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Old 27-06-2006, 09:21   #1
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LED Replacement for Flourescent Fixture

Here's a little project that I took on to repair / replace my recessed flourescent light fixtures. There are about 15 of these throughout the boat, and roughly half of those have failed / burned up. No replacements are available - at least I've not been able to find any. The previous owner actually resorted to soldering 3 incandescant bulbs together inside the assembly on some of these.

Anyway, here's what I started out with:
http://www.cruisersforum.com/attachments/gallery/1/6/9/8/OldAssy.JPG
Note the corroded frame - more proof that salt water and aluminum don't get along... and those burned spots don't look good to me
http://www.cruisersforum.com/attachm...lour_thumb.jpg

So off to the local air conditioning shop to have new sheet metal bent. Then I found a nice LED light bar at http://superbrightleds.com/
I used their LB1 light bar, but note that it is NOT suitable for auto/boat use right out of the box. The charging voltages are too high and will damage the unit over time if you don't limit the current flow to the light bar.

I did some experimenting and determined that a 6 ohm 1/2 watt resistor would limit the voltage sufficiently to protect the light bar. There really is no noticeable difference in light output when placing the resistor in line.

Here's the new assembly. I used two of the LB1s for a total of 24 LEDs. Note the terminal board - for the current limiting resistor.
http://www.cruisersforum.com/attachm...ssy1_thumb.jpg

And now we throw the switch.... current draw is 360ma at 13.8V input. The resistor drops the input voltage to the light bar to 12.6V maximum. I could probably use a 10 ohm resistor to keep this value closer to 12V.
http://www.cruisersforum.com/attachm...Assy_thumb.jpg

Next step is to install it aboard and see how the light output compares to the flourescent. In the garage it seems roughly equivalent, but it's how it looks aboard that matters. Since the chrome-plated plastic frames are corroded, my next phase is to build some replacements out of teak or mohogany - should add a nice touch to the light's appearance.

Here's my parts list & cost so far:
LB1 light bars, 2 @ $18.95 = $37.90
LB1 Jumper Interconnect (1) = $0.45
LB1 Jumper - Power (1) = $0.45
Light Bar Mounting Track 4@ $0.45 = $1.80
Double sided foam mounting tape, 6 @ $0.25 = $1.50
Terminal strip = $0.30
6 ohm 1/2 watt 5% resistor = $0.05
Sheet metal fixture pan = $5.50
Total = $47.95

I know I can get the cost down by buying in quantity, etc.

Next steps are to measure the current draw of one of the flourescent units for comparison. Then I'm off to fabricate some decent looking frames.

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Old 27-06-2006, 10:08   #2
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Mark, if you really want to optimize the life of the bars and their power consumption? SuperLEDs says the white LB1 units need to draw 113 mA per bar. The amperage draw is in some ways more important than the exact voltage for an LED. And your resistor is there more to limit amperage than the drop voltage. So, if you measure the amperage actually flowing in each bar (open the circuit between the resistor and the bar and insert ammeter leads there) that's the target for you to work with, 113mA per bar. If your resistor is allowing more current to flow when the engine is on--you are overworking the LEDs, they'll burn brighter but fewer years. Whatever the current flow is when the engine is on (14.4v in the system) it will be less on just battery...so there's some range and some trade-off to be made. I'd guess 125mA with the engine on will still give you many years of good service, but they might have more specific numbers for you.

Nice upgrade, gonna look good with the new trim on it. When do they go up on eBay?<G>
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Old 27-06-2006, 10:35   #3
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Actually, I did check with the folks at SuperbrightLEDs to verify this exact point. I was concerned about over-driving the light bars. They told me to limit the voltage and disregard the current draw - and when you think about it, it makes perfect sense.

The key point is that current draw will vary depending upon input voltage. I have these 2 bars wired in series to each other. So you might expect them to draw a combined 226ma at 12 Volts, and in fact I measured 250ma at around 11.8V. Bump up the voltage to 13.8 and the current draw will be different. Also, the total current draw I stated is for the entire circuit - including the resistor.

Input voltage to the light bars is the important factor, and it is about 12.6 Volts when applying 13.8V to the assembly - meaning that the voltage drop across the resistor is about 1.2Volts.

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Old 27-06-2006, 10:37   #4
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Hey Mark.

I'm not trying to tell ya what to do or anything. But those photos would be a whole lot nicer for the close up details on what you're doing. If ya posted bigger photos!!

Sounds like a great project that you got going for yourself. Keep up the good work!!
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Old 27-06-2006, 10:44   #5
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Interesting. Looking at their pix, apparently there is a current limiting resistor on each bar already--so as long as you're knocking the voltage down to what their circuit expects...their resistor is limiting the current. I wonder how your overall power consumption would vary, if you used a low-loss voltage regulator instead of a resistor to supply that dropped voltage to them?

Ah well, as the old proverb goes "There are many ways to skin a cat, only one way to turn it into soup."
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Old 27-06-2006, 11:08   #6
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Let's try using the image tag as suggested in the help thread...




Eureka!!
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Old 27-06-2006, 11:11   #7
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Much better this way....

