How to silence an EL Wire AC Inverter?
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How to silence an EL Wire AC Inverter?

by abritinthebay on Sun Jan 02, 2011 5:30 am

I love EL Wire to bits, but god damn that squeal is annoying from the inverter.

I've seen the suggestion of using foam tape inside the part of the inverter with the board, but that doesn't seem to help much (it does a little, but not a lot).

Does anyone have any better ideas to silence it, or at least make it a lot quieter? I've been thinking of filling the compartment with something... maybe a resin, epoxy, or some kind of foam but thought heat might be an issue if I did that...

I'm using the AA Inverter that comes with Adafruit's EL Wire kit btw (one day I'll mod it to use LiPo or NiCad batteries, maybe a USB charger...).
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Re: How to silence an EL Wire AC Inverter?

by adafruit on Sun Jan 02, 2011 11:48 am

they are -all- going to be squeaky. The only thing you can do is dampen it by wrapping it in some sort of foam or fabric. usually we have the inverter in a pocket, like the TRON bag and you can't hear it.
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Re: How to silence an EL Wire AC Inverter?

by abritinthebay on Sun Jan 02, 2011 1:15 pm

Right, I expect nothing I can do will shut it up completely. I am being realistic here :)

I'm just trying to work out an optimal way to decrease the squeaking - something that works better than the foam tape.

Do you foresee any problem with filling that cavity? I figure that reducing the chamber acoustics and sound transmission abilities with a spray insulation foam or epoxy would work rather well - if be rather undoable - but I'm concerned about damaging the board or causing problems with heat.

I don't know if the particular unit I have is just rather loud, but even with foam and in a pocket (I'm doing the Tron Bag idea myself) it's still really clearly audible... like passerby "what the hell is that noise?" audible.
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Re: How to silence an EL Wire AC Inverter?

by adafruit on Sun Jan 02, 2011 1:23 pm

it might overheat, not sure. you should make sure its closed all the way, a small crack in the casing will make it very loud
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Re: How to silence an EL Wire AC Inverter?

by abritinthebay on Sun Jan 02, 2011 1:38 pm

Yeah, it's as closed as it's going to get realistically.

I figured I could "pot" it, much like people used to do by hand for guitar pickups (for much the same reason actually). However the traditional way of potting a pickup is easy, but probably not usable for this (basically dunking the whole thing in hot wax on a stove for quite a while so it gets inside everything, then cleaning off the outside).

I guess some kind of thermally conductive epoxy would probably work out ok as long as I wasn't using loads of the stuff... and I really only need to target the transformer as that's the part that's buzzing. I guess I was curious if anyone had done it themselves or had any better ideas :)

I have two of them, so I suppose I can try it and see if I break everything ;)
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Re: How to silence an EL Wire AC Inverter?

by franklin97355 on Sun Jan 02, 2011 4:25 pm

You might try a conformal coating or thinned silicone on the transformer for a start.
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Re: How to silence an EL Wire AC Inverter?

by abritinthebay on Sun Jan 02, 2011 9:21 pm

Right now I've got the entire board part of the inverter basically filled up with Epoxy resin and setting in my banned.

I tried just coating the transformer but that didn't do much; it took the buzzy edge off but no noticeable decrease in volume. So I've gone for the nuclear option. We'll see...

Any improvement will be good though. I'll let you know the results.
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Re: How to silence an EL Wire AC Inverter?

by abritinthebay on Sun Jan 02, 2011 10:40 pm

Conclusion - not really worth it.

The epoxy needed to completely fill the cavity to make any difference, and even then it probably only decreases the volume by 50%. Also it interferes with the action of button which, while it still works, means it's less useful of a solution than I had hoped.

I'd call it a disappointing success, but a success I guess. However one that I wouldn't recommend.

Just coating the transformer in addition to another method might be useful, but I think a better filling material than epoxy could probably be found.
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Re: How to silence an EL Wire AC Inverter?

by abritinthebay on Mon Jan 03, 2011 1:56 am

Video - if anyone is interested in it.

http://vimeo.com/18381790
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Re: How to silence an EL Wire AC Inverter?

by adafruit on Mon Jan 03, 2011 9:22 am

the noise isnt from the inductor, its from the capacitors (we think)
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Re: How to silence an EL Wire AC Inverter?

by westfw on Mon Jan 03, 2011 6:08 pm

You can change the inverter design to do the voltage step-up at a much higher frequency (beyond audio range), similar to the circuit in the ice tube clock, and then you an H-bridge to convert to AC (and provide another effective voltage doubling.) This is the way that the tiny EL backlight driver chips in watches work. But it adds a lot of complexity to the typical boost circuit.
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Re: How to silence an EL Wire AC Inverter?

by stinkbutt on Mon Jan 03, 2011 6:36 pm

Does anyone know of the design for the inverter? Looking at the posted video where the guy "potted" the transformer it doesn't look like a particularly complex circuit: Maybe 8 or 9 parts tops. Working with the design it shouldn't be too terribly complicated to alter the circuit.

