process. That is, the designed circuit is built as calculated and actual measurements are made. Voltage drops across components are measured, new current values are calculated or measured and the circuit component values are adjusted so the designer is confident that all components are operating well within the electrical parameters as specified by the manufacturer.

    A more elegant solution is to use a voltage regulated power supply or a transistorized voltage regulator on the output of the wall wart. That opens a very thick book on an interesting topic that we will not explore further here, perhaps in another article.

    Somewhere around this point in the process the DIY’er will experience that wonderful feeling of satisfaction that was briefly mentioned at the beginning of this article. It is all worthwhile.

    Part 8 – In closing.

    At the end of it all we wish to have a working electrical circuit that is functional and looks really neat, a great addition to the aquarium.

    But we did mention safety, as indicated earlier, electricity and salt water are a dangerous combination. Here are some guidelines for finishing the project.

    There should be no exposed wires when you call the project complete. Any solder joints or wire connections should be covered with a water proof material.

    A good practice is to coat the connections with epoxy then cover with heat-shrink tubing that overlaps the insulation on the wires and components. Heat shrink tubing is available at any electronic supply shop, the same place where you purchase your resistors or LED’s. It is a hollow tube that is cut to length and when placed over the solder joint, and heated with a soldering iron or heat gun, will shrink to about 20 percent of its original diameter to provide a tight fit. If the joint is covered with epoxy and heat shrink tubing applied, the joints can be considered watertight for all intents and purposes. One must remember to slide the heat shrink tubing over the unsoldered wire before completing the electrical connection.

    Lastly and most importantly, all projects, regardless of the level of simplicity, must be protected by a fuse. Not only is the circuit protected, also the user, the aquarium stock and possibly the house that contains both groups of lifeforms is also protected. It is irresponsible to not use fuse protection in your designs.

    A fuse should be sized to not greater than two times the total circuit current and not less than 1½ times the total circuit current. The fuse is placed at the positive terminal of the power supply output.


    Like this.






    Figure 11: Fuse protected LED circuit.

    So that is it. All you need to know to build a DIY LED Moonlight system for the marine aquarium.

    Please read and understand all the information presented here. I have tried to make the descriptions and diagrams as clear as possible. Be careful and have fun.
    Last edited by kgolem; November 1st, 2007 at 02:10 AM.

    Set up date: June 2005 Display Size: 130 Total Water In System: 170 Do you have a sump? NO
    Do you have a refugium? YES Amount of Liverock: 220 lbs. Depth of Sandbed: 6 inches of Bomix Salinity level: 1.025
    ALK level: na Ammonia: 0 Nitrate: 0 Nitrite: 0
    What lighting are you using? What skimmer are you using? Calcium Reactor? Calcium level: na
    6 X T5 HO DIY fixture. DIY No
    Fish and Inverts: Hippo Tang, Coral Beauty, Tank Raised Perc Snowflake Moray, Bubble Tip Anemone. Corals: Common Mushrooms, Small Toadstool. Brown Gorgonian Frags, Green Open Brain
     

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