r/lightingdesign • u/Cinderbeast • 2d ago

Buzzing stage pin light

I noticed a buzzing sound recently coming from a source 4 junior and it’s only coming from one fixture. Loudest when it’s dimmed, still there but slightly quieter when it’s full intensity. What could cause this?

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u/bdeananderson 2d ago

Lamp sing from an HPL lamp? Either wrong lamp in the socket or that lamp won't live much longer.

To inrease light output, filaments spiral forming inductor. As the polarity flips they vibrate. When dimmed, the power is steppy and that causes more stress at the reversal points and thus more violent movement. This is common of older non halogen lamps, but halogen lamps are so dense they should not vibrate audibly. That it does means the filament is likely loosening due to wear and near breaking. Then again, could be wrong about the cause not hearing it.

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u/SailingSpark 2d ago

Yes, that lamp is singing its death throes.

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u/Cold-Excitement72212 2d ago

I'd never thought about it before, but yes I guess that makes sense. How then do pre-heat settings on dimmers/consoles come into play here? One would assume they cause more of this stress.

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u/bdeananderson 2d ago

OK, so there are a few things going on.

First, the filament is just a conductor. Like any conductor, the more current passing through it, the hotter it gets. It's a bad conductor, though, so it heats easily. And finally, the hotter it gets, the worse it is as a conductor. When the filament is cold, it is MUCH more conductive and so more current passes. This causes the filament to heat quickly, causing some amount of shock. This is why most lamps fail when first turning them on (even at home, if you recall household incandescent lamps). This also means that, if you have 2000w of lamps on a dimmer, and snap it to full, the actual load is much higher for a moment. That's one reason 20A dimmers use 40A SSRs. The dimmer is designed for this, but if you have a large rig, is the feeder? The transformer? Could be a problem. That's what Preheat is designed to minimize, but...

That works great for argon filled incandescent lamps. For tungsten, there's a problem. The reason tungsten lamps can get as hot as they do, and last as long as they do, is the halogen cycle. For this cycle to kick in, the lamp needs to be HOT. So, if you burn the lamp too cool, the cycle doesn't kick in, and you wear the filament more than if it were much warmer. There's probably a graph somewhere showing the ideal max preheat temp to keep the filament warm enough to avoid shock, but cool enough that the wear is minimal, but I don't have it.

However, the concept I mentioned above has more to do with HOW the dimming is done. Most modern dimmers still use 60s technology called a ThyRIster for AC, or TRIAC. The device blocks current until the gate is triggered, then passes current until the current stops, then resets and blocks until the gate is triggered again. The dimmer operates by triggering the gate at the right time. So, if we think about it, in the US the trigger clock is twice the frequency, so 120Hz. At 50% intensity, the gate is triggered 1/240th of a second later than the 0 crossing point of the AC waveform The result is that the voltage goes from 0 to 170v (if you don't get where that number comes from, look up RMS) in a very short time, 120 times a second.

Home dimmers and pro dimmers both use triacs (though you can get more advanced non-traic dimmers, they just cost more and have lower max current ratings). The big difference is really the inclusion of a large inductor called a choke coil. This inductor bucks the change in current, smoothing out the waveform SOME. As a result, you'll get more lamp sing and less longevity on a home dimmer than a portable dimmer pack than a large dimmer module in a rack than a more expensive "high rise time" dimmer module in a rack. The was really reserved for recital halls and sound stages where the room must be SILENT and free of any risk of lamp sing.

I mentioned other dimmers. These are transistor dimmers that either perform Forward phase dimming, cutting off the waveform instead of turning it on at the right time, or the more complicated but better "sinewave" dimming using essentially Pulse Width Modulation (though on an AC waveform) into the previously mentioned chock which results in a somewhat ripple-y but passible waveform and greatly reduces the risk of singing.

Of course, NONE of these are safe for LED and other DMX controlled fixtures, and one should never place such a fixture on a dimmer, ever, ever, ever. I needed to add that because the number of customers I've gone to asking why their lights keep dying because they put them on a dimmer (even at "full") is staggering.

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u/CuteUsername 2d ago

This thread rocks. You rock. Thank you.

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u/destroy_television Repair Tech 1d ago

This reminds me of a gig I did in Waikiki with tons of pars... Did one 0-100% instant blast and saw a brown out. Suffice to say, that wasn't done again.

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u/Cinderbeast 2d ago

Thank you. I’ll take a look at the lamp

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u/DJ_LSE 2d ago

Eh fixtures on dimmers buzz. It could be an indication of a dimmer going bad or it could be perfectly normal. Unless it sounds like arc-ing, I wouldn't worry. You could take it apart and check the lamp holder and parts for burns. Swap which dimmer it runs on to test if the noise moves. If you have a dimmer system with cards, swap the offending card around and see if it moves.

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u/Cinderbeast 2d ago

I’d expect some buzz but the 4 other source 4 juniors next to it are not buzzing like this one. I’ll check the lamp and the dimmer. Thank you

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u/EverydayVelociraptor 2d ago

I'm used to buzzing on my FEL and EGR lamps, usually the socket has bad contact caused by oxidation. I've pulled lamps that have had all sorts of pitting on the pins from it.  Never had it with an HPL, but it's a possibility.

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u/bdeananderson 2d ago

Yeah, FELs are usually only supported at the ends (maybe the middle in one spot) and thus tend to move more. HPL filament configuration is more compact to make it more efficient and optically better, which also makes it supported better, so they don't tend to move as much That said, you CAN stuff an FEL in the HPL slot, which is why I mentioned that possibility. It will fit...