Reverse Polarity on LED bulbs? Worst case scenario.

I’ll keep it brief. I have a lamp with L, N, and PG wires to install. I will be using LED bulbs, if that makes a difference. Will it work with the L and N reversed? Basically, the hot wired to the neutral. Assume the ground wire will go unconnected regardless. Hypothetical; you’re not my electrician, etc…


The led “bulb” I’ll assume is the standard A19 shaped lamp with an integral driver. I doubt it will care that the hot and neutral are reversed and without the ground I can’t see any way for it to know which is which.

The LEDs themselves will not work on reverse polarity. At least not the ones I have tried. But I was using a battery and a bare LED.


The actual LED takes direct current and acts as, well, a diode.

Right. Diodes in general absolutely care about polarity. However, they’re also DC devices, and the LED bulbs in question are AC devices which are transforming and rectifying and regulating the incoming AC into the preferred flavour of electricity of the diodes themselves. Those transformers and rectifiers don’t give a crap about the “polarity” of the incoming AC, if it even makes sense to talk about the polarity of AC.

There are good electrical safety reasons to take care not to get the hot and neutral wires mixed up, but the lamp will work fine regardless.

yes, but if this is a normal lamp socket (whether Edison screw base or bayonet) then you have the dangerous condition of the “shell” of the socket being energized.

I think only the Edison screw. A bayonet base has the contact pins within the base, not as part of the socket.

Don’t do that!

If this were a Bad Science Fiction Film, reversing the polarity will cause the LED to function backwards, and it will suck all the light out of the room!

well, all the light in its wavelength range, anyway.
In reality, your LED just won’t light up. Try it sometime with an LED, a battery, and a current-limiting resistor.

Just noting that Amazon sells LED replacements for automotive bulbs, for dome lights and such. Some of the review are from folks complaining that many of the “bulbs” received don’t work at all. Well they would if you just reverse it in the socket!

Nah, Cal, they’ll just produce antiphotons.

While it’s true an LED is a diode, it should never be used (or tested) as a diode.

See, a standard silicon diode can withstand a pretty high reverse voltage across it, depending on the type. Here are the max reverse voltages you can put across the (very common) 1N400X didoes:

1N4001: 50 V max reverse voltage
1N4002: 100 V max reverse voltage
1N4003: 200 V max reverse voltage
1N4004: 400 V max reverse voltage
1N4005: 600 V max reverse voltage
1N4006: 800 V max reverse voltage
1N4007: 1000 V max reverse voltage

The max reverse voltage you can put across most LEDs is around 5 V.

Transients that impose a reverse voltage across an LED can kill the LED. So whenever I use an LED in a circuit, I put a 1N4007 in parallel with it. Opposite polarity, of course. :slight_smile:

Or, if you do actually intend a diode at that point in the circuit, I assume you put a standard diode in series with it.

But if there’s a reverse voltage, will most of the voltage be across the diode? Or will most of the voltage be across the LED? Or will the voltage be evenly split between the two? It all depends on the ratio of the leakage currents of the diode and LED when they’re under reverse-bias, and each is very temperature dependent. Suffice to say, you can’t reliably predict how the reverse voltage be split between the two.

So even if you place a diode in series with an LED, you should also place another diode in parallel with the LED. The anode of this diode (the one in parallel with the LED) should connect to the cathode of the LED, and the cathode of this diode should connect to the anode of the LED. That way, if there’s a reverse voltage, the max reverse voltage across the LED will be around 0.7 V.

It depends on the neutron flow.

While you might get into trouble with an LED intended to be used at higher voltage, ain’t nobody gonna be hurt by hooking up a 5 volt LED the wrong way on a battery, and there’s no need to put anything in parallel with it.

And, realistically, if you want to run LEDs on AC, just use two in anti-parallel.
In fact, such ac-powered LEDs exist.
I’m using these in two places - a day/night outdoor light, and a wall sconce that is on 24/7.

My stepfather has a socket that was wired in reverse, as well as the neutral being switched, by a previous jackass “electrician”. A better quality LED bulb, presumably with a switching power supply, worked fine in it.

It wasn’t knob and tube was it?

The bulbs in question were standard LED filament bulbs with E27 and E14 sockets. I decided to just get one of these instead of playing the guessing game and rolling the dice with the wiring.

Thanks for all the input!