Led help please?

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votelibertarian
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Led help please?

Post by votelibertarian »

I would like to make an anchor light for a boat that is several 8000-12000 mcd white leds and a solar cell and battery. I would like to use a 12v battery. I know very little about electronics. I discovered through experimentation that they won't run in series to compound voltage like a battery. Should I use a resistor and if so of what value. I read somewhere that you can't run these Leds parallel but I don't understand why. they should pull the same voltage if there is one or many. I am thinking a 2 seperate loops of copper tube for hot and ground to solder the leds to and a resistor on each led. would this work and if not how can I do this? Thanks for any help offered.
cato
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Re: Led help please?

Post by cato »

1. Yes you should use a resistor.
2. You CAN string them in series OR parrallel. I would suggest series. That will be easier on your battery.
3. The value of the resistor is governed by ohms Law V=I*R. <p> I don't know the voltage drop of the LEDs you have in mind. The typical drop on an LED is about 1.2 volts at their rated current (which can be anywhere from 2ma to 20 ma). The drop on white LEDs might be very different. But I will use 1.2 for the following example.<p> Lets say you want to string 8 leds in series and that each drops 1.2 Volts. That means the drop across the string is about 9.6 V. That leaves 2.4 V from the 12V battery to drop across the resistor. If you want 20mA to flow through the resistor (and therefore the LEDs) you flip Mr. Ohms equation around to get V/I = R or 2.4/20ma (or 0.020 A) = 120 ohms.<p> You should also consider the wattage of the resistor. Watts are Volts times amps or 2.4 * 0.02, in our case, or 0.048 which means you should be fine with a 1/4 watt resistor which is the standard size that you are likely to find in Radio Shack.<p> Whats an Anchor Light?<p>Now, lets talk about why your experiments showed you "can't" string them in series. <p>I'm guesing 1 of 3 things happened.<p>1. You hooked up one or more of the LEDs backwards. LEDs are Light Emitting DIODES. Current only flows one way in a diode. If one was hooked up backwards, it prevented current from flowing in all the diodes. Therefore there was no light<p>2. All the diodes were connected in the right direction, but you didnt use a resistor and a huge current flowed, destroying all the diodes. I think there would have been a brief flash of light in this case, and the LEDs would be discolored.<p>3. You used too many diodes. Their voltage drop totaled more than the voltage of the battery and only a very small current flowed. It wasn't enough to generate a visable amount of light.<p>Whadayathink?<p>[ December 14, 2003: Message edited by: cato ]</p>
josmith
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Re: Led help please?

Post by josmith »

Considering that most white leds need around 4.5 volts each i would say that #3 in the last answer would be likely. For 12 volts you could run pairs of two in series with a resistor. YOu could run the pairs in parallel.<p>Other solutions include going to superbrightleds.com and buying a led replacement 12v tailight lamp for around $5 they have more than their worth in led's all in a turnkey package. There are also solar led driveway lights that charge in light and come on when it's dark. YOu could never duplicte one for the $10 price.
josmith
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Re: Led help please?

Post by josmith »

The reason you can't put led's in parallel without a resistor is that the forward voltage varies due to manufacturing tolerances. The one with the lowest forward voltage will take most of the current and the others will be dim.
JoshBensadon
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Re: Led help please?

Post by JoshBensadon »

Cato,<p>I think an anchor light is the same as
park lights in a car. It's usually the
light on a pole at the back of a boat
so the boat may be seen at night where
ever it is anchored. Just a beacon light.<p>What I want to know is why does he want it
made with solar panels?!
cato
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Re: Led help please?

Post by cato »

Thanks, I thought it was a set of lights on the anchor itself so you could see it in the water....as far as YOUR question goes: I'd guess that he doesn't want to drain the battery so that he can start the motor the next time he gets a chance to go sailing.
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Chris Smith
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Re: Led help please?

Post by Chris Smith »

"The reason you can't put led's in parallel without a resistor is that the forward voltage varies due to manufacturing tolerances. The one with the lowest forward voltage will take most of the current and the others will be dim"
<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr> <hr></blockquote>
*********************************************
LEDs are Current driven, not voltage tolerant. The resistor limits the current to the right level, and if you ignore individual values, a LED will over draw current and burn out or shorten its life. <blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr> <hr></blockquote><p>[ December 15, 2003: Message edited by: Chris Smith ]</p>
perfectbite
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Re: Led help please?

Post by perfectbite »

quote:<p>LEDs are Current driven, not voltage tolerant. <p>I'm having difficulty understanding this, aren't all electrical/electronic devices built based on a voltage (inherently not voltage tolerant) and draw their current as they require? Or are LEDs special somehow?
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MrAl
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Re: Led help please?