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Old 27-06-2006, 19:23   #8
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Mark, on a boat you should really use a current regulator rather than a resistor. I know it's more expensive but it would be well worth it.

nice pics...how's the autopilot coming along???

Deep.
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Old 29-06-2006, 09:48   #9
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Very nice work you've done there Mark.

How many of them are you planning to install onboard your boat? And how far til you got to completing all of them?

And roughly how much did this set ya back? (If ya don't mind me asking that one!!)
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Old 29-06-2006, 13:36   #10
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CaptainK, the price breakdown was on Mark's very first post on this thread...

Mark, I know that it seems prudent to just use a resistor instead of a current regulator. However, you will be feeding these things variable voltage. And a Resistor is NOT a current limiting device in a variable voltage cct.

All a resistor does in a cct. is drop voltage. How much does it drop? Well, that depends on the voltage fed to the cct. and the resistance of the other components in the cct.

An LED is a current device. The current limit specified for an LED is usually the absolute max. current that should flow through the LED. You should operate the LED at a lower limit than the specified max. and that is why you really should use a current regulator to drive LED lights.

I looked at current regulators for LEDs a while ago and could only find them with a max. voltage limit of 14 volts, so I figured I had to use a voltage regulator to drive the current regulator to drive the light. No big deal, regulate the voltage going to all your LED lights and include a current regulator within each light fixture. Something like an LM-317 should do as a voltage regulator for your application. May need to add a driver, heat sink, etc. Regulate your voltage at 9 volts (that way the voltage stays stable when the battery hits 12 volts, which should never happen). Actually, there are votage regulated power supplies available for just this thing.

hth,
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Old 29-06-2006, 14:34   #11
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Deep - a resistor is ALWAYS a current limiting device. Try reducing the resistance in a circuit and see what happens - current goes UP. Input voltage stays the same - it is the current flow through the circuit that varies inversely to the resistance.

The key here is that the resistor is sized so that at max input voltage the current through the two light bars has been reduiced to an acceptable value. Vary the voltage all you want - as long as I don't exceed the 13.8V design specification the unit will be fine.

While you are correct that adding a current limiting device is the 'best' approach, in my application it is truly not necessary since we know the expected / possible range of voltages.

A plain old cheap resistor will limit the current flow to this assembly to an acceptable value when the charger is on (max input voltage). When it's off, the resistor will still reduce current flow - but the resultant light output reduction is not that significant since light output is not linear at that end of the curve. Yes I can see a change in intensity at 12V compared to 13.8V, but the lower light output is still acceptable.

So for my purposes it just was not worth the added expense / hassle to add a current limiter. Yes I know if I take a big power spike I'll be in trouble. But if I keep these things cheap enough I'll be fine.

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Old 29-06-2006, 15:47   #12
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"a Resistor is NOT a current limiting device in a variable voltage cct."
In the absolute sense, that's correct. It is not. And then again, in the absolute sense, that's wrong. The resistor is still a current limiting device, but the current will be limited to a range that varies with the voltage variation. Knowing that a marine "12v" system will actually run from 14.4V at engine speeds down to 10.8v before someone decides the battery is dead...You can set the resistor value to give you the maximum tolerable amperage at one end of that scale, and let it go where it may at the other. Sometimes that's a workable solution.

But since the light bars already are apparently using a current-limiting resistor on each bar--the overall design becomes a little more complex, or a little more work, or a little more wasteful, depending on what you want to invest in it.
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Old 29-06-2006, 15:49   #13
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Mark,

I know nothing about electricity but I do know if you have a smart battery charger or smart regulator for your alternator the initial voltage will be above your limit of 13.8v. Also if you ever equalize your flooded batteries the voltage might get up to 15v. How do you get around this? Keep them switched off during charging?
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Old 29-06-2006, 17:37   #14
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Mark, try connecting your little "current limiting device" to 28 volts and see what happens to your current. Yes, I know ohms law. A resistor in a cct like you are proposing is fine if you are at home and have a 12 volt power supply plugged into 115 volts. But it isn't right in a 12 volt cct on a boat. (but then again it is your boat)

I have replaced resistors that were toast (you probably have too) because some engineer decided he could limit the current with this little 1/4 wat resistor. Then when you look at the cct. you see that the voltage varies (or the downstream resistance changes, ie: transistor...geez, I just gave away my age) and poof, resistor goes up in smoke. Hmmm, kind of like the burn marks on your old units.

By the way, do you think there was some current leaking into your old fixtures and eating away that aluminum?

Oh, on one of my ham xcievers I just replaced the 1/4 wat resistor with a 1/2 wat resistor. I gues we all have slips at one time or another...

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Old 30-06-2006, 13:08   #15
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I guess we just agree to disagree then. I'm comfortable with the design for my application and trying to meet the "lowest cost" design goal I set.

So when you come aboard for drinks (I make a killer margarita) I'll be sure to switch the overhead lights off so you won't feel too anxious

Now working on my next project - building another light fixture using 5mm LEDs in a 4x12 array. I may just go the current limiting route on that one.

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