AC / DC Light

[DerbyBorn]

At about 4:00am this morning I found myself wondering if a lightbulb would be brighter on DC than on AC at a given voltage. Any ideas?

[Bob Minchin]

The definition of RMS ac voltage is linked to the equivalent DC power so for a standard incandescent light bulb, the power input and hence light output should be the same.

Most if not all electronic light bulbs will have some form of regulation in the power supply and will possibly make no difference to the light output either.

In practise, light switches and dimmers will not appreciate a DC supply if you were going to experiment.....

[meow2222]

The heating effect is the same either way, assuming dc voltage = rms ac v oltage. There is one difference though, filaments last longer on ac, on dc they wear out at one end. So a given filament size with a given life expect ancy is actually brighter on ac, at marginally higher voltage than on dc.

The situation is completely different for other bulb types.

NT

[Archibald]

If the current is the same through the filament, why would one end wear faster?

AB

[harryagain]

AC voltage (and current) is specified at the"RMS" voltage/current which is the equivalent DC voltage/current to give the same power.

RMS = Root of the Mean Squared. (A sort of average)

Filaments last longer on DC. With AC the varying magnetic field causes the filaments to fail sooner due to "vibration".

[Archibald]

That I can comprehend. The bit I dont understand is the wear at one end.

AB

[Ian Jackson]

In message , harryagain writes

I'm just wondering if another factor is that, on AC, the filament is subjected to 100Hz pulses of a higher peak current than the steady, constant DC current?

[Vir Campestris]

I know old DC lamps used to go dark at one end.

As metal evaporates off the filament it's often ionised. In an AC lamp there's no standing electric field (it keeps reversing) but in a DC lamp there's a standing voltage difference which will accelerate the ions one way.

Andy

[Archibald]

AAAh! possibly.

I wonder if the ions tend to be repelled from the point of highest Voltage i.e the bit electrically furthest from ground. This could give rise to the filament loss at one end i suppose?

I would guess that the dark bit would be the none grounded side. [If they did that sort of thing with DC of course :-)]

AB

[Vir Campestris]

Metal ions are almost certainly positive, and would head for the negative end regardless of absolute voltage.

Andy

[meow2222]

Yeah, I suspect its ionic bombardment that does the damage on dc.

NT

[newshound]

Because the ends are at different polarities, and therefore attract (or repel) ionic species in the gas differently. It won't be "wear" so much as "oxidation" which is different. The positive end will attract and neutralise oxygen and hydroxyl ions which will tend to oxidise the filament. The oxygen level will be pretty low (most filament bulbs are filled with argon) but it won't be zero.