Post by MrAl »

Hello there,<p>When we go out to buy a replacement bulb for the house, we
dont just go by voltage alone--we also go by wattage. If we
need a brighter bulb we might select a 100 watt model, if
not we might go with a 60 watt model. In any case, we dont
go by the voltage alone.<p>When we go out to buy a replacement bulb for the car we
go by number. That is, we buy the same number as the old
bulb. Getting the same number means we will be getting the
same voltage, current, base type, and size, which is
usually what we want.<p>When we need a new bulb for the flashlight we go out and buy
either the same number or we get a bulb that matches the
type and number of cells in our flashlight.<p>From this we can see we never buy a bulb for its voltage
alone, and sometimes we even go by something as abstract as
a number which has nothing to do with the voltage, current,
or even size sometimes.<p>Now what about Leds?<p>If we were to go out to buy a replacement Led for a flashlight
we already had we would want to get the same part number.
Getting the same part number means we would get an Led that
runs at the same voltage and current, and is the same
physical size. That's the right way to do it really.
So again, we went by a number rather then a voltage or
current.<p>Now what if we were to build our own Led flashlight,
where we find an Led available somewhere and want to
supply some power to light it up? This is where we have
to know a little about the actual specs of the Led in
order to make sure we get all that we can from it
and dont burn it out in the first five minutes.<p>Leds are spec'ed according to various parameters, most
noteably are these three:
1. color
2. forward voltage
3. forward current.<p>For a flashlight we usually choose white so we can see
other colors normally when we shine it on something.<p>The other two specs, voltage and current, are electrical
specs that usually explain the 'nominal voltage' and
the 'recommended current'.<p>What we mean by 'nominal voltage' here is the voltage that will
be measured across the Led when the 'recommended current'
is flowing through the Led. This means somehow we want to
get the recommended current flowing, and thus get the
nominal voltage, although in practice we dont worry as
much about the voltage as we do the current once we get
the required current established.<p>So the next question is, how do we get the recommended current
flowing through the Led? This is where the nominal voltage
plays a big role. We look at the specs and we see, for example:
"3.5v @ 20ma"
which of course means 3.5 volts at 20 milliamps.
So this tells us two things:
1. 3.5 volts is the nominal voltage, and
2. 20 milliamps is the recommended current.<p>Now we can use these two pieces of information in the following
manner:<p>Knowing the voltage is 3.5 volts we know right away that we
need a voltage source of GREATER then 3.5 volts. This means that,
say, two AA cells in series wont work because they only add up to
3 volts total. We need at least three cells in series (1.5v each)
in order to light the Led properly. Three cells in series gives
us 4.5 volts. But wait, 4.5 volts is higher then 3.5 volts, so
we cant use the three cells directly in most cases. What we have to
do is provide another component--a resistor--and connect it in series
with the Led. This drops some of the voltage (about 1 volt) and
simultaneously limits the current to the Led. So this brings us
now to the current again....<p>Knowing the recommended current is 20ma tells us that if we are
going to use a source of 4.5 volts (three cells in series) we
need to select a resistor that will drop 1 volt (4.5v-3.5v=1v)
and at the same time provide 20ma. This is very easy to find:
Since we need to drop 1 volt and we need 20ma, we divide 1 by
0.020 to get the answer. 1/0.020 equals 50, so we need a
50 ohm resistor.<p>Thus, we found we can use three cells in series (like AA size)
if we wire a 50 ohm resistor in series with the Led. This combo
will give us the recommended current (20ma) through the Led,
and if the voltage is a little different (say 3.55 volts) we
dont worry about it.<p>If we wanted to do so, we could measure the current after we
hook this initial circuit up and see if the current is really
20ma. If it's a little higher, like 25ma, we let it go. If it's
a little lower, like 18ma, we also let that go. If it's very
different, say 40ma, then we might want to go up on the
resistance a little--perhaps try 75 ohms next--but if it's too
low, say 10ma, then we want to go down on the resistance a little
--try 40 ohms. We do this until we get the current to
be about 20ma.<p>With the higher power Leds, it's also a good idea to calculate
the wattage requirement of the resistor. We do this by
multiplying the voltage across the resistor (we had only 1 volt
this time) by the current through the Led (20ma). In this
case we would get 0.020 watts, which means you can use a resistor
like a 1/8 watt model if you wanted to, or even a surface mount
part.<p>This is basically how you build your first Led flashlight
or other type of light. If you use multiple Leds, you
use one resistor per Led or try to match them up so
you can use one resistor for one group of Leds.
When you get bored with it, check out Candlepower Forums,
where we discuss this kind of thing almost every day :-)<p>Last discussion was whether or not this exact kind of
flashlight could benefit from using four cells instead
of three (with a different value resistor of course).
Is the fourth cell a waste, or does it provide extra
runtime?<p>
Take care,
Al
LEDs vs Bulbs, LEDs are winning.
josmith
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Re: Led help please?

