LED lights and Polarity

[AnneW]

Having seen a reference to the use of LED lights as an alternative to normal bulbs (use much less power and last longer) I tracked some down in England but they were £44 each!

So I found some 12 cluster LED bulbs on the internet for $6.99 each that had the same bayonet (15b) fitting and said they were suitable for 12v. I ordered them on and they were duly delivered. I put them in and they didn't work. So I contacted the manufacturer who pointed out that I needed the correct polarity in my light fitting. Apparently positive at the tips and negative for the casing. So I took the light fitting to pieces to see if it was possible to change the polarity and then chickened out as I don't know anything about 12v systems other than there is polarity on the whole system. Is it possible to change the polarity on my light fittings or should I just give up and chuck the bulbs I've got away?

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[HaraldS]

just go ahead and change it. for regular bulbs polaritty doesn't matter. a proper wired socket has the positive in the middle and negative on the case. Some have both connectors at the bottom and you may need to try in this case. LED don't get damaged when polarity is wrong, they just don't light up. So they are a good polarity checker.

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[stevebirch2002]

Hi

Can you let me know where you purchased the LED lights as I am interested in making some complete lights.

steve@albinvega.com

<hr width=100% size=1>Albin Vega "Southern Comfort" V1703

 

[Johnjo]

I am also interested,

mike

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[jollyjacktar]

Why not change the polarity on your boat light fitting and make it more conventional. It will be connected with two wires. Just change the wires around. The one on the centre pole and the earth on the housing. Unless your boat is metal and uses the hull as a common earth or some such, in most cases it will not make any difference to the rest of the circuits.

May I respectfully suggest that as a boat owner, you learn a little more about simple 12v circuits etc. You may find it useful and interresting and the knowledge could perhaps save your life one day. If you have a problem on the water, sometimes the RAC will not come to your assistance.

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[AnneW]

I bought the bulbs from www.superbrightleds.com.

Point is well made that I need to increase my knowledge on 12v systems. I've bought the 12v bible but there is a little matter of yachtmaster exams (starting tomorrow!) that I have to get out of the way first!!

When you say just change the wires round do you mean the two wires that go into the fitting or the two wires inside the fitting that go into the bulb housing??

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[tome]

Change the wires that go to the fitting, don't alter the internal wiring.

Good luck with the exams!

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[Plum]

Please do let us know if you get the bulbs working and what you think about the brightness copared to the conventional bulb. and how was the exam? Hope it went well.

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[jurjen]

I think the bulb AnneW is referring to is a

1157 White LED bulb (?)

According to the site specs, this has a power consumption of 55-85 mA, and 78720 mcd.

Can anybody advise as to how this compares to a regular bulb?

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[qsiv]

An alternative (but more expensive) explicitly aimed at nav lights is

http://www.deepcreekdesign.com/

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[mldpt]

Wow that had not occured t me I have often wondered how the AC pannel on my boat knew when the polarity was reversed, as is very often the case in marinas in France and Spain, its just a LED wired in the incoming cable just after the main switch, if the polarity is correct it does'nt light but if its incorrect it does light. Thanks for that
Mike.

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[HaraldS]

Mike,
unfortunately not that simple. Polarity is a misleading term in conjunction with AC, which changes it's polarity 50 times per second. An LED would always light up on AC, as one half wave will supply it with power.

With AC the term ploarity is used relative to the neutral (earth potential) wire and the phase or hot wire. In this case the LED can be used to show a "hot" neutral wire. It is then wired between neutral and ground and the LED doesn't light up as long as neutral has the desired ground potential, if the poles are reversed, the LED lights up.

Harald

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[qsiv]

The maths isnt that tricky - assume you have a 10w bulb ...

P=I x V
or P/V = I
so 10/12 = current drawn for a 10w bulb - in my head I'd call it about 0.8A --- or 800 mA, so its about 1/10 th the current drain.

Almost as important (at least for masthead nav lights) is the 50,000 hour mtbf (mean time before failure).

