Incandescent Lamp Burn-in - Page 1
Forum section: electronics / projects

author: David Hess
author_rank: Super Contributor
timestamp: August 16, 2025, 08:20:58 pm
content: A couple months ago I performed some maintenance on my Tektronix FG502 function generator, including replacing the T-1 incandescent lamp used as a power indicator. I used a 28 volt 24 milliamp T-1 "grain of wheat" bulb and added a series resistor to operate it at 20 volts figuring it would last forever; it lasted 2 weeks, with maybe a day worth of being powered.Now that particular bulb was probably just bad. Tektronix had internal part numbers for similar bulbs where they "aged and selected" them, presumably for better reliability.So my question is, what would be the best way to "burn-in" a small incandescent lamp for better reliability? I just have my remaining bulbs running at their design voltage for a couple days, (1) as shown below, (2) but should I be doing something different? I was thinking of adding a circuit to blink them.I also find these little incandescent bulbs useful as fuses and PTC resistors. Unfortunately over the past 10 years they have at least tripled in price.(1) Oddly enough, over a couple of days they drew slightly *more* current. I thought it would go down if anything. I guess nice bulbs don't go down.(2) Voltage reduced to take photograph.

author: tggzzz
author_rank: Super Contributor
timestamp: August 16, 2025, 08:40:58 pm
content: I would have thought the best ways to increase lifetime are to reduce the voltage and to minimise inrush current when they are cold. That's when most domestic bulbs fail.There appears to be useful pointers to sources inhttps://pubs.aip.org/aip/adv/article/12/10/105116/2819829/Incandescent-lamp-design-and-lifetime#81928685Wackypedia indicates lifetime is proportional to V-16(!)https://en.wikipedia.org/wiki/Incandescent_light_bulb#Light_output_and_lifetimeEven if the "-16" shouldn't be taken too seriously, "-8" would be significant.ISTR introducing "coiled coils" was used to increase the lifetime of domestic incandescent light bulbs; the extra inductance reduced the inrush current.

author: Gyro
author_rank: Super Contributor
timestamp: August 16, 2025, 08:54:35 pm
content: Slightly increasing current probably implies a slight reduction in filament temperature as it's pretty unlikely that the Tungsten filament is lowering in resistance, either through increased thickness or shortening - it might be that the coil relaxes slightly though, increasing the spacing between turns which would allow more cooling. Either that or a subtle change in internal atmosphere, I don't know whether lamps that small are Argon filled or evacuated. Personally I would go with the relaxing theory.With regard to burn-in testing. I would certainly include some cycling, as cold filament surge current should weed out any weak sections (and the terminations of the filaments to the supports).The other thing I would do is put some thermal insulation around the lamps. Running them in the open air like that isn't representative of actual mounting position and won't allow the glass envelope and seals to reach peak temperature

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: August 16, 2025, 09:10:22 pm
content: David those lamps look like 12V running at 11.8V.My 24V lamps are 1/2 that brightness at 11.8V.Jeff

author: David Hess
author_rank: Super Contributor
timestamp: August 16, 2025, 09:29:55 pm
content: Quote from: Jeff eelcr on August 16, 2025, 09:10:22 pmThey were blooming my camera so I reduced the voltage from 28 to 12 volts. They have been running at 28 volts for a couple days now.Quote from: tggzzz on August 16, 2025, 08:40:58 pmThat is roughly correct, and why I thought running 28 volt lamps at 20 volts would extend their operating life to essentially forever, but the bulb burned out shortly after I installed it, which got me to thinking about why Tektronix was aging and selecting bulbs. The original bulb was too bright anyway.I could replace the bulb with an LED, but my question is about bulb reliability since I use them for other things.

author: mikerj
author_rank: Super Contributor
timestamp: August 16, 2025, 10:17:31 pm
content: Quote from: David Hess on August 16, 2025, 08:20:58 pmI may be misunderstanding this, but the FG502 service manual shows the lamp runs off a 20v rail so why did you need a series resistor?

author: tggzzz
author_rank: Super Contributor
timestamp: August 16, 2025, 10:38:12 pm
content: Quote from: David Hess on August 16, 2025, 09:29:55 pmCould the replacement bulbs simply be out of spec, e.g. a lower voltage?

