Steady DC power for LEDs

[[email protected]]

Hello! I have installed various configurations of LEDs in our
sailboat as indicators and light sources (single LEDs, multiple LEDs,
clusters). Each is fed from the boat's DC circuit, but they don't
work very well because the input voltage varies so much. After we
have been aboard and anchored for a couple of days, it's time to
charge the boat's batteries and the voltage may have dropped below
12V. When they're charging, the voltage is around 14.4V.

So, what's the best way to have constant light output with minimum
current drain? I was thinking of using a 7805 at each location where
there are LEDs, with the appropriate dropping resistor for 5V. How
much would this increase the current draw? I have basic electronic
and mech. skills but am just learning how to build useful gadgets,
etc. Could I use a zener diode instead, and how would it act
differently? Is there a better way? I was reading about PWM power
supplies but they seem kind of complex - I want a simple but reliable
solution since I have plans for more LEDs as indicators and cabin
lights.

 

[Nigel Heather]

Best way would be to regulate to a voltage suitable for each of the LED
units - choose a voltage which is higher than the need of the greatest
unit - use a 78xx as you suggested.

You don't say whether the LEDs units are home made or commercial units. If
commercial units for example they may be designed to run off 12V and have
built in resistors to suit that.

I douby that adding a regulator circuit will barely effect the current draw.

Need to choose a regulator circuit that can handle the total current. You
may also need to consider the power dropped acroos the regulator (Vbattery -
Voutput)*TotalCurrent.

Cheers,

Nigel

 

[BobG]

7805 linear regulators drop voltage by heating up the regulator.
Inefficient. I'd look in digikey for a small dc to dc converter that
will accept any voltage from 9-18VDC, and produce the exact 3.72V or
whatever the white led wants with 80% efficiency or greater. Longer
battery life!

 

[Tom Biasi]

Hello! I have installed various configurations of LEDs in our
sailboat as indicators and light sources (single LEDs, multiple LEDs,
clusters). Each is fed from the boat's DC circuit, but they don't
work very well because the input voltage varies so much. After we
have been aboard and anchored for a couple of days, it's time to
charge the boat's batteries and the voltage may have dropped below
12V. When they're charging, the voltage is around 14.4V.

So, what's the best way to have constant light output with minimum
current drain? I was thinking of using a 7805 at each location where
there are LEDs, with the appropriate dropping resistor for 5V. How
much would this increase the current draw? I have basic electronic
and mech. skills but am just learning how to build useful gadgets,
etc. Could I use a zener diode instead, and how would it act
differently? Is there a better way? I was reading about PWM power
supplies but they seem kind of complex - I want a simple but reliable
solution since I have plans for more LEDs as indicators and cabin
lights.

Your solution would depend mainly of your current requirement. How much
total current does your LED array draw from your battery?
Its a simple matter to supply regulated voltage to the array buy the method
would depend on the current needed.

Tom

 

[Ken]

7805 linear regulators drop voltage by heating up the regulator.
Inefficient. I'd look in digikey for a small dc to dc converter that
will accept any voltage from 9-18VDC, and produce the exact 3.72V or
whatever the white led wants with 80% efficiency or greater. Longer
battery life!

LEDs need stable current, not stable voltage.

 

[Ecnerwal]

So, what's the best way to have constant light output with minimum
current drain?

The best way to have constant light out from LEDs with varying input
voltage is to regulate current, not voltage. There are simple devices
designed to just do that, though being a bit of a dinosaur I reach for
an LM317 adjustable voltage regulator and set it up as a
constant-current source when I want one, by force of habit. With
adequate headroom (about 3 volts) between the low end of your battery
(call it 11.5 volts, which is seriously discharged) and the maximum
voltage rating your LEDS might need at rated current (2V or so per red,
3.5 or more for some white) you can run several LEDS in series from one
current source, which is definitely more efficient than running them in
parallel. ie, 20mA @ 12V out of the battery can either dump most of its
voltage in a resistor and a little in an LED, or it can dump a little in
a regulator (which acts like a variable resistor with smarts) and most
of the voltage distributed across 2-3 white LEDS or 4-5 red LEDs -
resulting in 2-5 times the light out for the exact same current drawn
from the battery.