Keep in mind, however, I'm not yet saying it's a good idea. I'm just curious. The sound on the Vimeo video sounds to be about 3 KHz. Pushing that out of the audible range means quadrupling or quintupling the frequency. Unless, of course, you're under the age of 25. Then you're screwed, and you're talking about eight or nine times the frequency.

Is it possible? Sure, I guess, but what impact will that have on the efficiency of the driver? At 25 KHz the transistor's delay, rise, and fall times might start to become nontrivial. The 2N3904, which has a total rise-fall on the order of about 125 ns, (8 MHz) would be fine. But you're probably not using a 3904, not for that much voltage. The only reason I pulled the 3904 out of my ass is because it's a common transistor. And what if it's not a transistor at all, but some sort of H-bridge in there? What kind of response time do you enjoy for those things? How about when they're Darlingtons? And what if the sound we're hearing isn't the frequency of the inverter, but a harmonic of the frequency of the inverter that the capacitors resonate at?

None of those questions can be adequately answered without the schematic for the inverter. I'm a little surprised they don't already have it somewhere on the web site, what with this being an open-source joint. But no big deal, somewhere out there there's got to be a part number & a datasheet for the thing.
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Re: How to silence an EL Wire AC Inverter?

by John_NY on Wed Jan 05, 2011 3:54 pm

SB,
If you've opened up the inverter you can see that it's not an Adafruit product, and not from any high-end PCB fab. The PCB fabrication technology looks pretty cheap (it still works), though the price is nice and the enclosure is great. I like the AA batteries instead of a 9V that I've seen in another EL wire inverter.
I would definitely be in the market for a PCB upgrade though, since the noise is not trivial. If anyone knows enough about these circuits to make a schematic/board layout that would be interchangeable with the original PCB it'd be worth a few extra bucks for that upgrade. It'd be nice to have an option for a more expensive but "quiet" (high frequency) inverter, along side the cheaper 3kHz option.
Since there is a multi-function button on the circuit, I'm guessing there's a microcontroller in there, which makes things a little more complicated, right? I've seen an alternate design that uses a 3-position slide switch, which might remove the need for a microcontroller "brain", but I don't have access to it to take some pics. I'll post some pics if I can get them of the 9V/slide switch inverter PCB.
-John
EDIT: It looks like SparkFun also has a 3.3V EL inverter (different design, no battery enclosure) that according to a few commentators is 4kHz. So about the same frequency, same input voltage. I hoped for a schematic, but no luck: I'm guessing it's also a COTS part. I'm guessing it probably sounds the same.
EDIT2: This is another discussion of EL Inverter theory: http://el-wire.tribe.net/thread/293c400 ... ba9132e3bc . I see from the thread that westfw knows a bit about EL wire -- changing the frequency might be undesirable if it makes the EL wire dimmer.
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Re: How to silence an EL Wire AC Inverter?

by stinkbutt on Wed Jan 05, 2011 5:01 pm

Westfw - You've been stepping out on us, you scamp!

Yeah, these inverter circuits appear to be fairly uncomplicated. I'd be pretty surprised if the IC underneath is a true microcontroller. Seems more likely it's one of the inverter controller IC's, just one with adjustable boost and frequency. Or an appropriate native boost and frequency.

Can you post a nice big image of it on both sides? Might be possible to figure out how it works even with the IC being a black box.

Err, black blob.
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Re: How to silence an EL Wire AC Inverter?

by westfw on Wed Jan 05, 2011 7:35 pm

I did some experiments with some of the EL inverter chips a while ago. They're pretty awful when it comes to driving EL wire. A typical EL panel wants something like 100Vp-p at 400Hz, while EL wire is happier in the 300Vp-p at 2kHz range. The chips output the former, and aren't very adjustable. Most of the inverters I've taken apart have simple analog step-up circuits that self-resonates at the appropriate frequency and have output voltage determined by the transformer turns ratio. If they have any chips, they do things like gate the dumb inverter on/off or handle sequencing of multiple outputs.

I have a circuit on paper (one of the many needing "a round tuit") that adds microcontorller smarts, something like:
Image
(You can judge the age by the use of the attiny11 micro... Sigh.)

The boost section would operate at high frequency (not audible), and the H-bridge section could operate at SW-controlled frequency to vary brightness. Multiple channels could be sequenced (at independently controllable brightness levels) by duplicating the h-bridge section (the original idea behind the circuit was to be able to vary the brightness of an EL wire piece, which I believed difficult to do with ordinary PWM since AC was involved. Sparkfun seems to have poked holes in that theory!)
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