author: wraper
author_rank: Unknown
timestamp: August 16, 2025, 10:55:43 pm
content: Probably it's just shit bulb made in some shed factory. Barely anyone needs them these days, so nobody reputable will produce them in high volume. I'd simply use an LED, especially considering you're adding a series resistor anyway.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 16, 2025, 11:33:21 pm
content: Quote from: David Hess on August 16, 2025, 09:29:55 pmReplace those "other things" with LED's also.

author: kite31
author_rank: Regular Contributor
timestamp: August 16, 2025, 11:39:04 pm
content: Quote from: Doctorandus_P on August 16, 2025, 11:33:21 pmOr heaters.

author: Analog Kid
author_rank: Super Contributor
timestamp: August 16, 2025, 11:44:03 pm
content: Quote from: Doctorandus_P on August 16, 2025, 11:33:21 pmBut what if one of those other things is a Wien bridge sine-wave oscillator? LED won't work there.

author: David Hess
author_rank: Super Contributor
timestamp: August 16, 2025, 11:52:31 pm
content: Quote from: mikerj on August 16, 2025, 10:17:31 pmThe 28 volt bulb running on the 20 volt supply was still too bright.Quote from: tggzzz on August 16, 2025, 10:38:12 pmThe original #7220 bulb was 18 volts 26 milliamps and had a 200 ohm series resistor.I do not think the bulbs that I bought are incorrect. They are suppose to be 28 volts, and they draw about 24 milliamps like they should at that voltage.Quote from: Doctorandus_P on August 16, 2025, 11:33:21 pmLEDs make poor low current fuses, and worse PTC resistors.

author: wraper
author_rank: Unknown
timestamp: August 17, 2025, 12:23:54 am
content: Quote from: David Hess on August 16, 2025, 11:52:31 pm'PTC fuse' with such low hold current (quite a bit lower than bulb's rated current) pretty much does not serve any realistic purpose other than increasing component count for no good reason.

author: David Hess
author_rank: Super Contributor
timestamp: August 17, 2025, 03:05:33 am
content: Quote from: wraper on August 17, 2025, 12:23:54 amIncandescent lamps work great as either fuses or PTC elements to prevent destructive overload. Examples of where they were used include some high impedance oscilloscope inputs, and multimeter ohms converter circuits. Their low thermal mass allows them to react faster than a PTC.In some cases they can be replaced with one or two high voltage depletion mode MOSFETs and a resistor, but at greater cost. Low current fuses exist, but they are much more expensive.

author: tggzzz
author_rank: Super Contributor
timestamp: August 17, 2025, 08:17:28 am
content: Quote from: David Hess on August 17, 2025, 03:05:33 amHere's an example, in series with a NiCd batteryhttps://www.eevblog.com/forum/testgear/fluke-8125a-military-dmm-teardown/msg2540730/#msg2540730

author: oz2cpu
author_rank: Frequent Contributor
timestamp: August 17, 2025, 09:47:04 am
content: for indication : replace with LED, warm white or even yellow can look almost like a bulb,for oscillators you need bulb, but they dont run warm so they still last forever,IF you really feel you need a bulb for some sort of special indicator,put a resistor in series with them, this will limit turn on currentand this way they last much longer, it will also drop normal run volts a little bitthat is also what you want

author: wraper
author_rank: Unknown
timestamp: August 17, 2025, 10:38:49 am
content: Quote from: David Hess on August 17, 2025, 03:05:33 amIf used in multimeter for protection, a small low voltage bulb like this becomes a liability of itself. It will 'protect' it by burning itself and you'll have a faulty multimeter that needs a repair. As of oscilloscope, I don't see it working well in series with 50 ohm input. As of high impedance oscilloscope input, why in hell do you need it there to begin with? It will do nothing other than screwing up the signal to some extent.

author: Gyro
author_rank: Super Contributor
timestamp: August 17, 2025, 11:53:42 am
content: Quote from: Analog Kid on August 16, 2025, 11:44:03 pmIt will if you optically couple it to an LDR, ie. a Vactrol. There are some extremely low distortion oscillators using those, and they suffer less frequency change amplitude bounce than incandescent lamp stabilized ones(Implementation dependent of course).

