Hi,
I need to hack together a power supply for an "illuminator". Bulb is rated 6.5V 2.75A.
Easy solution is a DC "brick".
However, I think the original power supply was probably just a transformer (i.e., driven with AC, no regulation (?)).
This got me thinking as to whether there *is* a difference driving incandescent lamps with AC vs. DC -- i.e., what would the mains frequency have to be in order for there to *be* a difference?
And, how would that difference manifest itself, otherwise.
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For an ordinary bulb a transformer with an output of 6.5V/3A will do. As
6.5V may not be available, 6.3V will also do. Mains frequency does not matter.petrus bitbyter
Skin effect at higher frequencies would reduce the current, and therefore the brightness.
Incandescent lights strobe slightly when driven by AC, and that's certainly detectable, even if it's not obvious in most situations.
Sylvia.
If you go to DC, remember to size the brick for inrush at startup.
Steve
There will be electromigration issues at some level--using AC tends to reduce the rate of void formation, but I don't know how big an effect that is.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
You don't say what your input is, but you allude to a transformer, so I assume you're running it off the mains.
Four issues that I can think of with AC vs. DC:
1: DC never reverses -- if there are aging effects that are exacerbated or alleviated by the current always flowing one way, the lamp will age more or less with DC. 2: At low enough line frequencies the filament will be significantly heating and cooling each cycle, and will die an early death of thermal fatigue. I suspect you'd have to be in the single-digit Hz for this to happen, but it would strongly depend on the bulb. 3: As mentioned, at some high frequency you'd get high frequency effects such as skin effect, followed by the inductance of the leads to the filament, etc. 4: Some filaments are resonant, if you caught this resonant frequency you'd induce accelerated mechanical fatigue.But I doubt it's an issue at 50 and 60Hz, and perhaps not even 400Hz aircraft line frequency.
-- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
The bulb will be running way above its annealing temperature, so you shouldn't have to worry about metal fatigue.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Tungsten incandescent lamps typically last longer on AC.
The effect is called "filament notching" (aka DC notching). It's more of a problem with very low power long-life bulbs, IIRC, as opposed to something like a car tail light bulb.
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Thanks Spehro! That made my day!
It higher voltages, the positive end the the filament gets thinner quicker than the the rest of it. This makes the bulb burn out quicker.
You don't want the AC to match any mechanical resonant frequencies.
If you drive a lamp with quite DC voltage you can make a nice quite light source. (as long as it doesn't jiggle too much.) 'course not too many people care about that.
George H.
16mm film projectors used to use a high frequency AC source to power the sound lamp, to eliminate any hum.
-- Lead free solder is Belgium's version of 'Hold my beer and watch this!'
You've missed the question: "what would the mains frequency HAVE TO BE in order for there to *be* a difference"
Surely you don't mean to suggest that exciting the filament at
500Kc would be no different than DC?
Yes, though presumably at *much* higher frequencies.
Ah, crap! I forgot about that (I tend to perceive flicker more than "The Average Joe" -- or "Sylvia" for that matter! :> ) And, if I tried to control the brightness of an AC driven lamp using a typical "dimmer", I would be inviting this problem. :(
Apparently a significant effect -- if you chase down the URL Spehro provided. I had thought AC introduced mechanical stress on the filament (heating and cooling rapidly to effectively "vibrate").
I think other (hysterically speaking) power supplies have been little more than a tapped transformer. But, i don't have that so I am looking at options (hence the post).
I don't know how to gauge that effect -- other than to trash some bulbs and determine it empirically. My concern is that they are smallish bulbs (roughly the size of a "tail light" in an automobile) but use a fair pit of power (e.g., 20W).
Lifetime is typically only 100 PoH -- so I don't want to make a foolish choice in powering them and find myself burning through bulbs at an alarming rate (they are $5-$10 each and a PITA to replace/install)
Dunno. The low lifetime leaves me suspicious as to what may be going on with the bulb's design :-/
So, how do bulbs in autos achieve their long lives?
Excellent resource! Thanks!
I've used a pyrometer to observe incandescent lamps operating on 60Hz power. I would see something on the order of a 3% change in flux at 120Hz. It was a casual playing around at the time, though, and my detector system was filtered for
950nm center and a bandwidth of about 120nm, memory serving. Anyway, I gathered from this rough experiment that the filament in a typical 60W light bulb remains fairly hot (doesn't cool that quickly) but does have some observable variation when using an instrument to look more closely.Jon
I would imagine this to be worse at lower frequencies where the wire could actually cool down and heat up enough. At some point with higher frequencies, this 'vibration' wouldn't be physically possible.
Jon
I was just thinking of the old discs you put in a lamp socket to make them last. A diode I guess. I have to make and modify many lamps and microscope lamps to convert the AC to DC for light noise problems. If your working with photodiods or cameras, don't need 120 Hz. There is a fix using 3 phase to DC convereter. It sort of works with incandescents.
greg
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