It's described in the datasheet (or it used to be) for the LM317, but
here's a simple page that's slightly easier to find. 20 mA (use a 62 ohm
resistor) is generally safe for unknown LEDs - if you have LEDS that you
know precisely what model/make they are and have a datasheet for, you
might be able to go higher (30 mA not unusual - use 43 ohms). If you
drive an LED with too much current, you'll be replacing it, since it
will fry.

http://diyparadise.com/yhlmccs.html

The diagram there neglects some capacitors you might want to put in
between the input and ground, and (perhaps) the output and ground. Not
too critical, 0.1-25uF.

 

[Nigel Heather]

I know but his circuit would have been doing that already - battery >
resistor > LED.

His complaint is that the battery drops and therefore the current drops and
therefore the LED intensity drops.

If he puts a regulator in there then he is removing the effect of battery
drop and therefore maintaining a constant current through the LED.

The volt drop would now be across the regulator and a new current limiting
resistor. But the power dropped would be more or less the same as across
the original resistor. The only extra power would be that to operate the
78xx circuitry which is minimal.

The OP asked what would the additional current draw be - which is why I
answer barely anything extra.

Cheers,

Nigel

 

[Ecnerwal]

The OP asked what would the additional current draw be - which is why I
answer barely anything extra.

However, active power savings can be realized by using a 317 current
source to drive several in series - while that may seem tiny, tiny
things add up on limited battery power systems which the OP is using.
Something like 39 cents each at Newark for ON Semi's version in the
easy-to-wire (comparatively large, not surface mount) TO-220 package at
the moment. Of course there might be a better deal somewhere else, I
haven't really shopped it that hard, as I have a small pile on hand.

 

[Ecnerwal]

The best way to have constant light out from LEDs with varying input
voltage is to regulate current, not voltage. There are simple devices
designed to just do that

Dusted off brain - Supertex, CL2, (20 ma) CL25 (25ma) CL6 (100ma)

http://www.supertex.com/pdf/datasheets/CL2.pdf

http://www.supertex.com/pdf/datasheets/CL25.pdf

http://www.supertex.com/pdf/datasheets/CL6.pdf

Just need a capacitor, no resistor required, work on inputs from 5-90V
(seem to prefer 5-28V - in any case, fine for the OP's use.)

44 cents a whack for the little ones (in TO-92) at Mouser (Newark does
not carry them). $1.62 for the big guy in TO-220, if you have an LED
that will handle 100mA.

 

[[email protected]]

The Supertex CL's look like exactly what I need! Two questions about
using them: What will happen as the input voltage varies - will the
brightness of the LEDs go up and down? Also, how many LEDs could be
fed in series off one CL?

 

[Nigel Heather]

Q1 What will happen as the input voltage varies - will the > brightness of
the LEDs go up and down?

It's a constant current driver so the brightness will stay the same.

Q2 how many LEDs could be > fed in series off one CL?

Determined by the size of your input source. Basically if you assume that
each LED needs 2V, the device will need some voltage across it to operate
(the graph in the PDF suggests this around 5V) and then you need to consider
the lowest input voltage you want to operate at.

Now the device is rated upto 100V - that will drive 47 LEDs

But it's not good news if you want to run of low voltage. You say your
boat's batteries vary between 14.4V and 12V. So working with a minimum of
12V, take 5V away to drive the CL, that leaves 7V. Each LED needs 2V, so
you could drive 3 LEDs in series off one CL.

To be honest if you have a lot of LEDs that is going to be quite a lot of
circuits and probably expensive. A regulated supply still seems the best
answer to me.

Cheers,

Nigel

 

[Ecnerwal]

The Supertex CL's look like exactly what I need! Two questions about
using them: What will happen as the input voltage varies - will the
brightness of the LEDs go up and down? Also, how many LEDs could be
fed in series off one CL?

No variation in brightness with input voltage, so long as there is
adequate voltage to drive the load. The CL drops anywhere from 5-90
volts as needed to maintain the current out. But it appears to need to
drop 5 volts to operate at all.

From 12V, (thus 7V available to drive the LEDs) probably 3 reds or 2
whites/blues. 2 or 3 greens/yellows - depends on the part to some extent
- the voltage drop varies with color from low at red through high at
blue & white (some whites you might only be able to drive one of)

The LM317 only drops about 3 volts, but does add the resistor and is a
little more fuss to install - it would bump you up to 9V available at
12V battery.