author: David Hess
author_rank: Super Contributor
timestamp: August 17, 2025, 07:00:17 pm
content: Quote from: oz2cpu on August 17, 2025, 09:47:04 amThe power indicator on the FG502 is green, so I was going to use a green T-1 LED if necessary, but I would have to order them. (1) The white T-1 LEDs I checked are weird.I did put a resistor in series with the replacement bulb, but it burned out within a couple weeks, with only hours of operation. I suspect the particular bulb I installed was bad in some way, hence my burn-in experiment and my post.(1) There are other reasons to have a small number of various color LED available built on different semiconductor processes; they are handy as temperature compensation diodes.Quote from: wraper on August 17, 2025, 10:38:49 amI would rather replace the bulb, which can still protect multimeter's ohms converter without the bulb failing for moderate overloads, than repair the ohms converter itself.50 ohm inputs require a different sort of protection.Usually high impedance inputs use about 500 kilohms of series resistance bypassed with a capacitor for input protection, so why use a lamp or very low current fuse instead? The bypassed 500 kilohms of series resistance adds considerable low frequency noise, which is covered up by the oscilloscope's high flicker noise. But if the input buffer has low amounts of flicker noise, then the resistive protection circuit ruins the noise performance. A lamp or fuse can be used for input protection in a low noise design, and the fuses are much more expensive and more likely to blow.High impedance voltmeter inputs suffer from the same noise compromise, suggesting that a lamp, fuse, or dual high voltage depletion mode MOSFET could be used for lower noise in place of the bypassed high value resistor. Most digital voltmeter do not require this because their high impedance buffer is already high noise, but some designs might benefit from this.Quote from: Gyro on August 17, 2025, 11:53:42 amAre modern photocells as good as the CdS ones? Their availability is suspect.

author: Gyro
author_rank: Super Contributor
timestamp: August 17, 2025, 07:37:37 pm
content: As far as I can establish, LDRs are still CdS, eg:https://uk.rs-online.com/web/p/colour-light-sensor-ics/9146710?gb=s- Technical Reference section: contains Cadmium. The same with other big distributors*.I don't know how they managed to squeeze through the crack, maybe because there is nothing better than CdS and in the grand scheme of things,volumesquantities are low.*Farnell / Element14 list no less than 6 parts, all CdS (including the ubiquitous ORP12) and all with large volumes in stock.

author: wraper
author_rank: Unknown
timestamp: August 17, 2025, 09:15:55 pm
content: Quote from: David Hess on August 17, 2025, 07:00:17 pmI would rather have a multimeter that does not fail at all if a voltage of magnitude of a mains voltage is applied in ohms range.

author: David Hess
author_rank: Super Contributor
timestamp: August 18, 2025, 02:51:25 am
content: Quote from: wraper on August 17, 2025, 09:15:55 pmSure, if that is a choice. But how is it accomplished?My oldest two multimeters use two different methods. One has a 1/16th amp fuse, which can be had now for $3; on one of these I replaced the blown fuse with these incandescent bulbs because they would work better for less cost. Another has a 3AG fuse sized 120 volts AC incandescent lamp which will sustain a mains level overload indefinitely, old school but very effective.The PTC fuses I checked in that current range have operating voltages below power line levels; I think I have seen modern designs with many of these fuses in series. Power depletion mode MOSFETs could be used, but are also subject to thermal limits so it could take quite a large device.

author: TimFox
author_rank: Super Contributor
timestamp: August 18, 2025, 03:01:17 am
content: Quote from: Gyro on August 17, 2025, 07:37:37 pmIf they ban CdS, will GaAs be next?

author: David Hess
author_rank: Super Contributor
timestamp: August 18, 2025, 12:14:12 pm
content: Quote from: TimFox on August 18, 2025, 03:01:17 amYes, but do not help them by giving them ideas.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: August 18, 2025, 10:26:59 pm
content: What type of support does the filament have?Jeff

author: David Hess
author_rank: Super Contributor
timestamp: August 19, 2025, 04:57:34 am
content: Quote from: Jeff eelcr on August 18, 2025, 10:26:59 pmInside of a T-1 lamp envelope? There is no support that I can see.

author: Gyro
author_rank: Super Contributor
timestamp: August 19, 2025, 08:56:30 am
content: Quote from: Jeff eelcr on August 18, 2025, 10:26:59 pmWell we're all rooting for them.[Hopefully no more failures, David]

author: David Hess
author_rank: Super Contributor
timestamp: August 19, 2025, 11:01:38 am
content: Quote from: Gyro on August 19, 2025, 08:56:30 amI reinstalled the bulb yesterday after about 2 days of continuous burn-in, so far so good.Next time I place a parts order, I will get more bulbs, and some suitable LEDs which can also be used for temperature compensation.