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Old 23-12-2010, 07:10   #1
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My DIY LED Tri Light Project

I made a masthead light and hope someone else may benefit from my learning experience. There is not a lot of power to spare on Waltzing Matilda and I have wanted LED running and anchor lights for some time but $150 bulbs are not in the beer budget. In true cruiser fashion I have fashioned some lights myself and run into a few small difficulties I would like to share.

My 1st attempt was pitiful but free, I discovered a bar in Key West (Fat Tuesdays) sells these goofy light up plastic mugs with red, green and blue LEDs in the base, I scored a few from the dumpster, pulled them out of the original circuit board and soldered them parallel to a DIY board (used a safety pin to drill bits from the mugs) with some foraged white LEDs to make a tri light. I used a automotive cell phone charger to drop 12V to 4.5V, and viola..... not bright enough.

The second attempt was not free but less pitiful. I went to SurplusGizmos.com and purchased 16 70ma Piranha LEDs, after shipping it was a total of $20 worth of beer money. Since I am home for the holidays I have better tools to use than I can carry in the boat including a great adjustable power supply from a model train set.

I soldered 4 red, 4 green and 4 white in parallel arrays and powered them up to test. The red was nice and bright, the green and white, not so much. If I disconnected red the green and white lit up. Once the red was on it sucked all the power, so it seems, the red array draws twice as many amps as the green and white combined. Using a multimeter as an adjustable resistor I found I could get all three arrays to light up by limiting the power that went to the red array on the negative line but had no way to see how many amps were being used (I only have 1 multimeter).

I found a resistance calculator for LEDs online and according to it I need a 30ohm resistor to solve my problem and make all 3 arrays light. If anyone that reads this and knows more about LEDs than me I'd appreciate if you can contribute some info, I don't know much about electronics. I know what the markings on a the resistor I need look like, I'll just have to find a toy in the dumpster to find the right one.

I haven't encased the new lights in epoxy yet, but they will be before I climb the stick

PS I am not affiliated with SurplusGizmos.com
PPS the 4 extra white lights are for the stern
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Old 23-12-2010, 15:36   #2
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You really need to use a current regulator instead of a resistor. Don't forget, the voltage on your DC bus will vary quite a lot depending on state of charge of the batteries. If you have an alternator it will be putting out something over 14 volts, while sitting at anchor your batteries will be as low as 12.2 volts (hopefully no lower). A current regulator will ensure that your LEDs get the proper current no matter what voltage is on your bus. A resistor can not do that.
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Old 23-12-2010, 19:47   #3
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thanks for the reply, since I am stepping the voltage from 12vdc to 4.5vdc via the cell phone charger wouldn't that regulate the current?
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Old 23-12-2010, 20:05   #4
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It might, but I don't think it is a good permanent solution. Okay to test your lights though.

Have you read some of the threads on DIY LED lights on this forum? If you scroll right down to the bottom of this page you will see some links to other threads.
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Old 23-12-2010, 20:24   #5
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Wire 3 to 4 LEDs of each color in series with a resistor. Select the resistor value to limit the maximum current to that specified for the LEDs. Use 16V in your computation, not 12 or 14, as that is the typical maximum voltage in a boat.

Using the cell phone charger has no advantage. The resistor wastes a small amount of energy but it is not important.

The voltage across the resistor is 16V minus the combined forward voltage of the series LEDs, about 8V.

An active current regulator can save some energy, but seems beyond your current expertise and budget.

See Marine LED Masthead and Running Light Bulb Replacement Assembly for some very reliable beer-budget lights well respected by cruisers.
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Old 24-12-2010, 02:55   #6
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Using a simple resistor on a unregulated supply from a battery is a path to premature failure. The cell charger isnt a bad idea once it can supply the total current however it will probably fail if used a lot.

Unfortunately OP powering high power LEDs reliably and effectively is not as straightforward as it seems. ( and I'm a electronics engineer!)getting them to light up initially is easy , keeping them going for any length of time requires design expertise

( also using 16v as the voltage in the resistor calc will leave the LEDs very underpowered in low voltage conditions ( ie around 12v). Equally designing for a lower voltage leaves the LEDs outside their current limits at higher voltages thereby shortening their life.

Also these high current LEDs will need a good heatsink to handle the heat output just encasing them in epoxy will hasten their death.

In most cases the little LEDs you see every day are powered from at very least a regulated power supply this leaves the resistor current limit a viable option. High powered LEDs have more complex drive scenarios and really need constant current drive sources with an ageing algorithm to compensate for light output declining over time ( and redundant LEDs support and status monitoring etc etc). It's why good high performance high output led arrays are expensive.

