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senormechanico · 29-12-2010, 13:40

It would also be more efficient to put the LEDS in series before designing a regulator.
That way there is less voltage drop being used in resistors and better use of the available current, i.e. actually producing light.

goboatingnow · 29-12-2010, 13:40

Using a series pass regulator like that is very inefficient

dave

SoonerSailor · 29-12-2010, 14:58

When comparing efficiency, keep in mind that constant current regulators typically are only 80% to 90% efficient. If the added total of forward voltage drop of your string of LEDs is fairly close to the supply voltage, you won't be losing a lot of your power in an appropriately sized dropping resistor.

In some instances, the dropping resistor will compare very favorably to a constant current regulator in terms of efficiency, and win hands down in cost, simplicity, and lack of RFI production, while losing somewhat in constancy of light output with supply voltage swings.

And Dave above is right, using a linear voltage regulator is going to be very inefficient.

It all depends on what you are trying to do and what kind of design you can come up with.

goboatingnow · 29-12-2010, 17:11

Quote:

In some instances, the dropping resistor will compare very favorably to a constant current regulator in terms of efficiency, and win hands down in cost, simplicity, and lack of RFI production, while losing somewhat in constancy of light output with supply voltage swings.

Well its not "somewhat in constancy", it will be quite a big swing in light outout, why , well firstly you have to design conservatively, hence the dropping resistor will be sized for say 16V , given that you could see 11.8V its going to be quite dim. ( example using one of CREE BRIGHT WHITE leds, this would result on a reduction from 1.5 to 1.0 in relative luminosity, a very significant loss of output( approx a quarter), in real life probably more due to preferred resistor sizing

The reasons for constant current, especially in high current high luminosity LED are two fold (a) to avoid breaching the maximum forward current and (b) to get consistent light output.

Its worth noting two further things, firstly the forward current of a batch of LEDs varies quite considerably for a given forward voltage, in white LEDS with a 3.4V forward voltage current varies from 10mA to 40mA typically with corresponding changes in light output. Constant current sets a consistent current, hence consistent luminosity

Secondly maximum forward current needs to derated to account for ambient temperature or more correctly Die temperature, derating is necessary for long life, again this is hard to do with resistors on a unregulated supply. Derating can be very severe for high temperatures and is often the cause of premature failure. lack of adaquate heatsinks, unexpected ambient temperatures, localised heating etc all contribute to these issues. in reality for long life active monitoring of the Die temperature is a useful addition.

driving high intensty LEDS, ie thoese with 20 mA or more is not a straightforward excercise, as anyone whose has bought cheap LED fixtures will tell you.

Dave

SoonerSailor · 29-12-2010, 18:53

I'll have to disagree with Dave about luminosity. While a reduction of 33%, from 1.5 to 1.0 as in his example, might seem great, to the eye and brain it is perceptually a far smaller change than that.

Further, as others have mentioned, designing for an operating voltage of 16v is unnecessarily conservative on the high side, and 11.8v is probably lower than most of us will routinely allow our battery voltage to fall.

I'll agree that proper heat relief is important, and is the most frequent source of LED failures in cheap fixtures. One must read the data sheets on the devices you choose to use.

I also agree that a constant current regulated supply is the ideal for LEDs, but I'll continue to say that it isn't necessary or even desirable for every application, particularly where an LED doesn't need to be driven all the way to its design limits.

daddle · 29-12-2010, 20:10

Quote:

Originally Posted by SoonerSailor

Further, as others have mentioned, designing for an operating voltage of 16v is unnecessarily conservative on the high side, and 11.8v is probably lower than most of us will routinely allow our battery voltage to fall.

Perhaps for those who cruise at room temperature. Cruisers in cold weather will see charging voltages far above the folkloric 14.4V ... often near 15V ... or higher. Thus my reference to 16V being a reasonable voltage for designing a reliable lighting system. This assuming a temperature compensating smart charger.

Likewise, typical discharge voltages less than 11.8V may be experienced by tropical cruisers where cabin temperatures soar to the 40s. A happy full charge voltage of 12.6V might be only 12.1V when the equatorial sun makes the decks burn bare feet ... like today.

conachair · 30-12-2010, 01:55

Quote:

Originally Posted by DeepFrz

LEDs are current devices, not voltage devices. You really need a current regulator to drive them properly, not a voltage regulator with resistors in series with the LED. You use LEDs for 2 reasons. Longevity and efficiency. By adding dropping resistors you are negating the efficiency to a large degree.

i doubt doubt what your saying , but how does it work? if the voltage is constant then how can the current vary? Temperature?
Would a good driver output constant current and voltage regardless of temp?
On a cruising boat with solar the voltages could well range from below 12 to 16v

Not disagreeing, just interested.
Ta

Edit; just looked and some replacements i have and they use this driver..
http://pdf1.alldatasheet.com/datashe...AMC7150DL.html

BjarneK · 30-12-2010, 03:26

Many excellent points have been made above, so let me try to sum it all up and explain why current regulation is preferred for LEDs. Here is a graph of the current as a function of voltage for a high power white LED (Cree EZ1000):

It is from these specifications: http://www.cree.com/products/pdf/CPR3EC.pdf

The maximum allowed sustained current is 1000 mA. As you can see, the curve is very steep in this range so a small change in voltage means a large change in current. So if we design a system to run at 800 mA to get the desired light output, a voltage that is slightly too low could mean a significant decrease in light output. A voltage that is slightly too high could mean that the (expensive) LED is fried. These high power LEDs are very vulnerable to heat damage and cooling is a major design consideration. To complicate matters even further, I would think that the exact needed voltage would vary a among indivudual LEDs and probably also with temperature as suggested by other posters above (though the temperature at the emitter is much higher than ambient when on).

