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Thread: Electrical Circuit for DIY LED, need input

  1. #1
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    Electrical Circuit for DIY LED, need input

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    Which is better?

    - High Voltage with Low Current
    With a bridge ($5) I convert 240Vac to 240Vdc I can power up 63 LED (3.8V per LED) connected in Series at 0.1mA

    - Low Voltage with High Current
    With a 12Vdc Adapter ($50 - $100), I connect 21 parallel sets of 3 LED (3.8v per LED). but as the connection parallel, I need to supply 21 x 0.1mA... meaning I need 2100mA

    Power consumption is the nearly the same Pwr = Voltage x Current = 24Watts

    Many advise me against playing with 240Vac to 240Vdc with a bridge. it might be cheap but dangerous. But I can add on more parallel connection anytime I want.

    Using a 12Vdc adapter, on the other hand is more expensive. But don't think it is much safer with 2.1A running round the circuit. Moveover I need to buy more adapters ($50 to $100 for high current) if I need to create a few more sets.

    Your advise is greatly appreciated
    Baby Steel!

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    LED is current drive, thus it is typically connected in series for uniform brightness.
    -Robert
    Aquascaping is a marriage between Art and Farming
    My Blog: http://aquatic-art.blogspot.com/

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    Sorry for the typo... should be 0.1A or 100mA instead of 0.1mA

    Bro, LED need voltage to operate... you can't connect 63 LED in series to a 12Vdc source.

    Let me rephrase my Question. Assuming the resistor value (ohm and wattage) is already known, just want some opinion as to which to choose.

    BTW In case anyone want to know what is a bridge. it is used to convert AC to DC
    Baby Steel!

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    of course need voltage but the brigness is control by current pass throught it. If you connect it in parallel, some LED might draw more current than the other thus some LED might brighter than the other.
    -Robert
    Aquascaping is a marriage between Art and Farming
    My Blog: http://aquatic-art.blogspot.com/

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    With the same number of LEDs and the same resistor connected in parallel. the current passing through each parallel connect is same ± v.small % (due to resistor % error, Silver 10% and Gold 5%, temperature also affects the resistance) in theory .

    I have load compare LEDs side by side drawing different current. looking from an angle away from the light path and at a distance with my naked eye, I can't tell the different, until the current gap is big 10mA vs 100mA then i can see significant difference. So it is not a concern.

    DO NOT LOOK DIRECTLY INTO THE HEART OF THE LED WHEN YOU POWER THEM UP... I remember seeing dark spots for several min when the high power LED light took a direct hit on my eyes!
    Baby Steel!

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    Seems you already make up your mind.

    For what it worth, You might be interested on Maxim application note:
    http://www.maxim-ic.com/appnotes.cfm/an_pk/3256

    There was also another article related to LCD backlight using LED, but can't find the link
    -Robert
    Aquascaping is a marriage between Art and Farming
    My Blog: http://aquatic-art.blogspot.com/

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    Quote Originally Posted by Shadow View Post
    Seems you already make up your mind.

    For what it worth, You might be interested on Maxim application note:
    http://www.maxim-ic.com/appnotes.cfm/an_pk/3256

    There was also another article related to LCD backlight using LED, but can't find the link
    No I did not make up my mind, Just disagree with you on the inconsistent brightness issue base on the parallel connection.

    In fact, I am inclined to the direct 240Vac - Diode Bridge - 240Vdc then connect 63LEDs in series then later add on more sets of 63LEDs in parallel when I have the $$$ to spare.

    In case you wanna know where I come up with 63 LEDs, Dealer indicated that each LED need 3.4 to 3.8V to work. Extreme case of 3.8V, i can connect 63 LEDs (MAX) which is in total 239.4V.

    So I need a resistor that can drop the voltage by 0.6V at 100mA. R = V/I
    Resistor is 6 Ohms with a power (P = IV) rating of 0.06W. So a cheap 10cents 6Ω ¼W will do the job.

    Just need someone to tell me extra facts, e.g Warning: Direct 240Vac conversion to 240Vdc, very dangerous, Diode Bridge not stable will explode or something. Or Advise: connect ??? way, it is safer and can save money.

    Main concern here is, how come people spend $$$$ to buy a step down AD-DC adapter when I can use a $5 Diode bridge for the job.
    Baby Steel!

