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Old 24-12-2011, 21:31   #1
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Bayonet Bulb Polarity

The BA15s bayonet base is supposed to have 12 volts hooked to the center contact where it is fairly well isolated from contact with exterior points. It would seem that reversing polarity such that the bayonet base cylinder was connected to 12 volts would possibly create a shorting hazard, but I am having trouble finding a reference stating that. Any thoughts or references would be appreciated.

Thanks,

Bob
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Old 24-12-2011, 21:53   #2
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Re: Bayonet Bulb Polarity

Why would you want to do that?
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Old 24-12-2011, 22:03   #3
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Re: Bayonet Bulb Polarity

Some LEDs are polarity sensitive. Some have reverse polarity protection. Others don't.

The worry would seem to be blowing out the bulb, not so much a short.

Bill
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Old 25-12-2011, 05:16   #4
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Re: Bayonet Bulb Polarity

Ziggy: I am trying to establish why you would not want to hook up the base with reverse polarity from an authoritative source such as the folks that developed the standard for the socket.

Bill: Thanks. I believe that polarity sensitive LEDs have diode protection to prevent damage, but my question applies to both incandescent and LED bulbs.

Thanks for your comments.

Bob
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Old 25-12-2011, 09:39   #5
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Re: Bayonet Bulb Polarity

An LED (exclusive of PWM power supply regulation built in) IS a diode.
Reverse polarity won't hurt it, it just won't light.
Forward bias needs current regulation via a resistor or the above mentioned regulation to prevent damage.
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Old 25-12-2011, 11:18   #6
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Re: Bayonet Bulb Polarity

The reason you don't reverse polarity on a bulb socket is the outer case then becomes positive which increases the chance of a short.

Is it an LED? Some answers assumed it is. All diodes are polarity sensitive. Allowing current to flow in only one direction is what defines a diode.
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Old 25-12-2011, 12:15   #7
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Re: Bayonet Bulb Polarity

Quote:
Originally Posted by rabend View Post
Ziggy: I am trying to establish why you would not want to hook up the base with reverse polarity from an authoritative source such as the folks that developed the standard for the socket.
I think the likelihood of a short is marginally greater with reversed polarity, but normally the outer shell is hidden inside the fixture anyway.

The same convention applies to the 110V AC circuits. The base is normally hot, and the outer case is neutral. This reduces the chance of a shock from accidentally contacting the socket.

Same convention also applies to cigarette lighter plugs.

It makes sense to me that there should be a convention for the polarity of sockets and connectors, and if you have to make a choice, negative outer shell seems more sensible than the reverse. If you want an authoritative source, you should contact ABYC. Good luck getting a response.
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Old 27-12-2011, 06:27   #8
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Re: Bayonet Bulb Polarity

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It would seem that reversing polarity such that the bayonet base cylinder was connected to 12 volts would possibly create a shorting hazard,
It would only create a shorting hazard if the bulb socket is not electrically isolated/insulated from the rest of the light fixture. There is reference to this in ABYC A-16 Electric Navigation Lights. In the case of metallic fixtures, the positive and negative connections must be identified and polarity must be observed to avoid energizing the case.

Eric
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Old 27-12-2011, 07:29   #9
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Re: Bayonet Bulb Polarity

FYI, although LEDs are a diode they have a very low "Reverse Breakdown Voltage", frequently less than 12 volts so don't rely on being a diode to protect them against reverse polarity. In circuits where there is a possibility of reverse polarity I always and a regular diode in series.
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