Subject:
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Re: Need Electrical Help
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Newsgroups:
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lugnet.trains
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Date:
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Mon, 20 Aug 2007 15:40:29 GMT
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Viewed:
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5936 times
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In lugnet.trains, Rob Hendrix wrote:
> Typically, most LEDs have a mA rating of around 20mA. Let's assume this is
> the case for yours as well. I have seen some 1.5v LEDs so I 'm taking your
> word that yours are 1.5v like you say, instead of them being the more
> normally found 2.1v LEDs (assuming you're using red, green or yellow LEDs).
> So now we have all the components we need for the formula to figure out what
> ohm (and what wattage) resistor you need. I'm also assuming you are only
> hooking up 1 LED per 1 resistor.
>
> To calculate required LED current limiting resistor (R):
>
> R = (Vs - Vl)/I
>
> R = Resistor ohm
> Vs = Voltage source
> Vl = LED forward voltage
> I = LED current in amps (remember 20 mA = .02A)
>
> For your LED running at 9 volts, lets put the numbers in the formula:
>
> (9 - 1.5) / .02 = 375 ohm
>
> They don't make 375 ohm resistors, so choose 390 ohm which is the nearest
> standard larger value. Now let's figure out the required wattage of the
> resistor we need.
>
> P = (Vs - Vl) * I
>
> P = Power (in watts)
> Vs = Voltage source
> Vl = LED forward voltage
> I = LED current in amps (remember 20 mA = .02A)
>
> (9 - 1.5) * .02 = .15 watt
>
> That's too much for an 1/8w resistor (.125 watt), so choose a 1/4 watt (.25
> watt) which is the nearest standard larger value.
Let me check I understand the theory of this - you want 1.5V across the LED,
which means the resistor has to take the other 7.5V. You then pick a resistor
that gives you a 20mA current at that figure of 7.5V. Then you double-check
the power so you don't cook the resistor?
So two LEDs in series would drop 3V, so you'd work out the resistor values based
on the remaining 6V but still with 20mA going through the circuit?
Jason R
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Message has 1 Reply:
 | | Re: Need Electrical Help
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| "Jason J Railton" <j.j.railton@btinternet.com> wrote in message news:Jn2wvH.BoL@lugnet.com... *snip* (...) You are correct, In your case (assuming those LEDs are 1.5v and not your typical 2.1v variety), Vl would be 1.5v + 1.5v, or 3v So in our (...) (17 years ago, 20-Aug-07, to lugnet.trains)
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Message is in Reply To:
| | Re: Need Electrical Help
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| "L. Andrew Reynolds" <andrew_reynolds@uhc.com> wrote in message news:JMvuv7.AF2@lugnet.com... (...) Typically, most LEDs have a mA rating of around 20mA. Let's assume this is the case for yours as well. I have seen some 1.5v LEDs so I 'm taking (...) (17 years ago, 17-Aug-07, to lugnet.trains)
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