Light output of an incandescent light bulb changes with the AC frequency. - Page 1
Forum section: electronics / beginners

author: BeBuLamar
author_rank: Super Contributor
timestamp: August 30, 2025, 07:04:47 pm
content: I used a 60W incandescent bulb at the light source to test my camera shutter. I avoid using LED or CFL because they can flcker. Suprisingly on the scope the light output of the incandescent bulb does change very slightly with the voltage resulting a sine wave of 120Hz ripple like signal from the sensor.

author: TimFox
author_rank: Super Contributor
timestamp: August 30, 2025, 07:28:36 pm
content: Why would it not?The relatively long (compared with 60 Hz) thermal time constant greatly reduces the variation in temperature of the hot filament, but does not eliminate it.The resulting change in optical emission is at 120 Hz, since the maximum temperature occurs for both positive and negative peaks of the current.If you need constant illumination, you need to heat the bulb with DC, 120 V DC at 0.5 A to achieve the same mean power as 120 V rms.

author: Benta
author_rank: Super Contributor
timestamp: August 30, 2025, 07:46:00 pm
content: Nothing surprising in that. Incandescent were always so, and sensitive people notice it.

author: coppercone2
author_rank: Super Contributor
timestamp: August 30, 2025, 07:52:32 pm
content: Quote from: TimFox on August 30, 2025, 07:28:36 pmIt might be better to use a constant current source to make it just a little more stable

author: Benta
author_rank: Super Contributor
timestamp: August 30, 2025, 08:06:40 pm
content: Quote from: coppercone2 on August 30, 2025, 07:52:32 pmWut?

author: CaptDon
author_rank: Super Contributor
timestamp: August 30, 2025, 08:10:59 pm
content: Since filament resistance changes with temperature, age and other things a constant current source is a bad idea. Constant D.C. voltage with a controlled ramp up to not over current a cold filament is the way to go.

author: mikerj
author_rank: Super Contributor
timestamp: August 30, 2025, 08:34:01 pm
content: Quote from: coppercone2 on August 30, 2025, 07:52:32 pmConstant current would make itlessstable. The resistance of tungsten filaments increases with temperature so as the filament heats up a constant current drive would increase the power dissipated by the filament, causing it to heat up more which gives a positive feedback loop. A constant voltage drive is a stable configuration, reducing the filament's power dissipation the hotter it gets.The carbon filaments used in early incandescent lamps had a negative tempco so they were very sensitive to supply voltage .

author: Benta
author_rank: Super Contributor
timestamp: August 30, 2025, 08:46:14 pm
content: Quote from: CaptDon on August 30, 2025, 08:10:59 pmNeedless over-engineering.The incandescents in my house (230 V, E14/E27, 25/40/60 W) generally last 5...10 years.OTOH, the highly-praised LED bulbs (Osram/Philips) burn out within 1...3 years.Go figure.I'm glad I hoarded incandescents back then. I still have most of them, they just don't burn out.

author: TimFox
author_rank: Super Contributor
timestamp: August 30, 2025, 08:51:49 pm
content: Quote from: Benta on August 30, 2025, 07:46:00 pmRecently, I have attended several concerts where I sat close enough to the stage to see the pianist's fingering.Of course, auditorium lighting is usually not incandescent, but I can see the stroboscopic effect when a good pianist is playing like a bat out of hell.

author: SteveThackery
author_rank: Super Contributor
timestamp: August 30, 2025, 09:10:46 pm
content: Quote from: Benta on August 30, 2025, 08:46:14 pmI think you have misunderstood. The OP isn't talking about incandescent bulbs for domestic lighting; he is using it to test his camera shutter.I'm surprised by your experiences with incandescents for lighting - it certainly doesn't match mine, and almost all incandescent bulbs have a design lifespan of 750 - 2000 hours.

author: Benta
author_rank: Super Contributor
timestamp: August 30, 2025, 09:14:01 pm
content: Quote from: SteveThackery on August 30, 2025, 09:10:46 pmYes. Using a 60 W domestic incandescant bulb.

author: BeBuLamar
author_rank: Super Contributor
timestamp: August 30, 2025, 09:20:37 pm
content: Well I think I would get some 12VDC or so LED and run them on DC. It's easier to have something like 12VDC supply than 120VDC power supply.

