[simpits-tech] Resistor for LEDs?

[fitchett]

You can get around the 1 resister/LED by wiring them in series, if you
just want to light up a whackload of LEDs.  You will have to increase the
voltage you are powering them with depending on how many LEDs are in
series, about 2 volts per LED.  At the end of the chain, you put two
resistors, each going through a transistor (the optoisolated outputs are
good for this) to ground.  If you have both outputs off, the LED chain is
dark, one output will run the chain dim, and both outputs will run the
chain bright.  You can get away with only using one output by wiring one
resistor to ground directly, and switching the other resistor.  This will
give you dim by default, and bright under software control.

David Allen uses this to do bright/dim, for bulbs.  He is using a zener
diode to 'drop' a known voltage.

http://www.737flightsim.com/28v16v.html

Putting LEDs in series also simplifies the wiring for backlighting
operations.

Just don't drive LEDs in parallel without resistors for each LED; unless
you are using an constant current LED driver.

Chester

> Hey cool,
> so far this is very interesting and I actually understand it and will use this
> idea for my panel back lighting too.
>
> My question would be, how do I modify this so that I can control the level of
> brightness. Im thinking either a set day/night switch, or a variable resistor
> but where would I need to put it?
>
> Chris.
>
> > Ahhh, that makes sense...
> >
> > Yes on resistor per led or you would have to calculate the ohm value
> > over again when ever you add more led to the "bus"
> >
> > or as an alternative, if you are using a ATX PSU, it will have a 3.3
> > volt output aswell, and if im not mistaken you kan drive 50w (but im
> > not
> > 100% sure, but the Athlon uses something in this region) from that 3.3
> > line, this gives you (acording to this law : P=U*I) 15A, enough to
> > drive
> > more then 800 leds
> >
> > http://www.akasa.co.uk/enhance/atx1125b.gif
> >
> > /Stig
> >
> > -----Original Message-----
> > From: Joseph Fagner [mailto:crease-guard@attbi.com]
> > Sent: 23. december 2002 21:35
> > To: simpits-tech@simpits.org
> > Subject: RE: [simpits-tech] Resistor for LEDs?
> >
> >
> > Q2. I understand what you are asking. He wants to know that if he set
> > up a power strip (i.e power coming into a bus bar with multiple
> > connecting site to feed power to multiple LED's) would he need to have
> > a
> > resistor for each LED or could he put one resistor between the bus bar
> > and power line? I believe you would need to use a resistor for each
> > LED, as if you start lighting several LED's, you will be running an
> > increasing current through that one resistor as the multiple LED's
> > produce an increasing load. If the resistors wattage is rated high
> > enough you could probably get away with just one, but I think it would
> > be safer to use a 1/8 watt resistor for each LED.
> >
> > Any one disagree? I'm no electronics expert but this is what I plan on
> > doing.
> >
> > Jay
> >
> > -----Original Message-----
> > From: simpits-tech-admin@simpits.org
> > [mailto:simpits-tech-admin@simpits.org] On Behalf Of Stig Joergensen
> > Sent: Monday, December 23, 2002 1:18 PM
> > To: simpits-tech@simpits.org
> > Subject: RE: [simpits-tech] Resistor for LEDs?
> >
> > Mike
> >
> > q1:
> >
> > do you know this law: U=R*I that is Voltage = Risistance * Amps
> >
> > you need a voltage drop of 5-3.2 = 1.8v and the amps is about 18ma this
> > will give you 1.8 = R * 0.018 eg. R = 1.8/0.018 = 100 ohm
> >
> > q2: i dont understand what you are asking?
> >
> > /Stig
> >
> >
> > -----Original Message-----
> > From: Mike Williams [mailto:mikewilliams3@comcast.net]
> > Sent: 23. december 2002 19:53
> > To: simpits-tech@simpits.org
> > Subject: [simpits-tech] Resistor for LEDs?
> >
> >
> >
> >
> > Going to be using a PC power supply for my LEDs for my pit, using the
> > 5v
> > line, what
> > resistor would I use to bring the voltage and amps down to 3.2v, second
> > part
> > of the question, if I feed a terminal strip with the 5v line to feed
> > all
> > the
> > lines to the LEDs, (panel backlighting) could I use just one on the
> > incoming
> > 5v line, or would I use them on all lines from the terminal strip to
> > each
> > LED, sorry if this question is simple, I'm just not much of an
> > electronics
> > guy.
> > Thanks
> >
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>
> Chris Brace
> V(H) +64 4 973-6378
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> E chris.brace@paradise.net.nz
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