Incandescent Lamp Resonance

I've got to come up with a new "automotive" (12 volt automotive type system) lamp dimmer to replace the old linear design that's been on the market for a very long time. I'd like to do PWM but I have also seen some PWM designs where the filament in a small "dial light" type incandescent bulb actually gets to swinging back and forth, rapidly decreasing the life of the bulb.

Anybody got data or experience what the lowest frequency is that can be used without this filament resonance phenomena?

Jim

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RST Engineering (jw)
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You can use any frequency you like if you include a large enough inductor and/or capacitor to smooth out the ripple. But I'm guessing that this is supposed to cost a few cents per unit, so you want minimum component count.

Ultimately, the frequency depends upon the thermal time constant of the filament. If this isn't in the datasheet, measure it with a photodiode and an oscilloscope.

But why aren't they just using LEDs instead of filament bulbs? Is selling replacement bulbs at extreme markup part of the business model?

Reply to
Nobody

Because to the FAA, replacing anything that doesn't use 1930s technology is way too far advanced to be safe.

Jim

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Reply to
RST Engineering (jw)

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No, I don't have direct data on the resonance, but I'd wager you'll find this site well worth your while:

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HTH, James Arthur

Reply to
dagmargoodboat

True, although in the case of automotice headlights, I don't think any LED manufacturer has anything that comes close to the 50W+ power required in anything close to the

Reply to
Joel Koltner

He operates the PWM switch at 250 Hz.. That's fine if you don't have to contend with audio getting back onto the battery line. In this case, the battery is used not only for lights but for general audio amplification in the vehicle. I'd probably raise cain with somebody's audio switching panel or intercom if I choose a frequency that is audible.

Above or below is fine, but in the human auditory band, not so very good. I'll probably put it up at 20 kHz. or so and put a low pass at both input and output. I had considered putting it way down around 30 Hz. but for interference with navigation gear that operates with a 30 Hz. phase sensitive signal. Much lower than that and you get into the 24 Hz. eye flicker response.

Jim

Reply to
RST Engineering (jw)

On a sunny day (Wed, 25 Mar 2009 11:19:20 -0700) it happened "RST Engineering \\(jw\\)" wrote in :

My SEPIC with PIC drive works great with car headlights,

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have not seen _any_ vibration of the filament. I have tried 19 kHz as minimum, it is a hysteretic controller, so it may occasionally pulse only ever so often... The heat in the filament takes care of that... it cools too slow to be a problem.

Reply to
Jan Panteltje

Headlights are irrelevant here. OP was talking about dimming stuff like dashboard lights.

At least I hope that's what it was about! ;-)

Cheers! Rich

Reply to
Rich Grise

Where did you ever get this idea? Please tell us where "I have also seen some PWM designs where the filament in a small "dial light" type incandescent bulb actually gets to swinging back and forth,...".

As far as I know, the freq. of PWM for incandescents is irrelevant, primarily because, to make a filament vibrate, you'd need a magnetic field through the bulb, a la those candle-emulating Xmas bulbs that lasted about a week. ;-)

Cheers! Rich

Reply to
Rich Grise

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Sure. Your question was what the lowest possible usable frequency was. There, at that website, you have an example application at 250 Hz.

It's used commercially in flashlights, and especially by spelunkers. They're very demanding users--it must be reliable.

Willie Hunt further dishes on the life-extending advantages of soft-starting the filament, improving reliablity even more.

I can't imagine PWM at 20KHz causing resonance problems; the mass, length, and construction of the lamp filaments I have in mind would preclude that.

I s'pose if you want to know for sure, get a lamp and try it.

Even better: convince those neaderthals at the FAA to certify LEDs.

Cheers, James Arthur

Reply to
dagmargoodboat

Ah, my mistake, sorry about that.

Reply to
Joel Koltner

Because I've written about a dozen magazine articles on dimmers and for SOME PWM frequencies at SOME pulse widths for SOME lamps the filaments heat and expand, then cool and contract, then heat and expand ...

With some filament construction (mostly those rather small ones that don't have a center "hangar" for the filament) this expansion and contraction gets the filament to swinging about merrily.

I was just hoping that somebody had done their master's thesis on it and would have a better handle on it rather than my admittedly unscientific observations.

Jim

Reply to
RST Engineering (jw)

Jim, these have existed for many years, and looking at prior art would show you what is available. Why re-invent the wheel? As to your physically swinging lamp filament, two issues: one is that the frequency is simply too low, and the other is why use an incandescent lamp at all, use LEDs and have a better, more reliable, adn probably cheaper product.

Reply to
PeterD

I would think that as you vary the pu;se width, you should be able to hear the resonance mechanically coupled through the base. I know that my family room light dimmer causes the bulb to be quite audible at certain points. I would guess the bulb would fail early if run a long time at that mode.

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RFI-EMI-GUY

I measured the instrument illumination PWM frequency in my Toyota at 400 Hz. Nothing comes over the sound system.

I think auto sound systems' power supplies are filtered pretty well so that they'll reject the variable frequency ripple produced by an alternator. Plus all the commutated motors and other crap cars tend to run.

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Paul Hovnanian P.E.

How about the equivalent of spread spectrum; (semi) random PW and PRF?

Reply to
Robert Baer

A long long time ago when the 2N3055 first appeared I made a 12V 5A PWM operating at 500Hz for my car. It could control heater fan speed, instrument lights, etc., and for an experiment I connected it to a single 55W fog lamp. It was auduible from several feet away at everything except zero and full power.

Graham H

Reply to
Holloway,Graham (UK)

You have to distinguish 2 basic things:

1) You have a case without feedback, so a simple PWM. In this case there likely will be no oscillations if the PWM frequency is much higher then the thermal constant of the smallest light bulb. 2) You have feedback. In this case, if you use a PID scheme, there may be a instability when the time constants are close to those of the light bulbs used.

As load (type of bulbs switched on at any time) is highly variable, the PID approach is not recommended. I have seen instability experimenting with light bulbs and PID, or even low pass filters in a hysteretic setup.

As light bulbs have a resistance that goes to up to 10x the value when hot, there is plenty of room for instability. Also this _absolutely_ requires a current limiting circuit for startup, and slow start, slowly increasing the pulse width at startup, is also nice,

So you want no filters (time constants ) in your feedback path, leaving only hysteretic controllers if you _must_ have feedback.

Finally there is the harmonic issue. I do not know what your EMC requirements are, a simple PWM needs no inductors... But maybe some filtering would be in order. RF leaks from the most unlikely places, it amazes me every time. Yesterday I put my cellphone with bluetooth in a metal drawer in a metal cabinet, while listing to mp3 on it via a bluetooth headset, it simple kept working. The range was still a few meters! There are only narrow slots between the drawers and the cabinet... waveguides is weird stuff! An that is about 2.4 GHz IIRC. So you may need to filter.

Reply to
panteltje

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to op Mr RST..

above 20 kHz is fine but if you go lower be aware that the eye flicker response is most sensitive when the object is moving. Some new car LED tail lights when dimmed, if i look right at them I do not see any flicker. But when the car is moving past me, it clearly leaves a dotted line trail in my vision and is VERY distracting. This can also be a problem in LED displays that are multiplexed and viewed under vibration. Keep the freq above a few kHz. I agree audio interferene can be an issue so you are up to 20 kHz.

Mark

Reply to
makolber

[snip]

Audible through your car's sound system? You need better filters in the power supply.

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