Post by josmith »

That post could use clarification. An led is a diode, which has the characteristics of a semiconductor junction. In the forward mode, a junction requires a minimum amount of voltage before current starts to flow. This value is the "forward voltage". If more voltage is applied, the current will increase rapidly.This forward voltage varies considerably among "identical" devices. <p>Now consider a circuit with two leds in parallel. One with a forward voltage of 4.2 the other with a forward voltage of 4.5. A constant voltage of 4.5 is applied to them. The former would be conducting a large amount of current while the latter would be just beginning to light.
In practice each led would have it's own resistor. If for example you were using a 12 volt supply and wanted your leds on 20 ma, you would size the resistor by deducting the nominal 4.5 volts from the 12 leaving 7.5. Divide 7.5 volts by the desired .02 amps to get 375 ohms. You would use the standard 390 ohm resistor. The first led would draw (12-4.2)/390=.02 amps the second would draw (12-4.5)/390=.0192 amps which would give you nearly equal brightness.<p>For Chris I should have said"In parallel without individual resistors"
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Chris Smith
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Re: Led help please?

Post by Chris Smith »

quote:
LEDs are Current driven, not voltage tolerant. <p>I'm having difficulty understanding this, aren't all electrical/electronic devices built based on a voltage (inherently not voltage tolerant) and draw their current as they require? Or are LEDs special somehow? <p> <blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr> <hr></blockquote><p>LEds will draw what ever you give them,all the way past their friendly value, to the way of burnt, with out question. <p>AND if that voltage or current happens to be "with out limits" or above the led values, it will burn out. <p>Placing a simple resistor in line [a choke], with a given value based upon a given line voltage, it all adds up to "Three out of Three", which is known as Ohms law. <p>The problem with LEDs is that if they are based upon 1.7 volts[one example] and they will draw 20 milli at that exact voltage, then applying two batteries at 3 volts will allow them to go wild, drawing twice that [apx], because its twice the voltage needed, and poof, they suck up twice the current! <p>Water pipes, if you squeeze a water pipe in half, the water flow is half. LEDS, WANT all they can get, and they want to burn out! <p>You must coddle them, to a limit. If you coddle the voltage, the current acording to ohms law, will follow.
Bernius1
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Re: Led help please?

Post by Bernius1 »

Chris, I feel your pain. Think of it this way; a diode is OFF at 0V, and acts as a SHORT over .7V (1.7V > 3.5V for LED). That's because forward conduction is an exponential curve (toward infinity). So, as long as you account for the fwd drop (.7V), you can disallow any 'current consumption' within the device. Only the series resistor is limiting current. That's why when you double voltage, you're doubling the current. The resistor sees double voltage. The diode can be considered a 'short'.
Now, run several in parallel, like a ladder. You can put one resistor in the bottom leg, or a resistor at each rung. At each rung, and you ensure each has equal current. In the leg, when the LED with the LEAST FORWARD DROP ( say 1.69V, vs. 1.73V )starts conducting, it will start to act as a short. So most of the current will divert to THAT LED, up to the limit of the resistor. 'Current hogging' is one way to put it. So, if you draw your circuit, and then re-draw it with the diodes removed, you'll see the distribution of current by the resistors alone. Hope this helps. Merry ....
Can't we end all posts with a comical quip?
toejam
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Re: Led help please?

Post by toejam »

there are several solar powered anchor lights available.(check google) If i were to make them i would consider using three nicad cells in series to get 3.6 volts and a solar battery made up of ten .5volt 500ma solar cells in series to charge them during the day. bg micro sells white leds and nicad's, and electronic goldmine sells solar cells.
good luck and merry christmas and holiday greetings
tj.
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Chris Smith
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Re: Led help please?

Post by Chris Smith »

Ohms law says that a current is what flows, [not voltage]..... and the voltage will follow. <p>Thats why led are current devices. <p>You can place 100 volts at the diode junction, for 100/ of the time frame of one second, and the same amount of current will flow, as if it were one volt, for one second, asuming you got over the threshold values. Its current values, that these devices run on, not voltage. <p>Voltage in this case only apply to threshold values, or poping the cork in the pressue cooker to get the ball rolling. <p>
Voltage FOLLOWS Current.
josmith
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Re: Led help please?

Post by josmith »

chicken? egg? who knows???
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