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JOHNNIE36 · 01-08-2009, 05:52 PM

If anyone out there is interested, the thing that a few years back got me going on CFL's was my wife's constant complaining about how hot it got in her bathroom when she was blow drying hair and putting on makeup. It was all those incandescent bulbs--so I changed them out to CFL's and she hasn't complained since. Thinks I'm a genius!

NW-Bound · 01-08-2009, 06:21 PM

Quote:

Originally Posted by SteveL

ERD50 is correct, except that Ohm's Law is for direct current. You have to convert AC to get a direct current voltage equivalent. The easy way to do this is to multiply the AC voltage by .707. This gets you close to the correct voltage to calculate amperage.

For our convenience, AC voltages and currents are usually already quoted as rms (root-mean-square). This means that the common 115VAC is really 115VAC rms, not 115V amplitude or 115V peak. It has a peak voltage of 115*sqrt(2)=162V. This means that it swings sinusoidally between -162V to +162V, not between -115V to +115V.

Quote:

Originally Posted by ERD50

100V, 100 W bulb: Requires 1 Amp, and a 100 Ohm filament.

50V, 100 W bulb: Requires 2 Amp, and a 25 Ohm filament.

So, if we want to pick on ERD50, all we need to add is the word rms as follows, and he is still correct.

100Vrms, 100 W bulb: Requires 1 Amp rms, and a 100 Ohm filament.

50Vrms, 100 W bulb: Requires 2 Amp rms, and a 25 Ohm filament.

mathjak107 · 01-12-2009, 05:19 PM

25 watt bulbs are much more expensive than 40 , they sell very few in comparison

ERD50 · 01-12-2009, 07:18 PM

Quote:

Originally Posted by wrigley

I go into Home Depot last night to get some light bulbs for the house.

$2.97 for a 2 pack of GE soft white 25W bulbs, and

$.88 for a 4 pack of GE soft white 40W bulbs.

Quote:

Originally Posted by Want2retire

I don't know! Maybe GE sells more 40W bulbs (than 25W bulbs), bringing the price down? Doesn't sound like a likely explanation but I can't think of any other.

Quote:

Originally Posted by mathjak107

25 watt bulbs are much more expensive than 40 , they sell very few in comparison

I've noticed this too and it is odd. They definitely sell more 40/60/100W bulbs than they do 25W bulbs, and that explains part of it. But that is a big delta. Maybe 40/60/100W light bulbs are sold as a "loss leader" (or at least "low profit" leader?) to get people in thee store and buying, but they feel they don't need to do that with the less popular sizes?

A 25W bulb is just a variation of a 60W bulb, it's not like they really need to charge a multiple to re-coup their engineering, or capital expenditures, etc. You don't pay 2x or more for a car in a less popular color. These are not special or custom orders, just less popular variations.

In fact, this has had a negative effect. I have a few places where I just need some light, 25 or 40W would be fine. But I look at the price and get the 40W - I'll never recoup the savings in electricity with the price delta. And before you say CFL - these are on an electronic timer that can't use a CFL. I feel bad "wasting" energy, but I'm deciding to make it a financial decision.

-ERD50

ERD50 · 01-12-2009, 07:37 PM

Quote:

Originally Posted by NW-Bound

So, if we want to pick on ERD50, all we need to add is the word rms as follows, and he is still correct.

100Vrms, 100 W bulb: Requires 1 Amp rms, and a 100 Ohm filament.

50Vrms, 100 W bulb: Requires 2 Amp rms, and a 25 Ohm filament.

Well, as long as this thread came alive again, I'll respond to this. I started, but my post got eaten up during the maintenance problem, so I dropped it. Wasn't that important. Anyway...

Just to be technical about our technical discussion

, there is nothing 'incorrect' about saying 100V x 1A = 100W. Since the descriptor was not specified, it would be assumed (convention might be a better word) that they are all the same units. You would not mix RMS and Peak (for example) in the same equation w/o specifying them.

So:

100V RMS x 1A RMS = 100W RMS
100V DC x 1A DC = 100W DC
100V Peak x 1A Peak = 100W Peak
100V Peak-to-Peak x 1A Peak-to-Peak = 100W Peak-to-Peak

are all as true as

100V x 1A = 100W.

As a reference point, the venerable Ohms Law states simply that E=IR. It does not specify RMS, Peak, etc. And if Georg Ohm had it wrong, I've lived my adult life as a lie!

-ERD50

NW-Bound · 01-12-2009, 07:41 PM

Quote:

Originally Posted by ERD50

As a reference point, the venerable Ohms Law states simply that E=IR. It does not specify RMS, Peak, etc.
-ERD50

Very true. I simply pointed out to SteveL that the rms is usually implied when we talk AC currents and voltages.

mathjak107 · 01-13-2009, 01:51 AM

Quote:

Originally Posted by ERD50

I've noticed this too and it is odd. They definitely sell more 40/60/100W bulbs than they do 25W bulbs, and that explains part of it. But that is a big delta. Maybe 40/60/100W light bulbs are sold as a "loss leader" (or at least "low profit" leader?) to get people in thee store and buying, but they feel they don't need to do that with the less popular sizes?

A 25W bulb is just a variation of a 60W bulb, it's not like they really need to charge a multiple to re-coup their engineering, or capital expenditures, etc. You don't pay 2x or more for a car in a less popular color. These are not special or custom orders, just less popular variations.

In fact, this has had a negative effect. I have a few places where I just need some light, 25 or 40W would be fine. But I look at the price and get the 40W - I'll never recoup the savings in electricity with the price delta. And before you say CFL - these are on an electronic timer that can't use a CFL. I feel bad "wasting" energy, but I'm deciding to make it a financial decision.

-ERD50

im an electrical distributor and manufacturers of lamps have tier pricing based on how many they make and how long they have to warehouse stock before it sells... and we do the same.... we have to pay for those 25 watt bulbs we stock in 30 days....they sell so infrequently our money may be tied up for 90 days before we sell some,hense they cost more..... theres other factors besides manufacturing cost that goes into pricing.

in distribution its all about product turns, if i used that money insted to buy 12/2 romex in those 90 days i may have sold that romex 6 or 7x and made a profit each time ,instead those 25 watt bulbs turn once

ERD50 · 01-13-2009, 09:03 AM

Thanks for that explanation Mathjak107, I didn't consider the inventory turns issue that fully at the retail level. Makes sense.

I would think it would be less of a factor for the big box places (but still a factor). I would think they could turn a case of 25W bulbs pretty fast, and might order 10 cases of 40W for every case of 25W. But I suppose there are still qty discounts, similar issues up and down the line, etc. And in a small store, where they might have to order the same number of 25W as 40W just to hit some minimum, it would be a bigger deal.

-ERD50