[Public-List] LED Bulbs

[George Dinwiddie]

Mike,

LEDs, themselves, require about two volts across them to work. When 
powering them on 5 volts, it's common to put a resistor in series to 
drop the other three volts across the resistor. The value of the 
resistor sets the current, which relates to the brightness of the LED. 
Too much current, of course, turns the LED into a DED (Dark Emitting 
Diode). By experimentation, I once found if you continue to increase the 
current through the DED, it will turn into an incandescent light bulb, 
though not a very efficient one.

When you're powering the LED with 12 volts, the inefficiency of a 
resistor is too much. Generally there is some circuitry to regulate the 
current. They might also put multiple LEDs in series, since each has a 
2-volt voltage drop.

The same sort of thing applies to 120 volts AC, though the circuitry 
also has to rectify it to DC.

I don't know what sort of circuitry is in your LED bulbs. It's likely 
that some resistance network is used to make a voltage reference for the 
current limiter. Since it's being powered on one-tenth of the voltage, 
that voltage reference is off and therefore the current allowed through 
the LED is off, resulting in the lower brightness. It would be possible 
to design the current limiting circuitry to work equally well for 12VDC 
and 120VAC, but that would cost slightly more.

I can't imagine any mechanism whereby using a 120VAC LED bulb in a 12VDC 
circuit could cause any danger.

  - George

On 3/29/19 9:28 AM, Mike Lehman via Public-List wrote:
> We have several light fixtures on the boat that take standard base bulbs
> (like those at home). Naturally on the boat, the voltage is 12V while at
> home it is 110V. I plan to change out the bulbs on the boat to LED (have
> already made this change to all of the bulbs at home). So as an experiment,
> I took one of the LED (Home) bulbs, that was purchased at IKEA, and tried
> it on the boat not expecting it to work...but it did! Not quite as bright
> as it is when connected to a 110 lamp, but bright enough to read by. I was
> very surprised.
> 
> Can anyone shed some light (pun intended) on this topic? Why did this work?
> is it safe? It sure as hell is a lot cheaper than 'Marine' 12v bulbs.
> 
> 

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