[Public-List] Public-List Digest, Vol 3831, Issue 2

[Stephen Gwyn]

I have a bunch of LED bulbs at home that are explicitly
marked as being 12VDC/120VAC/.

In general, as George points a LED meant for home use
will have a rectifier circuit to turn AC into DC.
The circuit is just as good at turning DC into DC.
However if is expecting 120 V and only getting 12V
depending on how it's set up, it might not be very bright.

The main potential problem with LED bulbs is RFI.
Some LED bulbs cause RFI feedback that can make
VHF radios have more static. But as far I can determine,
it the brighter LEDs that tend to have this problems,
because they also have thermal regulation circuits
which, when the LED gets too hot, dim the LED by
turning it on and off very quickly.

Most of the bulbs on my boat are LED. Only the masthead
anchor light causes trouble.

S





On 2019-03-29 1:04 p.m., public-list-request at lists.alberg30.org wrote:
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> Today's Topics:
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>     1. Re: LED Bulbs (George Dinwiddie)
>     2. Re: LED Bulbs (Kris Coward)
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> ----------------------------------------------------------------------
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> Message: 1
> Date: Fri, 29 Mar 2019 14:33:18 -0400
> From: George Dinwiddie <gdinwiddie at alberg30.org>
> To: public-list at lists.alberg30.org
> Subject: Re: [Public-List] LED Bulbs
> Message-ID: <b15f0285-64c2-29fd-dea8-9f6fc6251ed7 at alberg30.org>
> Content-Type: text/plain; charset=utf-8; format=flowed
> 
> 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.
>>
>>
>