author: golden_labels
author_rank: Super Contributor
timestamp: August 30, 2025, 09:25:06 pm
content: BeBuLamar, consider the equivalentthermalcircuit to see, why the light output is not flat:(note: the source is full-wave rectified, not sine)I hope that explains the pulsing you see.As TimFox suggested, you need DCvoltageequal to the RMS AC voltage. But if you merely rectify your line voltage, you’ll get the same effect. Slightly smaller, because now both electric (reservoir capacitor) and heat (filament) capacitances are combined, but not completely eliminated. May be enough, may be not: depends on your needs.So perhaps consider using 12 V automotive lightbulbs? They’re readily available around the power you need. A battery may be borrowed from a car. Just remember about safety: car batteries can deliver enormous currents, never short the terminals, preferably cover them while battery is moved or not used.(edit: I see you got the same idea while I was writing my post)

author: Zero999
author_rank: Super Contributor
timestamp: August 30, 2025, 09:29:55 pm
content: Quote from: BeBuLamar on August 30, 2025, 07:04:47 pmJust use LED strip run off a decent quality DC power supply. It will give a more steady light output than a mains powered incandescent lamp, use a fraction of the power and not get so hot.

author: coppercone2
author_rank: Super Contributor
timestamp: August 30, 2025, 09:30:37 pm
content: I thought in the end they settle better from constant current. maybe halogen bulbs?for optical spectrum meter here, they use special incandescent bulbs because of their output spectrum. Its a 100 dollar... tiny incandescent bulb, that gets put into top end very expensive equipment . looks no different then a automotive bulb for a panel light

author: BeBuLamar
author_rank: Super Contributor
timestamp: August 30, 2025, 09:57:41 pm
content: Quote from: golden_labels on August 30, 2025, 09:25:06 pmI have regulated power supply 0-36V @ 5A max should power the LED to same brightness as a 60W incandescent bulb.

author: tooki
author_rank: Super Contributor
timestamp: August 30, 2025, 10:04:52 pm
content: Quote from: coppercone2 on August 30, 2025, 09:30:37 pmHalogen bulbs are just a subtype of incandescent bulb optimized for higher operating temperatures. mikerj already explained why constant current driving would have a destabilizing effect on incandescent bulbs, and that applies equally to halogen bulbs.

author: Circlotron
author_rank: Super Contributor
timestamp: August 30, 2025, 10:06:11 pm
content: A three-phase incandescent lamp with three separate filaments would fix the problem.

author: Analog Kid
author_rank: Super Contributor
timestamp: August 30, 2025, 10:07:41 pm
content: Quote from: BeBuLamar on August 30, 2025, 09:57:41 pmWhat kind of LED exactly? COB?Certainly not a little-bitty T-1-3/4 LED.

author: Analog Kid
author_rank: Super Contributor
timestamp: August 30, 2025, 10:10:07 pm
content: Quote from: Circlotron on August 30, 2025, 10:06:11 pmWhat a great idea; all you need is a 3-phase converter.That oughta only set you back, say, $400-500.And where does one get 3-filament bulbs? Never seen one, but I guess they must exist somewhere.

author: BeBuLamar
author_rank: Super Contributor
timestamp: August 31, 2025, 01:05:29 am
content: Quote from: Analog Kid on August 30, 2025, 10:07:41 pmSomething like this should work.https://www.amazon.com/ENERSYSTEC-12V-60V-Voltage-Medium-Lighting/dp/B0DLBTYWSD/ref=sr_1_4_sspa?dib=eyJ2IjoiMSJ9.532WXqWyAwRkzj4XSLinvIMWF8y0gvs5lNGb9UFc-qojA5wFzGnrwnOw_VmdJiIh_2XWyl7dieugVa9ifanfJjnB97A_5mheeKk2HfnZ6o57fL_UzYOakHG8knYxRdJ4goC4QVXvqcuO3AkGB3uoX2nKkAtoIbR2H2fYbvSbMPENUROv5xyoU5PVAe-xh_FF2sEx4jNHBzbr0ZGRrBD9oR6_pQo4UoEqp9DoknAVa88Prk4ao4HEuOAhsovmqQTNo2bTnER2uIfFgzqCw_cJRecT-e9RbqJpI9qfGgXcR5k.TzQe5hNm6iuIn7iXihTMyrPjyXbnqzX4Gz1rBuACypM&dib_tag=se&hvadid=694355725629&hvdev=c&hvexpln=67&hvlocphy=9026816&hvnetw=g&hvocijid=6283972230784419511--&hvqmt=b&hvrand=6283972230784419511&hvtargid=kwd-306020268612&hydadcr=24657_13626628&keywords=24+volt+light+bulb&mcid=82a4c61f9305345982c0d60ccc5a0280&qid=1756602224&sr=8-4-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1

author: Analog Kid
author_rank: Super Contributor
timestamp: August 31, 2025, 07:55:15 pm
content: Quote from: BeBuLamar on August 31, 2025, 01:05:29 amJust as a side note, that horribly long URL can be shortened down to this:https://www.amazon.com/ENERSYSTEC-12V-60V-Voltage-Medium-Lighting/dp/B0DLBTYWSD/ref=sr_1_4_sspaTrim everything off starting with the "?". All that junk is just irrelevant stuff that applies to your web browser's "session", nothing that we out here need to see.(Just a whole bunch ofvariable=valuestatements, separated by "&"s.)