To be honest I'd stick with bulbs.

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Old 24-12-2010, 08:24   #7
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Using a simple resistor might actually work just fine, in my opinion, and I think the OP should give it a try just to see how it works for him. Let me just mention a couple of things to keep in mind. First, the eye and brain have a hard time detecting a change in brightness of a light unless that change is more than 50%, or two lights are compared side by side. The other thing to keep in mind is that an LED driven at 35 mA is MORE than half as bright as one driven at 70 mA, i.e. light output vs. power consumed increases as drive current is reduced.

Take the example of three LEDs, a red, a green, and a white. You are saying you want to drive them at 70 mA maximum, and we'll use a 16v supply. Put all three in series and add up their Vf, or forward voltage drop at rated current. This is typically 1.9v + 2.1v + 3.0v respectively, for a total voltage drop across the diodes of 7v. Plugging this into an LED calculator, a resistor of 128 ohms in series with these LEDs would allow a drive current of 70 mA with 16v input, and 39 mA with 12v input. That is a drop in drive current of less than 50%, and a drop in light output of even less than that. The resistor itself will be dissipating a bit over 1/2 watt, so should be of the 1 watt variety (it will be dissipating about half of the total power input itself).

This is just an example of how the calculations might be made. In fact, you would probably want two red leds to every green one since red is perceived as less bright to the eye than green. In that case the total Vf would be 8.9v and you would want a 101 ohm resistor to give 70mA at 16v. You would only have about 30 mA at 12v in that instance (although because the Vf of an LED drops as current through it decreases, you might see more than 30 mA).

You could put any number of the above circuits in parallel with each other for more total light output. The dropping resistors in each circuit would prevent any one from "hogging" a disproportionate amount of current.

Also keep in mind that the Vf of individual LEDs of the same color can vary, so actual current flow can vary a bit from calculations depending on how those variations add up.

To protect the LEDs, even though they have a dropping resistor in series, I would also put a 16v TranSorb across the leads supplying your "module" to prevent damaging voltage spikes that might appear on your boat's 12v bus.

I'm not saying this would let you make a satisfactory tricolor light, but you wouldn't know for sure unless you tried.

Chip
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Old 24-12-2010, 10:43   #8
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If you have any electronics experience, try flashing them. At 50/60Hz your eye won't see the flicker, but you can up the current and apparent visual light output dramatically. At 100 Hz on time can be sub milliseconds. Low duty cycle (on time vs off time) and high peak (on time) current can give optimum brightness without any average current drain (or power dissipation) increase in the LEDs.
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Old 24-12-2010, 13:07   #9
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Originally Posted by harmonytek View Post
If you have any electronics experience, try flashing them. At 50/60Hz your eye won't see the flicker, but you can up the current and apparent visual light output dramatically. At 100 Hz on time can be sub milliseconds. Low duty cycle (on time vs off time) and high peak (on time) current can give optimum brightness without any average current drain (or power dissipation) increase in the LEDs.
This is not true. The Eye/Brain is a light integrating system. If an LED is on 50% of the time at a fixed current, the eye will not see more light than if it were on continuously at half the current. In fact, the LED might appear less bright in the 50% pulsed instance, because LED efficiency falls with increasing current (apart even from heat issues).

PWM (pulse width modulation) is well used in LED circuits because it is easy to build dimming capability into them, but not because they drive LEDs more efficiently than constant current circuits. You also have to make the on/off frequency much higher than 60Hz in PWM circuits if you want to avoid bothersome LED "flicker" with eye movement.
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Old 24-12-2010, 13:16   #10
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"Use 16V in your computation, not 12 or 14, as that is the typical maximum voltage in a boat." No good, daddle. The voltage at the alternator should never exceed 14.4 volts (except during charge equalizing) and if it does, there's been a regulator failure. The voltage up the mast should never be that high since there will always be wiring losses.
If you use 16V as your maximum and pick a resistor for that, you will have insufficient current at 12.6 volts (high battery voltage) and the masthead lights will be TOO DIM. And that's ignoring the question of using cheap resistance to limit current to the LEDs in the first place.

Using resistors as current limiters works, but there's more than a 10% variation from 'low battery' to 'alternator' voltage on a boat, so you've got at least a 10% variation in brightness if you do it that way. Too bright, shorter life. Too dim, can't be seen. Not very effective.