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    If you read the aplication note, from the first figure you will see that at the same forward voltage different LED draw different curent. Nothing you can do about it except screening it. It is due to the variation in semiconductor process. Thus for LED it prefered to drive them with constant current.

    When applying white LEDs for display backlighting or other illumination applications, there are two reasons to drive them with constant current:
    1. To avoid violating the Absolute Maximum Current Rating and compromising the reliability.
    2. To obtain predictable and matched luminous intensity and chromaticity from each LED.
    -Robert
    Aquascaping is a marriage between Art and Farming
    My Blog: http://aquatic-art.blogspot.com/

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    erm. it is not wise to do series led circuit with so many LED together. u have say u are playing with 240VDC!!! any short on your circuit's it will instantaneously catch fire man!!! don't play play. think u better start from square1. try serial+parallel LED circuit. and the 12VDC/2Amp or more adapter that cost $50 to $100 normally will have overload protection circuit built-in and it is still much safer then the 240VDC/100mA bridge. and normally a 12vdc/2amp adapter w/o overload protection circuit built-in should cost less that $30 buck cause it just a step-down transformer only.and even u really want to do a bridge .

    it not that simple like the diagram u show. u still need to add in some other component ..like diode-|>|-,cap -||- to the circuit to filter always the ripple to make the DC wave to be more stable and noise free .And add in an overload protection circuit.base on the add on of component added to your bridge design, u end up spending more then saving. if w/o all the filter and overload protection circuit, u will have blinking led and every now and then replacing spoil resistor and LED due to the unstable DC voltage that cause by the $5 buck bridge.

    just my 2cent
    Last edited by sacy; 25th Dec 2008 at 12:07. Reason: add more word
    一个相机走天涯..

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    Agree with that, when you are working with hight voltage you might want to start thinking about safety. Adding fuse for example, in case acidentally shorted.

    You might want to take a look on the LED tube that NA sell. If I seeing it correctly it does not even have a diode, just 1big and 3 small resistor to limit the current. It worked because LED is essentially diode, but you might see the 60 Hz flicker.
    -Robert
    Aquascaping is a marriage between Art and Farming
    My Blog: http://aquatic-art.blogspot.com/

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    Quote Originally Posted by limsteel View Post
    No I did not make up my mind, Just disagree with you on the inconsistent brightness issue base on the parallel connection.

    In fact, I am inclined to the direct 240Vac - Diode Bridge - 240Vdc then connect 63LEDs in series then later add on more sets of 63LEDs in parallel when I have the $$$ to spare.

    In case you wanna know where I come up with 63 LEDs, Dealer indicated that each LED need 3.4 to 3.8V to work. Extreme case of 3.8V, i can connect 63 LEDs (MAX) which is in total 239.4V.

    So I need a resistor that can drop the voltage by 0.6V at 100mA. R = V/I
    Resistor is 6 Ohms with a power (P = IV) rating of 0.06W. So a cheap 10cents 6Ω ¼W will do the job.

    Just need someone to tell me extra facts, e.g Warning: Direct 240Vac conversion to 240Vdc, very dangerous, Diode Bridge not stable will explode or something. Or Advise: connect ??? way, it is safer and can save money.

    Main concern here is, how come people spend $$$$ to buy a step down AD-DC adapter when I can use a $5 Diode bridge for the job.
    Quote Originally Posted by sacy View Post
    erm. it is not wise to do series led circuit with so many LED together. u have say u are playing with 240VDC!!! any short on your circuit's it will instantaneously catch fire man!!! don't play play. think u better start from square1. try serial+parallel LED circuit. and the 12VDC/2Amp or more adapter that cost $50 to $100 normally will have overload protection circuit built-in and it is still much safer then the 240VDC/100mA bridge. and normally a 12vdc/2amp adapter w/o overload protection circuit built-in should cost less that $30 buck cause it just a step-down transformer only.and even u really want to do a bridge .

    it not that simple like the diagram u show. u still need to add in some other component ..like diode-|>|-,cap -||- to the circuit to filter always the ripple to make the DC wave to be more stable and noise free .And add in an overload protection circuit.base on the add on of component added to your bridge design, u end up spending more then saving. if w/o all the filter and overload protection circuit, u will have blinking led and every now and then replacing spoil resistor and LED due to the unstable DC voltage that cause by the $5 buck bridge.

    just my 2cent
    i seconded bro limsteel and sacy, it is not wise to have 63 led light connected in a series circuit, you will have problem trouble shooting a blown bulb just in case any 1 of the led light failed you.
    Regards
    Robin Tan

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    that very easy, just use multimeter
    -Robert
    Aquascaping is a marriage between Art and Farming
    My Blog: http://aquatic-art.blogspot.com/

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    ya multimeter will do the job ^^.