author: Laval
author_rank: Regular Contributor
timestamp: August 31, 2025, 08:16:10 pm
content: QuoteIt doesn't match my experience either. I have been using LEDs instead of incandescent bulbs for 13 years now and I have replaced my first one a few months ago. Incandescent bulbs are also very susceptible to vibrations. I had one outside close to a door and it was failing many times a year because of vibrations.The only thing that gets complicated with LEDs is when you want to use dimmers.

author: golden_labels
author_rank: Super Contributor
timestamp: August 31, 2025, 09:37:51 pm
content: Same here, but…While it’s hard to overheat an incandescent lightbulb, LED and CFL bulbs are vulnerable to temperature. If Benta had them mounted where they got cooked, their life might indeed be very short. The problem is made worse by the market being flooded with ultra-cheap models, made at absolutely minimum cost and killed quickly by normal inrush currents, and by trouble with adequate cooling (mostly in the past), since all obvious solutions were blocked by patents.Of course I still doubt the “10 year” reports. Even my grandma, who almost didn’t use lamps even when it was getting dark, never reached such values. And it gets even worse now in places with light source efficacy laws. To feed on people’s belief they need an incandescent lamp, vendors are now selling bulbs that run at much higher power.

author: BeBuLamar
author_rank: Super Contributor
timestamp: August 31, 2025, 10:07:45 pm
content: Quote from: golden_labels on August 31, 2025, 09:37:51 pmWell all LED bulbs designed to work with normal household power have power supply in them and I found the power supply often failed not the LED themselves.

author: TimFox
author_rank: Super Contributor
timestamp: August 31, 2025, 10:14:03 pm
content: Do the LEDs used in automotive signals and brake lights use switching power supplies to work with 12 V DC?For your shutter tests, presumably from 1 sec to 1 msec or so, you want to avoid ripple, and a bare LED is more sensitive to ripple than an incandescent bulb.

author: floobydust
author_rank: Super Contributor
timestamp: August 31, 2025, 10:18:43 pm
content: This thread has some scope/phototransistor measurements:20W Halogen bulb viewed by a photodiode

author: Analog Kid
author_rank: Super Contributor
timestamp: August 31, 2025, 10:19:00 pm
content: Yes, ripple.Having had some experience in this area--I built a shutter-speed timer, and found that using CFLs (LEDs hadn't yet made it into the consumer light bulb market) would throw the tester off because of their 120 Hz pulsations, whereas incandescent bulbs worked just fine.So I'm guessing that whatever light-output ripple there is in an incandescent bulb will be negligible, and shouldn't interfere with the operation of your timer. Sure, you might be able to see it on your o'scope, but I don't think the shutter tester will be bothered by it.

author: BeBuLamar
author_rank: Super Contributor
timestamp: August 31, 2025, 11:25:47 pm
content: The ripple of the incandescent bulb isn't bad enough to make the measurement not working. Plain LED should be OK with a CV/CC power supply.

author: Analog Kid
author_rank: Super Contributor
timestamp: September 01, 2025, 12:04:03 am
content: Quote from: BeBuLamar on August 31, 2025, 11:25:47 pmAs pointed out by others, current regulation is not needed at all for your application.Probably not even voltage regulation. If the power supply is only powering your LED, then there's no need to worry about light intensity fluctuations.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 01, 2025, 01:32:17 am
content: Quote from: TimFox on August 30, 2025, 07:28:36 pmThe traditional way for e.g. cinematography applications was to balance the lights across three phases. Obviously that doesn't work if you don't have three phase power available.

author: TimFox
author_rank: Super Contributor
timestamp: September 01, 2025, 03:33:43 am
content: Quote from: ejeffrey on September 01, 2025, 01:32:17 amA simple full-wave, single-phase rectifier (without filter capacitor) has 100% ripple, but a six-diode three-phase rectifier has about 30% ripple, if I remember correctly (peak-to-peak compared with peak DC).However, I have never seen a single bulb with a three-phase filament: in cinematography, you are using multiple bulbs.