The deepcreak lamps have very sophisticated drivers, that chop the input to the LED, effectively flashing it, but so fast you cant see it. The net effect is that they consume even less power pro rata as they spend a significant percentage 'off'.

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[jurjen]

Sorry the V/A/W story I do (did) understand!

I was wondering about how the luminous intensity in Candela's compared to regular bulbs, as I got a bit confused with Candle, candela's, lux, flux, lumen, emittance. luminance.... etc. (if you're also confused read http://www.caves.org.uk/led/foot1.pdf for an explanation)

In the meantime the site of http://www.caves.org.uk/led/led-myths.html explained most of my questions, I am continuing to learn!



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[Gordonmc]

A subsidiary question... are the individual LEDs in a cluster wired in series or in parallel?
My cabin lights use "tube" incandescent bulbs and I have been toying with the idea of using a piece of circuit board cut to size with half a dozen LEDs soldered on.
Your answer might save some trial and error.

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[bedouin]

Either - or both. It depends a bit on the supply voltage.

The current generation of white LEDs seem to require about a 3.5V supply - so use in a 12V environment might typically involve 3 wired in series, along with a resistor. If you needed more than that you would wire identical configurations in parallel to give 6, 9, 12 LED configurations.

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[HaraldS]

Most LEDs operates at about 2-3V. Their resistance is very non-linear, they are almost ópen'at lower their resistance sharply above the operating voltage. This measn they easily sel-destruct if forced to a higher than regular oparting voltage.

In pratical terms this measn that they will always need to be run with a resistor in series, which will stabilize the situation. For efficiency reasons it makes sense to keep the volatge drop over the resistor as low as possible, because that part just ends up as heat.

Consequently running an LED at it's 2V and 'burning' 10V in the resistor wouldn't be too good. That would be the case if all of them were connected parallel.

If you put 5 in series, the volatge drop over the diodes would be 10V and you would just burn 2V in the resistor, which would be a good configuration. If you want more than five you can go in multiples of those.

Now whether 4, 5, or 6 diodes in series are best for a 12V ship supply depends on the on the individual diode type. The typical operating voltage and current can be found in therelated data sheet.

You may want to put a regular diode in series to protect the LEDs against wrong polarity. The ready-made clusters have that, and that's why I adviced that you can safely reverse polarity. That is not the case with individual LEDs, like any regular diode they conduct in the 'other' direction above about 0.7V. Didn't want to vercomplicate matters explaining what's in the cluster; but you asked.

Also don't get seduced into running them without any resistor. Say you find that 6 of them work just great on 12V, the whole thing will burn out as soon as you start the engine and voltage goes up.

Given the wide voltage range of the 12V ship supply, you'll never get the ideal setup, as you will have to loose some 20% in the resistor. A more sophisticated regulation is to switch the diodes on and off very fast, and vary the on and off time so that the desired average current is maintained. Some super efficient flash lights do that.



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[bedouin]

NB While "Most LEDs" due indeed run at about the 2-3V area, the current generation of High Brightness White LEDs don't, they are typically run at 3.5V, with a 4V max input voltage. I wouldn't recommend trying to run more than three in series.

Even that is less than ideal 3 x 3.5 = 10.5V - given that you have to allow for the supply voltage to go up to 15V that means you would have to drop between 1.5V and 4.5V across the resistor - it we size the resistor to allow each LED to drop 4V at 15V supply implies 3V across the resistor, or twice the current compared to the 12V input!

With these figures it is probably worth thinking about more active regulation, either a current-limiting transistor or a zener - or of course an LM317

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[jfkal]

For incandescent bulbs the polarity does not matter. It does however for LEDS. Convention says (even though it does electrically not matter) that + should go to the tip and - to the casing of the bulb. Now the one who wired up your lighting system did not seem to bother, that s why it did not work (. If you look at the socket, as long as there are two wires going into it (most likely one red and one black) just cut them 1-2 inch before the socket and reconnect red to black and black to red. It should work then.


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[Plum]

Is there a fixed voltage regulator that gives a steady 3.5V suitable for running LEDs from 10-15volts that needs no additional components and is small enough to fit into the cap of a bulb?

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