    NA got sell the 240vac T5/T8 LED tube lights?
    一个相机走天涯..

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    just abit of of track.. any idea do any where sell led reflector?? i have seen them at swarovski crystal shops..

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    63 LEDs won't be very bright. The brightness of the LED can be determined by the DC voltage used.

    I tried the brightness of Vdc powered LED light. The brightness is limited, compared to Vac powered LED light.

    NA does sell Vac powered LED tubes.

    For the start, you can get those LED spot light from lighting shops. They have variants of Vdc powered and Vac powered. I got a pair of Vac powered LED spot light(12 high-intensity LEDs) for $10 each from a lighting shop at Kelantan Lane. You can check out how it is connected.

    Personally, I feel the price of individual LED still very high. Do a search at alibaba website on LED lights, you will be able to see the number of commercially available LED lightings. Maybe you can get some idea from them.

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    Quote Originally Posted by khseah79 View Post
    just abit of of track.. any idea do any where sell led reflector?? i have seen them at swarovski crystal shops..
    swarovski crystal that gonna cost you a bom

    LED is only shine in one direction, no need reflector at the back.

    Usually LED come with build in lens, so you might need diffuser instead You can buy LED with different viewing angle.
    -Robert
    Aquascaping is a marriage between Art and Farming
    My Blog: http://aquatic-art.blogspot.com/

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    Quote Originally Posted by sacy View Post
    erm. it is not wise to do series led circuit with so many LED together. u have say u are playing with 240VDC!!! any short on your circuit's it will instantaneously catch fire man!!! don't play play. think u better start from square1. try serial+parallel LED circuit. and the 12VDC/2Amp or more adapter that cost $50 to $100 normally will have overload protection circuit built-in and it is still much safer then the 240VDC/100mA bridge. and normally a 12vdc/2amp adapter w/o overload protection circuit built-in should cost less that $30 buck cause it just a step-down transformer only.and even u really want to do a bridge .

    it not that simple like the diagram u show. u still need to add in some other component ..like diode-|>|-,cap -||- to the circuit to filter always the ripple to make the DC wave to be more stable and noise free .And add in an overload protection circuit.base on the add on of component added to your bridge design, u end up spending more then saving. if w/o all the filter and overload protection circuit, u will have blinking led and every now and then replacing spoil resistor and LED due to the unstable DC voltage that cause by the $5 buck bridge.

    just my 2cent
    Ah! that the kind of Reply I am looking for... I read the noise created by UPs and Downs for the DC wave. 1 way is to connect with a capacitor but I donno how many F is need to smooth out the ripple effect. The $5 bridge should cover the Diodes portion. As for the over load circuit, 3A fuse?


    Quote Originally Posted by Shadow
    Agree with that, when you are working with hight voltage you might want to start thinking about safety. Adding fuse for example, in case acidentally shorted.

    You might want to take a look on the LED tube that NA sell. If I seeing it correctly it does not even have a diode, just 1big and 3 small resistor to limit the current. It worked because LED is essentially diode, but you might see the 60 Hz flicker.
    1 "big resistor", it might be the Bridge. Bridge itself is a 4 diode circuit. If no capacitor, then the blink is 60 time a second... don't think our eyes can detect that. but the fluctuation might cause resistor failure as Sacy mentioned.


    Quote Originally Posted by ghim
    63 LEDs won't be very bright. The brightness of the LED can be determined by the DC voltage used.

    I tried the brightness of Vdc powered LED light. The brightness is limited, compared to Vac powered LED light.

    NA does sell Vac powered LED tubes.

    For the start, you can get those LED spot light from lighting shops. They have variants of Vdc powered and Vac powered. I got a pair of Vac powered LED spot light(12 high-intensity LEDs) for $10 each from a lighting shop at Kelantan Lane. You can check out how it is connected.

