Electrical Question need help

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    Electrical Question need help

    I know there are electrical wizards out there so hopefully you can help me here....I am trying to connect a radio shack 276-036c 5mm red blinking led to a 9v battery... Maximum ratings on the package are 2.5-5vdc supply voltage, 55-80ma ON-state foward current, 0.4vdc reverse current. Does anyone know with using a 9v battery what resistor i need to use????? Thanks for your help
    dan

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    Re: Electrical Question need help

    Unless my math is WAY off (it IS early in the morning) for 9V with an ON current of 55mA - 80 mA, you'd need a resistance of 1636 Ohms - 1125 Ohms. The higher the current, the less the resistance, since they are inversely proportionate.

    I'd say get a 1000 Ohm (1K) resistor and connect it in series with a 370 Ohm resistor. This would give you a total resistance of 1370 Ohms @ about 65mA using a 9V battery, which should be fine. It would be right in the middle of the range, so to speak. 1K and 370 Ohm resistors are readily available at Radio Shack.

    Hope this helps!

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    Re: Electrical Question need help

    fetthunter are you positive thats quite a statement...I know nothing about this stuff. So if you are sure then i will go for it....Fettpride you agree??
    thanks
    dan

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    Re: Electrical Question need help

    OPPI - Sorry, yes I do agree

    But If you want to suck every last ounce of brightness from the LED, you could go as low as just one 330 Ohm resistor,I do it a lot. I wouldn't do it if it were NOT a blinker. Since it is a blinker, the blinking keeps the constant load off of the LED. But Fetthunter's suggestion is the way to go for most applications.

    Chris

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    Re: Electrical Question need help

    I'll go with Fetthunter on this one. The LED chaser kit I used for my chest display has a 1 meg resistor, as well as a 3.9k resistor for each transistor. The above combo should work. My kit works with a 9 volt battery as well.

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    Re: Electrical Question need help

    Well, as a disclaimer, it HAS been a LONG time since I used my EE degree for anything useful. I jumped the fence in 1997 and wandered off into the world of Information Technology. However, some things like "R=V/I" you just never forget.

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    Re: Electrical Question need help

    Well i believe you guys...already had one led BLOW UP on me....wanted to see what this one looked like right away so i touched the led right to the terminals on the 9v battery.....WHOOOPS it lasted about 2 seconds then POP... and now i need another trip to the beloved radio shack....i knew better not to do that....but oh well....thanks everyone
    dan

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    Re: Electrical Question need help

    You listed the max specs for the LED ... do you have the nominal values on the package? Using a 1k resistor won't hurt anything, and you'll be safe. But as far as calculating the "ideal" value to use, you'll want to go with typical values (nominal). It's possible I can pull the files at work if you want.

    Again, the suggested values should be fine ... but as someone else mentioned getting max brightness, I'm not used to seeing red LEDs (blinker or otherwise) pull that much power. My guess is that your LED is pulling a lower voltage and less current, meaning that too high a resistance will leave you safe, but dim.

    ATM
    wild_wire@yahoo.com

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    Re: Electrical Question need help

    the nominals on the package are the lower numbers in the ranges above

    supply voltage 2.5-5.0vdc
    ON-state foward current 55ma
    peak wavelength 697 nanometers
    blink rate 0.5-3.0 hz
    1.2 mcd intensity

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    Re: Electrical Question need help

    1.2 mcd? Sheesh ... I knew there was something about blinkers I didn't like. You'll be doing good to see it unless you're standing in a dimly lit room.

    OK, using the nominal values you listed, a forward drop of 2.5V across the LED and a 9V source (battery), you're looking at the resistor having to drop 6.5V (when the battery is fresh). Given a current drain of 55mA, ohm's law would give you a resistance of 118ohms ... 120 ohms is the closest standard value you'll find. Those crazy EEs ...

    Seriously though, ohm's law (if you're interested) would work out like this:

    R = E/I R = Resistance, E = Voltage, I = Current

    But the voltage you use in the calculation isn't the voltage of the battery or what's dropped across the LED ... it's the voltage that's left over. What the resistor will have to drop, which is 9V (battery) - 2.5V (LED) = 6.5V (resistor). Take that voltage, divided by the current through the circuit (series circuit will have the same current flowing throughout), and there's the resiatance you need. As stated before, anything higher than this will ensure you're circuit is safe ... but will also give you a reduced output (dimmer).

    Oh, and you'll be dropping about 0.35 watts across the resistor. So if you want a "bullet proof" circuit, you'll want a 1/2-watt resistor ... but since it's pulsed, you should be able to get away with an 1/4-watt.

    Good luck,

    ATM

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