author: Zero999
author_rank: Super Contributor
timestamp: September 01, 2025, 07:15:45 am
content: Quote from: mikerj on August 30, 2025, 08:34:01 pmYes. Powering an incandescent lamp from a constant current power supply would most likely cause it to fail, assuming the voltage isn't limited to the rating of the lamp. It like powering a fluorescent lamp directly from a constant voltage supply and is why a ballast is used.Quote from: TimFox on September 01, 2025, 03:33:43 amFactories and workshops also have three phase lighting because it eliminates the stroboscopic effect which can make object rotating at the mains frequency appear static and result in accidents. In reality the 30% ripple is smooth enough not to cause a problem and will be further smoothed by LED PSUs, fluorescent ballasts and incandescent lamp filaments.

author: TERRA Operative
author_rank: Super Contributor
timestamp: September 01, 2025, 08:34:38 am
content: Quote from: Analog Kid on August 31, 2025, 07:55:15 pmYou can go even smaller:https://www.amazon.com/dp/B0DLBTYWSD

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 01, 2025, 10:57:45 am
content: Quote from: Analog Kid on September 01, 2025, 12:04:03 amIf I use a LED bulb designed for say 12VDC then there is no need for regulation because there is cicuitry inside the bulb to do that. If I just use a plain LED then I have to regulate the current but I don't think I would go that route. Using a regulated power supply to ensure there is no ripple.

author: Analog Kid
author_rank: Super Contributor
timestamp: September 01, 2025, 10:46:06 pm
content: Quote from: BeBuLamar on September 01, 2025, 10:57:45 amAll you need for that is awell filteredpower supply, not a regulated one.

author: TimFox
author_rank: Super Contributor
timestamp: September 01, 2025, 11:38:47 pm
content: Quote from: BeBuLamar on September 01, 2025, 10:57:45 amIf the LED bulb's internal circuitry is a switching circuit, there may well be ripple at higher frequencies, since the LED response itself is reasonably fast (although phosphors may slow it down for white LEDs).Ordinarily, there is no need in regular illumination for good ripple-reduction filtering at frequencies well above 100 Hz.If the internal circuitry is merely resistive, then a well-filtered DC supply should induce no ripple whatsoever.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 02, 2025, 12:27:32 am
content: Quote from: Circlotron on August 30, 2025, 10:06:11 pmThree filament bulbs are hard to come by. But fixtures to hold three different bulbs, each on a different phase, are easy and would do the trick. For someone with easy access to three phase this might make sense.Another special circumstance that would work is someone who has access to 400 Hz power, commonly used in aviation. Flicker would be reduced by a drastic amount.Finally, there may be another easy path if the OP is in a location that uses 220 V power. I don't know if filaments in 220 V bulbs are made longer, or thinner (or both) to achieve the same power output as 110 volt bulbs. If the 220 bulbs are thinner the thermal time constant would be shorter and using two 110 bulbs in series would give somewhat less ripple.Final comment: Running the bulbs on DC will certainly work, but many, many reports say that operating life is significantly reduced. Fun physics involved. This can be mitigated by periodically reversing the applied polarity. Which can be a PITA if not automated.

author: Analog Kid
author_rank: Super Contributor
timestamp: September 02, 2025, 12:55:43 am
content: If the OP wants to use an incandescent bulb for his shutter tester, then all this talk about light-output ripple is just so much hand-wringing over nothing.As I wrote above, I actually built and used such a tester, a microprocessor-based timer with a phototransistor to measure shutter speeds. Works fine.Under CFL illumination the results were unreliable because of flicker. But underanyincandescent bulb, the results were always fine. Whatever flicker there is with an incandescent bulb is negligible. (Worst case, you might have to do a little waveshaping on the photo sensor output, but probably not even that would be needed.)

author: 5U4GB
author_rank: Super Contributor
timestamp: September 02, 2025, 10:28:57 am
content: Quote from: Benta on August 30, 2025, 08:46:14 pmThat text describes the problem: "LED bulbs", i.e. trying to cram a pile of components and heat into a form-factor they were never meant for and that's perhaps not quite the pathologically worst you can get but close to it. LED downlights with decent components and where the whole body is a giant heatsink last essentially forever (the ones in the house here have been in use for 13 years with zero failures, ask me again in ten years).

author: Siwastaja
author_rank: Super Contributor
timestamp: September 02, 2025, 11:41:24 am
content: It was common to use standard lightbulbs fed with AC in animation/stopmotion work, but you then need to choose the shutter speed carefully: either exactly the length of a half-cycle (or integer multiple thereof), so that the same amount of light gets integrated no matter where in the phase you exactly start and stop (and you might still see variation during mains frequency excursions unless you actually sync to it) - or simply long enough so that contribution of any partial cycle is becoming negligible.To be sure, rectify and smooth it to DC. Remember the scary and possibly dangerous effect of DC on fuses/switches/plugs.Low voltage is recommendable. Lower design voltage also increases bulb efficiency, lifetime and color temperature. E.g. 12V or 24V.