    Personally, I feel the price of individual LED still very high. Do a search at alibaba website on LED lights, you will be able to see the number of commercially available LED lightings. Maybe you can get some idea from them.
    12 Volts support 3 LEDs - 240Volts Support 63LEDs what wrong with that? So connecting 63 LED in series will have the same brightness when there is enough voltage to push the current thru.

    LEDs are semi conductors, for LED, current can only pass in 1 direction unless the reverse Voltage is high enough to blow it. So there is no such thing as a Vac LEDs. there must be a circuit that convert it to DC.

    e.g:


    Quote Originally Posted by khseah79
    just abit of of track.. any idea do any where sell led reflector?? i have seen them at swarovski crystal shops..
    Agree with Shadow, LED shine in 1 direction. Depending on the LED you buy... standard LED have a light angle of 20° to 40° (Diffusing lens as Shadow mentioned). some high power ones has light angle of 80°, star LED have light angel of 120° to 160° (this one you need a focusing lens)

    So get the right LED for the job... If need Light to cover a large area, use a star LED with wide light angle. If you need to focus the light on something maybe a very light demanding SPS Coral then use once that has 20°-40° light angle.
    Last edited by limsteel; 25th Dec 2008 at 20:57.
    Baby Steel!

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    Dear Sacy, Just found a common electronic ballast for the Down Light in my house. it step down 240Vac to 12Vac transformer - 60W (COST $10). So now 12Vac, using bridge $5 to convert it to 12Vdc. OK? Safer?

    Still cheaper than using a laptop charger as the driver for the LEDs... OK I am cheapo! Econ Crisis must cut cost, but CUT is one thing, need to be safe too!
    Baby Steel!

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    Pulsing LED will not cause a fatigue/failure. LCD backlight on your mobile phone, PDA or others equipment are driven using PWM. Pulsing it will increase the LCD live also save power. That the beauty about semiconductor.
    -Robert
    Aquascaping is a marriage between Art and Farming
    My Blog: http://aquatic-art.blogspot.com/

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    Apologies on what I wrote that causes the misunderstanding. What I meant is not using 12 DC adaptor, but direct power from AC.

    Here is an extract from Wiki:
    "Power sources

    The voltage versus current characteristics of an LED are much like any diode. Current is approximately an exponential function of voltage, so a small voltage change results in a large change in current. It is therefore important that the power source gives the right voltage.
    If the voltage is below the threshold no current will flow and the result is a unlit LED. If the voltage is too high the current will go above the maximum rating, heating and potentially destroying the LED. As the LED heats, its voltage drop decreases, further increasing current. Consequently, LEDs should only be connected directly to constant-voltage sources if special care is taken. Series resistors are a simple way to stabilize the LED current, but wastes energy in the resistor. A constant current regulator is commonly used for high power LEDs. Low drop-out (LDO) constant current regulators also allow the total LED string voltage to be a higher percentage of the power supply voltage, resulting in improved efficiency and reduced power use. Switched-mode power supplys are used in some LED flashlights, stabilizing light output over a wide range of battery voltages and increasing the useful life of the batteries.
    Miniature indicator LEDs are normally driven from low voltage DC via a current limiting resistor. Currents of 2 mA, 10 mA and 20 mA are common. Sub-mA indicators may be made by driving ultrabright LEDs at very low current. Efficiency tends to reduce at low currents[citation needed], but indicators running on 100 μA are still practical. The cost of ultrabright LEDs is higher than that of 2 mA indicator LEDs.
    Strings of LEDs are normally operated in series LEDs, with the total LED voltage typically adding up to around two-thirds of the supply voltage, with resistor current control for each string. In disposable coin cell powered keyring type LED lights, the resistance of the cell itself is usually the only current limiting device. The cell should not therefore be replaced with a lower resistance type.
    LEDs can be purchased with built in series resistors. These can save printed circuit board space and are especially useful when building prototypes or populating a PCB in a way other than its designers intended. However, the resistor value is set at the time of manufacture, removing one of the key methods of setting the LED's intensity. Alphanumeric LEDs use the same drive strategy as indicator LEDs, the only difference being the larger number of channels, each with its own resistor. Seven-segment and starburst LED arrays are available in both common-anode or common-cathode form."

    Below is how the resistors are used on my LED lights:



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