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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Fri, 17 Aug 2007 15:08:15 GMT
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Viewed:
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5520 times
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"L. Andrew Reynolds" <andrew_reynolds@uhc.com> wrote in message
news:JMvuv7.AF2@lugnet.com...
> I am trying to setup some LED lights to run from the 9v track or a 9v
> battery
> pack for some train signals. I have 1.5v lights and the track is 9v
> (obviously)
> how can I step down the voltage. Any help would be great.
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.
I can go more in depth if you want or I can simply give you the answers. I
prefer teaching over telling... If you want to hook up more than one LED
either in series or parallel, just ask!
-Rob
www.brickmodder.net
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Message has 3 Replies:
 | | Re: Need Electrical Help
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| Yes Thank you very much. This is a big help I am electrically challenged in circuits. I am trying a Microwave switch to power a train signal and track for a manually movable bridge. I will try to post it later L. Andrew Reynolds (17 years ago, 17-Aug-07, to lugnet.trains)
| | | Re: Need Electrical Help
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| (...) 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 (...) (17 years ago, 20-Aug-07, to lugnet.trains)
| | | Re: Need Electrical Help
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| (...) I realize that this is an old post, but I jut found it...If I want to hook up a bunch of LED's (say they are 2.1 volt and I want to hook up 10) to a 9v system, what do I need to do? Only hook in four at a time or something like that? I could (...) (16 years ago, 22-Oct-08, to lugnet.trains)
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Message is in Reply To:
| | Need Electrical Help
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| I am trying to setup some LED lights to run from the 9v track or a 9v battery pack for some train signals. I have 1.5v lights and the track is 9v (obviously) how can I step down the voltage. Any help would be great. (17 years ago, 16-Aug-07, to lugnet.trains)
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