Which is why there are LED "driver" modules. Some of them are current limiters, others are more advanced and pulse the LEDs many times per second in order to effectively conserve power and extend life. They may cost $5-10 each and drive one or more LEDs. And generate RFI in some cases. Using a simple 3-pin regulator chip in current regulator mode will also work, cost is going to be similar but there's more work to be done building.

hillbilly, I'd suggest you knuckle down and search the internet for some of the many fine articles on how to properly drive and use LEDs. How to calculate the correct voltage and amperage to supply each one, and that's going to be very different for white LEDs vs. any other color. If you want the masthead light to be bright enough to be legal, with a wide enough beam to be legal, and last longer than a plain tungsten bulb? You've got to do some math and some soldering, and it won't be cheap.

The beer mug lights are a clever idea (wish I had that dumpster to dive<G>) but better suited for "patio" lighting in the cockpit than navigation lighting. LEDs that are bright enough to use as nav lights tend to get expensive, fast.

How to use LEDs "properly"? Honest, that's all on the web, and there are a lot of little details to be done right--or not.
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Old 25-12-2010, 19:02   #11
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"Use 16V in your computation, not 12 or 14, as that is the typical maximum voltage in a boat." No good, daddle.
I was advising on the idea of using resistors as ballast. Of course an active regulator would be better. 16V is the proper maximum voltage for safely specifying parts connected to a 12V boat system. Especially for LEDs, whose life is dramatically shortened when over-driven. Designing a device to fail catastrophically at 14.4V is not wise.

Yes, of course 12V will make for a dimmer lamp, but how can you know that it will be "TOO DIM"? Simply design the lamp to be 30% brighter than the regs at 16V.
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Old 26-12-2010, 19:44   #12
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"16V is the proper maximum voltage for safely specifying parts connected to a 12V boat system. "
Emphatically, no it is not. You might want to protect the circuit against overvoltage, but that's got nothing to do with the design point for a voltage dropping resistor.
The highest voltage that will be on a boat's electric system, IF THAT SYSTEM IS NOT DEFECTIVE, is 14.4 volts. Never 16 volts, unless the regulator has failed or someone is doing an equilizing charge without disconnecting the other systems--which is a screwup anyhow.
If you spec your dropping resistors using 16V as the starting voltage, you will simply be designing the circuit for a condition that never exists. The regulator will be putting out a maximum of 14.4 volts when the engine is running, NEVER 16.
It really is that simple and absolute. If you want to design a system "in case there's 16 volts" you might as well use the common 15-volt protective devices (zener diodes, zorbs, etc.) because a system that is seeing 15 volts is ALREADY IN DANGER from the failed regulator. And even the common instruments, radio, vhf, ssb, ham radios, all will start to show failures at 15 volts. They're all spec'd for 13.8V (which is nominal "low" alternator voltage) with a +-10% tolerance, for a mximum of 15.18 volts before things fail.
That's the industry standard for tolerances--and a nominal design voltage for dropping resistors, again using 13.8 as typical with 14.4 as a max.
All of which is one more reason not to use dropping resistors when proper current regulation is so easily available, and a 10% (or, considering low batteries at 11.7 volts and alternators at 14.4) or even 20-30% voltage variation being normal on boats. Brightness, longevity, and COST of the LEDs make it much more effective in the long run to do it right up front, and regulate the power.

" Simply design the lamp to be 30% brighter than the regs at 16V. " Yeah right. Either you use 3 LEDs when you only needed 2 (to gain 30% more brightness) or you buy more expensive LEDs. Either way, you are now spending 30% more money on LEDs and 30% more real estate in the lamp housing. That's a way to limp home, not a way to outfit a boat. A kludge job. And the LEDs that you can buy or find "cheap" enough to waste 30% of them? Sorry, you'll need dozens to meet any lighting standards in nav lights.
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Old 26-12-2010, 20:51   #13
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A little searching found this:

Circuit - Constant Current LED Driver - Circuits designed by David A. Johnson, P.E.

A grand total of 3 parts plus the LED.

You can change the resistor value if you want to put LED's in parallel.

A better way is to put them in series.

You don't need to change anything for that until your string of LED's exceeds the supply voltage's ability to supply the constant current.
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Old 26-12-2010, 21:19   #14
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Wouldn't it be easier to spend the $150? Plus, If you don't give the inventor/manufacturers their dues then no one will be able to buy them.
Just the amount of time spent on the forum here, ones wages could have made the purchase.
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Old 27-12-2010, 06:35   #15
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Funny how some people actually enjoy cobbling together their own hardware. Of course there is also a very great danger of learning something in the process. It really is much safer in that event to just spend the money.
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