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