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Steady DC power for LEDs

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.

Reply to
nancydow26
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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

Reply to
Nigel Heather
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!
Reply to
BobG

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

Reply to
Tom Biasi

LEDs need stable current, not stable voltage.

Reply to
Ken

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.

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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.

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Reply to
Ecnerwal

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

Reply to
Nigel Heather

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.

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Reply to
Ecnerwal

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

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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. 
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Reply to
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?

Reply to
nancydow26

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

Reply to
Nigel Heather

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. 
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Reply to
Ecnerwal

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