led's

I posted a question about how much power (solar and battery storage) it
would take to keep a simple 25mw led light on for 24/7..

I have got a lot of answers but more questions than answers...

So I will start over...

1- If I wired 4 led's in series, on a 12 to 14 volt dc system, would I
still need to limit the current using a resistor?

2- what is the most energy efficient way to wire up said LED's ? ( would
rather add another led to the string and have the LED's produce less light
than waste energy .

3- the system needs to be as simple as possible, using what I have on
hand....
Several hundred LED's
A small 4 amp morning star charge controller
3-3.3 watt solar panels and a damaged 85 watt panel (that still works but
glass is shattered but still has full output..so far) I do not trust the
broken panel in my main array but no sense in tossing it.
I would like to see what I can run off of the 3-3.3 watt panels first and
use the 85 if needed...The broken 85 would work great for the electric
fence...

What I would like to with the above ( spare crap) is run a wire along (on
top of ) a wire fence. With LED's on the top of the posts, to light up the
fence line.
I can build a dusk to dawn circuit to turn on and off the led's but I am a
bit confused as to how to wire them.
It seems that LED's wants the current limited and I am not sure if 4 LED's
wired in series would limit that current enough in a power source that
floats between 12.4 volts and 14.3 volts.

I could build a voltage regulator for them but I would rather keep the
system as simple as possible.

Most off the shelf items I have looked at has losses in the design greater
than or equal to the power usage of the LED's.

I just want to light/mark a fence line. Nothing that requires a great deal
of light, just something to mark the fence line so you can see the edge of
your land at night.

Ideas?

merlin-7 wrote:

25mW 24/7 = 576 mWh daily = ~ 4Wh weekly.

How much more info do you need ?

Can't you do the sums yourself.

Graham

merlin-7 wrote:

Yes. You can get away without using the resistor sometimes but if you
want to wire multiple LEDs in parallel the resistors will help balance
the current.

There are various web based calculators out there to help. Here is one.
http://led.linear1.org/led.wiz

I don't know what you mean by a 23mw led light. I know most Yellow and
Red LEDs run 2V to 2.4V while Green and Blue are 3.2V to 3.8V with most
all of them running between 20mA to 30mA. This gives us a mW range of
40mW to 114mW.

I would say 45 mW is about fair for yellow/red LEDs which is probably
what you are talking about. This is a little over 2V and about 20mA.

Using that series string calculator with 6 2V yellow/red LEDs in series
it says to use a 1 ohm resistor. If you want more than 6 then you wire
series strings of 6 (with 1 resistor) in parallel.

You would probably want to account for wiring resistance also. It may
well be that whatever wire you choose to use has more resistance than
you need to limit current.

The most energy efficient way is to wire them in series so that their
voltages add up to just slightly less than your supply voltage.

If you want to use even less energy then you could wire them up so they
blink. Blinking, flashing or pulsing LEDs are still visible but they
don't consume energy when they are not lit.

For instance, you could wire them up like holiday light strings with
the LEDs spaced every three feet and 4 strings twisted together so
the LEDs are then spaced every 3 inches but from alternate strings.
Light only one string at a time in sequential fashion and all the
lights will be visible but only consume 1/4 the energy.

Three 12V 3.3W PV panels with 4 sun-hours insolation and a total panel
to LEDs efficiency of some 64% would provide some 25.3 watt-hours. If
your nights are 12 hours long this means your maximum LED current is
about (25.3 / 12) 2.1 Amps. This should light (2100mA / 45mA) 46 LEDs.
You could probably push this to 8 strings of 6 LEDs, 48 LEDs.

Using the same numbers of 4.65 LEDs per W of PV panel the 85 watt panel
would add another 395 LEDs or 66 strings of 6 LEDs.

Anthony

Anthony Matonak wrote:

You can run them at however many mA you choose. Of course the more mA, the more
mcd of light output.

Graham

| 1- If I wired 4 led's in series, on a 12 to 14 volt dc system, would
I
| still need to limit the current using a resistor?

Yes. It's usual to think of any electrical load as having the dominant
property of resistance. Then, if you know the supply voltage you can
think of "adding" these resistances (by series connections) in order
to up the resistance / reduce current. But LEDs don't work like this;
shift your thinking to the idea of an LED not having the property of
resistance, instead replace it with "forward potential difference"
(i.e. - voltage drop across the LED). Apart from this voltage drop
they behave (to a very rough first approximation) as a short circuit.
The forward potential difference varies from type to type, and has a
dependence on temperature (you'll have seen other threads discussing
"thermal runaway"). You need to know what the forward potential
difference is for your LEDs (for a guide see
http://en.wikipedia.org/wiki/LED_circuit ). Then you can go about
designing a "branch" circuit (the calculator in Anthony's reply is
perfect for this).

| 2- what is the most energy efficient way to wire up said LED's ? (
would
| rather add another led to the string and have the LED's produce less
light
| than waste energy .

The most energy efficent depends upon the efficiency curve of your
LEDs. I suspect that what you're wanting to know is "is it
better/safer to run the LEDs at less than maximum capacity?". If
you've already investigated the calculator in Anthony's reply, you'll
see that you can run the LED at any current you like as long as it's
less than the rated maximum. (Before anyone bites - naturally you can
run them at greater than the rated maxmum - but they tend to make a
very cute little "pop"). At less than about one quarter of the rated
current you won't see much light...

| What I would like to with the above ( spare crap) is run a wire
along (on
| top of ) a wire fence. With LED's on the top of the posts, to light
up the
| fence line.
| I can build a dusk to dawn circuit to turn on and off the led's but
I am a
| bit confused as to how to wire them.
| It seems that LED's wants the current limited and I am not sure if 4
LED's
| wired in series would limit that current enough in a power source
that
| floats between 12.4 volts and 14.3 volts.

You're correct that the LEDs need their current limited. Even if your
LEDs had a forward potential of 3v and you wired 4 in series (across a
12v supply) the current would not be limited. Each LED would only
"see" 3 volts but, as LEDs don't behave like a resistor, the circuit
would behave like a short-circuit. Your battery will provide as much
current as it can, your wires will get hot and your LEDs will make
that cute little popping sound I mentioned earlier.

| I could build a voltage regulator for them but I would rather keep
the
| system as simple as possible.

No need if you're powering from a battery. Run a 12 volt loop around
your fence line and "branch off" 12 volts wherever you want a few LEDs
(as you're running around a wire fence you may be able to use the
fence as the return path of the circuit). Try for maybe 3 LEDs with an
appropriate current-limiting resistor per "branch". If you feed both
ends of the loop from the battery then the voltage drop shouldn't be
too great at the middle of the circuit. If you find that the voltage
drop is affecting LED output, then alter the resistor in those branch
circuits to suit the voltage at that point.

Hope this helps
Al