LED bulb polarity

oldbilbo

oldbilbo

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I'm recycling some parts of an Aquasignal masthead fitting, and seek to include an LED bulb as shown.


IMG_4121.jpg



I have a 'Red' and a 'Black' lead exiting the masthead ( it's an old boat! ) and I reckin I s=ought to be able to squirt some 12V flavour 'lecky up those. May I ask - does polarity matter and, if so, how do I determine which colour-cable to connect to which connector-terminal?

Thanks.... :)


( Er, apologies for poor focus. My camera seems to be having a 'senior moment'. )
 
More and more LED bulbs have bridge rectifiers in so will work either way.
In theory LEDs are not rated as rectifiers, they are entitled to blow up under reverse voltages as small as 3 or 4 volts.
In practice, they seem to withstand being reversed, but 'here endeth the warranty'.
 
if you have a multimeter with diode/transistor test on it you can use that to determine polarity, but unless the socket has some mighty clear indication of polarity it probably does not matter.
 
lw395 said:
More and more LED bulbs have bridge rectifiers in so will work either way.
In theory LEDs are not rated as rectifiers, they are entitled to blow up under reverse voltages as small as 3 or 4 volts.
In practice, they seem to withstand being reversed, but 'here endeth the warranty'.
Click to expand...

With diodes reverse voltage is non-destructive If the current is limited (usually with a series resistor) . The Zener diode operates in reverse breakdown.
 
Ours, from Bedazzled, also work either way as I found out when making a cockpit lantern with a spare one and an empty spice jar recently!
 
A bit off topic but I bought a LED bulb at a boat jumble recently. It doesn't seem to work whichever way I connect it. Is there any way of testing it with a multimeter?
 
padge said:
With diodes reverse voltage is non-destructive If the current is limited (usually with a series resistor) . The Zener diode operates in reverse breakdown.
Click to expand...

A typical LED bulb has 3 LEDs in series. In reverse breakdown, there is no reason to suppose the voltage would be dropped equally by the three diodes. So, the power, limited by the resistor, might be mostly in one die.
In practice though, if the LED is cold, it's likely to withstand a lot more than 12V anyway.
Might be different if the junction temperature is at maximum to start with.
I suspect most LEDs have a low reverse voltage rating for a reason.
 
ghostlymoron said:
A bit off topic but I bought a LED bulb at a boat jumble recently. It doesn't seem to work whichever way I connect it. Is there any way of testing it with a multimeter?
Click to expand...

No, the diode test function won't work, it will need something over 9V to work at all.
 
ghostlymoron said:
A bit off topic but I bought a LED bulb at a boat jumble recently. It doesn't seem to work whichever way I connect it. Is there any way of testing it with a multimeter?
Click to expand...

Some LED bulbs such as for side tail on a car which have a double contact on the base use one contact and the cap for each circuit. Try connecting this way if you have such a bulb.
 
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