Turning lightbulbs on and off makes it blow faster False or true?

[rbj]

Yes - there is a price to pay (duty-cycle) for on-again-off-again usage that is mitigated by continuous use. It's not just in light bulbs.

oh yeah, we know it's not just light bulbs. another example issue is the one between people who leave a computer (or perhaps some other electronic device) on always and people who turn it off every time they're done with it. I'm in the middle category - i turn on the computer the first time of the day that i use it and leave it on until i am done with it for the last time that day, then i shut it down. i just think that is better than leaving the hard drives spinning for 10+ hours at night without any use and sometimes i hear the read/write heads rachetting around when the computer just sits there.

 

[rbj]

I think Doc Al's reasoning is correct. Should be a relatively simple experiment to set up... on for 5 seconds... off for 5 seconds... repeat. If it really does have an effect on the life expectancy of a bulb, I think it'd be significant enough so that you'd get the bulbs to blow within an hour or so under such conditions.

it would take longer than an hour. could take weeks.

 

[cesiumfrog]

it would take longer than an hour. could take weeks.

What do you base that on?

I mean, the "one hour" was a complete guess. Order of magnitude at best. There was no justification given whatsoever, so no reason to think it meant specifically one actual hour. But since you think that guess is out by a specific amount, I've got to ask, what makes your refined estimate any more trustworthy?

 

[rbj]

What do you base that on?

i pulled it out of my butt (and I've replaced a lot of light bulbs in my 5 decades, you get an idea how long they last).

I mean, the "one hour" was a complete guess. Order of magnitude at best. There was no justification given whatsoever, so no reason to think it meant specifically one actual hour. But since you think that guess is out by a specific amount, I've got to ask, what makes your refined estimate any more trustworthy?

it's hardly refined. in fact it's very broad ("weeks" which could mean two or fifty) and qualified ("could"). but check out

http://www.gelighting.com/na/home_lighting/ask_us/faq_defective.htm

now if a typical light bulb might last for about 1000 hours (about 4 weeks) when simply left on (100% duty cycle), you might, under decent circumstances, expect it to last for 8 weeks at 50% duty cycle. but that's something like on for a few hours and off for another few hours. i really doubt it would last as long as that where the frequency of power cycling is 1/10 Hz.

what ballpark figure were you thinking of? one hour? one year? "weeks" is somewhere sloppily in between.

 

[Mk]

Didn't Mythbusters do this one?

 

[ShawnD]

No. AC in the US is at 60 cycles and if you use a dimmer to interrupt that the sine wave gets really notchy with a squared-off wave form.

Is that true? I always assumed dimmer switches were just variable resistors. A resistor will lower the overall amperage without affecting the wave in any way. A squared wave sounds a lot more like what a power inverter does.

And yes Mythbusters did do this one. They found that leaving the lights on does not save electricity (what a stupid myth), but they also found that turning the lights on and off dramatically reduces the life span. In their test of either 5 or 6 different kinds of bulbs, none of the bulbs lasted longer than 2 months when turned on and off at 2 minute intervals.
This is why schools and offices often leave the lights on 24/7; it's expensive to replace light bulbs when the guy replacing them is paid $20 per hour. It's a myth that janitors replace light bulbs; most companies have their engineering/maintenance department change the light bulbs.

 

[rbj]

Is that true? I always assumed dimmer switches were just variable resistors. A resistor will lower the overall amperage without affecting the wave in any way.

but what are you going to do with the heat dissapated by the resistance? and who's going to pay for the wasted power? zero-resistance (switch closed) has no power loss because there is no voltage acrossed it. infinite-resistance (switch opened) has no power loss because there is no current through it. (power is voltage x current.)

 

[ShawnD]

Increasing resistance lowers overall power consumption, it does not increase it as you are suggesting.

 

[Stingray]

Variable resistors aren't used in dimmers. That would generate a lot of heat and waste money. In the old days, variable inductors were used instead. Since the source is AC, the inductor presents an impedance to the circuit without dissipating (much) power. Modern dimmers use solid-state devices to apply a kind of pulse width modulation.

 

[ShawnD]

Modern dimmers use solid-state devices to apply a kind of pulse width modulation.

Weird, but interesting.

Wouldn't an inductance-style resistance cause incredibly high power loss? I don't know what scale you're talking, but I remember doing some calculations relating to electric motors and the power loss was very high if the circuit was not balanced out with capacitors.

I'm curious about resistor loss, so I'll work it out and see how it goes. I'm posting it so nobody else needs to waste time doing the same calculations
120W bulb would be (120V)^2 / (120W) = 120 ohms resistance
Let's double the resistance, so 120 in the bulb and 120 in the dimmer.
120V / 240ohm = 0.5amps
bulb power = (0.5amp)^2 * (120ohm) = 30W
dimmer power would be the same, so that makes 50% efficiency if the dimmer goes to 1/4 normal brightness.

I see your point.

 

[rbj]

Increasing resistance lowers overall power consumption, it does not increase it as you are suggesting.

i did not suggest that it would increase the power. i said it would waste power.

if the resistance in the dimmer is equal to that of the light bulb, then just as much power is used on your light bulb is also wasted in the dimmer. besides waste, that means heat. the dimmer gets pretty hot and you have to deal with that.

they haven't used resistive dimmers since the 60's. maybe even earlier. all modern dimmers use solid state chopper circuits. it's the only efficient way to do it.

 

[rewebster]

In our old house, we had a dimmer switch on the chandelier over the kitchen table and that fixture at light bulbs like candy. The power filtered through that dimmer was really "notchy" and if it was quiet in the house, you could hear the filaments humming as you dimmed the lights.

If it was the 'dimmer' that was 'humming' it could have been from magnetostriction.

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/magstrict.html

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(And which may, from a previous post, #15, add to the 'vibration' of the filament, too)

 

[turbo]

If it was the 'dimmer' that was 'humming' it could have been from magnetostriction.

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/magstrict.html

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(And which may, from a previous post, #15, add to the 'vibration' of the filament, too)

The dimmer was not humming. It was not making a sound. The hum was coming from the filaments in the light bulbs and I assume it was because of the dimmers changing the wave-form of the power to the filaments so instead of getting a smooth sine wave, the bulbs got energized in notchy pulses. The sound was quite distinct.

 

[u83rn00b]

TRU! ITLL BURN OUT WAYY faster!