This describes why direct voltage regulation is bad idea and current regulation is needed. Of course, current regulation can be achieved in several ways. One way (obviously) is to use a current regulator. This is fairly simple given that current regulators are available as components and it is also a very power efficient approcah. If the voltage is high enough, you can use one current regulator for several LEDs in series as they will all see the same current.

Alternatively, a voltage regulator can also be used as described by Don1500 in post #25 above. The reason that this works is the resistor which absorbs some of the energy. This ensures that a significant part of the effect of a small voltage change just results in more or less power dissipated in the resistor. But of course, it has the downside that more energy is wasted, but it is still more effecient than conventional lighting.

Let us consider a small example: take one of the above mentioned high power LEDs and connect it in series with a 12 ohm resistor. If we supply this setup with 15 Volts, the current will be about 925 mA (11.1 Volt drop in the resistor and 3.9 Volt drop in the LED as read from the graph). If we supply 11 Volts, the current will be about 620 mA (7.4 Volt drop in the resistor and 3.6 Volt drop in the LED). Notice how most of the change in voltage drop occurs in the resistor rather than the LED. This is because of the steepness of the LED current curve.

So this setup with a 12 ohm resistor could actually be used without any regulation on a 12 Volt supply. There are several reasons that this is a bad idea: 1) The resistor has to be able to dissipate 10 Watts. 2) Energy is wasted. 3) It is easy to fry the LED by accident. 4) There is still some variation in light output depending on the supply voltage.

Using a voltage regulated 5 V supply and a resistor is much better than using an unregulated 12 V supply. The power lost in the resistor would only be about 25%, which is not too bad. In fact most of four issues outlined for the 12 V supply would be resolved. But since we are using a regulator anyway, we might as well do the right thing and use current instead of voltage.

Viking Sailor · 30-12-2010, 05:29

Since all of my lights are on there own circuit, I have opted to provide a low voltage supply for my DIY LED lights. This supply is inserted after the DC breaker feeding the lighting circuit. Thus, a simple resistor in series with each of the LED's will provide an efficient lighting system. The power supply consists of a $4.00 DC/DC converter found at a local discount store. This supply provides 2 amps at 4.5 volts from a 10 to 16 volt source. That is enough power to drive 100 LED lights while consuming less then 1 Ah. The supply may need to be enclosed in a metal box to eliminate any RFI.

BTW, the eye can detect a 2% change in brightness.

SoonerSailor · 30-12-2010, 08:47

"BTW, the eye can detect a 2% change in brightness"

That would only be true for a rapid change in light level. A long used rule of thumb is that typically a 50% difference in light level is required before a change is noticed unprompted and unexpected. This also is going to be dependent on the speed with which the change occurs.

Don1500 · 30-12-2010, 11:10

You guys are going too deep into this. The main question is: Will you be able to power enough LEDs at an acceptable brightness without burning them out in a reasonable time, and without burning up your boat. The circuit I supplied will do that. You may need to tweak the load resistor to get the brightness but it will be stable and not burn out (with the right resistor) and it will be bright enough (with the right LED). And the Voltage regulator will keep the +5VDC within the parameters of the led so current draw will be controlled. And all this for less then 1amp (Depending on how many LEDs you are running.) AND it is simple and CHEAP.

As far as current, if you keep two of the legs of Ohms law constant the third will follow. The voltage regulator is not just a resistor, it is a micro circuit, it maintains +5VDC, with an input of 8-20VDC, up to 5 AMPs then it brakes down. If the load is below 5 amps then voltage is constant, and the load over the resistors and LEDs is constant, so the current is constant. If the input voltage exceeds 20vdc or goes below 8VDC than something is wrong with your boats battery supply. If the load changes for one LED it will have no effect on the other LEDs (There is no such thing as a shorted resistor so any change would be an open) But if for some reason, corrosion or mislaid screwdriver, the load is shorted the regulator will crap out. You could fuse the 5VDC I guess, but I think a fuse on the input to protect your boat is all that's necessary. If this were for an Anchor light with the wire running in the mast, I would have the fuse right after the switch at the nav table.

SoonerSailor · 30-12-2010, 13:20

Don - you will want to use a switching DC-DC converter to do as you propose.

The LT1003 and LM7805 you mentioned earlier are linear regulators that dissipate into heat the voltage drop at the current you are running through them. e.g. For a high power LED you wanted to drive at 1 amp, the LED would be using about 3.5 watts while the LM7805 would be dissipating (12.0-5.0)*1 or 7 watts, and the dropping resistor 1.5 watts if the system was supplied with 12 volts. At higher supply voltages, the system would waste even more power. The actual current wouldn't affect the percentage of wasted power, BTW.

**noelex 77** · 30-12-2010, 14:10

There are lots of commercial constant current drivers available. Pre made cheap. reliable and efficient.

They are available for 20ma, 1w, or 3w LEDs. Some are dimmable will drive multiple emitters etc.
For example.
BuckPuck - LED Supply.com

The challenge is always building the housing, not the electronics.
Navigational lights are particularly difficult. A led will not give the same light distribution or colour temperature as an incandescent bulb.
An anchor light with a white 360 distribution can be easily produced, but other navigation lights, particularly tricolor or port and starboard fittings are unlikely to be satisfactory without extensive and expensive trial and modification.
Even the time rich, cash poor cruiser is better with commercial units of these lights.
Experiment with interior lights. LEDs greatly superior to commercial units can be produced at low cost and you get the pleasure of DIY. Leave the navigational lights to the professionals .