Noisy power circuit

[Exie]

Hi,

I just picked up a new gadget the other day, and it came with a small power adaptor. When I fired it up, the adaptor makes a terrible high pitch squeeling sound.

I presume this is because its a cheapie. So I proceeded to dismantle the adaptor and see if there was anything I could replace. Inside I found a very small & simple circuit with the following:

1 x NPN Transistor (D882-P)
1 x 50v 4.7uf capacitor
2 x resistor
1 x small orange thing (capaciitor ?)
1 x Transformer

Any idea what might be creating the noise ?

most of the componants are un-labeled, but it takes 12v DC in and by my amature multi-meter skills, it seems to output about 2.3mv

P.S. I've attached a photo for what its worth.

 

[Dr.EM]

The small orange thing is a ceramic capacitor. I'm assuming this is a high frequency, low current DC to AC convertor. Its possible a transformer winding is loose and is causing the noise.

 

[Exie]

Hmm .... it could be. It powers a small electro-luminecent cable inside my PC actually.

So, if I went down to my local electronics shop, how would I know what to ask for ?

The transformer has 5 pegs ?!? but I know nothing of its properties.

My multimeter has a setting for "V~" and "A~" but it seems to register about the same under both, so it maybe AC but I dont really know what I'm doing.
For what its worth, my multi-meter is a M9508 exactly as seen here:
https://www.jameco.com/webapp/wcs/s...toreId=10001&catalogId=10001&productId=220791

I've never measured frequency before but for what its worth, I set the multimeter to "F" which has a scale of "2n" and tested the output and it registered about 0.034nF

Any tips to help identify what sort of Transformer I should get ?

... or this may be a poorly designed circuit .... maybe someone could suggest a new circuit to power the light and I'll get all the components and start from scratch ?

 

[Dr.EM]

Ah, yes, its an inverter then. It will output around 1200v AC for your cold cathode tubes. 5 pegs is common, it indicates either the primary or secondary has tapped windings. I presuming the primary and this uses the same system as a simple camera flash charger. However, with this prticular problem, its probably cheaper to buy a new unit than replace the transformer as that will be the bulk of the cost of it. If its faulty, the supplier should replace it. You 2n setting on the multimeter is probably for capacitance, 2 nano farads.

 

[Edmond]

The transformer winds and the core have got a vibration, and they make a hearable noise.

Hi,

I just picked up a new gadget the other day, and it came with a small power adaptor. When I fired it up, the adaptor makes a terrible high pitch squeeling sound.

 

[Oznog]

Electroluminescent wire actually often makes noise itself when powered. It requires AC in the audio range to function. The surfaces often function as a speaker.

 

[Exie]

Thanks folks!

I just fired it up, and found cupping my hands around the inverter module made the sound go away. So I'm still convinced its the power module and not the cable make the noise.

I'd still like to have a shot at fixing it if possible. As i brought it via mail order, it will cost me a bit to post it back under warranty etc.

If I wander down to my local electonics shop tomorrow and ask them about a 12v to 1200v transformer(inverter) will they laugh at me ?

I have the following shops handy:
radioparts.com.au
jaycar.com.au
dicksmith.com.au

... unless anyone else has any suggestions to source one ?

 

[Nigel Goodwin]

If I wander down to my local electonics shop tomorrow and ask them about a 12v to 1200v transformer(inverter) will they laugh at me ?

I'm afraid so, it's NOT a standard transformer, but specific to that one application.

 

[justDIY]

try some potting

this will be a fairly permanant solution... find or build a little box or containter that is larger than your gizmo. line the container with wax paper or plastic-wrap. fill it part way with clear or black epoxy glue, fiberglass resin, polyester resin, jb weld, wood filler, whatever... let that layer firm up for 15 min to an hour (check your instructions for "set time") ... then lay in your power circuit, and fill the rest of the way with epoxy... let it sit someplace for a few days, and then pop it out.

the potting should help to absorb any noise from the windings, and it may even have stopped them from oscillating, audibly at least.

 

[Exie]

Thats a top idea!

The inverter actually came in a small plastic box, so I could just pop it back in and "flood fill" it with resin.

Should I give any consideration to heat ? without any airflow, it might get hot and burn out ? <shrug> .... but thats a very doable solution.

 

[zevon8]

While I agree that potting it would likely help with the noise, 2 problems can arise, both fatal to the device.

Firstly, most epoxies will shrink as they cure. Maybe not much, but enough to be an all-encompassing boa constrictor on the electronics. Think cracked resistors, shattered capacitors, fractured IC's. Not good. There are specific potting compounds designed for doing this, many are urethanes, they remain slightly flexible after cure. Bringing up another point... thermal expansion. When the parts warm up, and can't expand...snap crackle and pop.

The second point, you mentioned also, heat. This device was designed to operate with natural convection to keep itself cool. Not gonna happen very well once it is intombed ( pun intended )

Therefore, if you want to pot this thing, get the proper material ( BTW, JB Weld is likely conductive, being metal filled and all )

One simple fix may be to get some varnish, apply a thin coat to the switching transformer. If nothing else you may change the mechanical resonance enough you don't hear it.

 

[Exie]

Good thoughts.

I was down at my local hardware store during lunch, and saw all the apoxy's and fibreglass resin's there. I dont know about the varnish idea. I dont think a thin layer of anything would really help (unless I did like 50 coats).

What I hadnt thought of was a silcon based filler. It sets kinda rubbery, doesnt conduct. It's flexible, allowing for heat expansion.

The only real questions is .... would it cook itself ?

I think it might be worth the experiment. I'll try it out tonight and post back if it works (and then if its still working in 1 week).

 

[justDIY]

the inverter (thats what you have) shouldn't even get warm, let alone hot, unless you're severely overloading it somehow.

my inverter, which also arrived sealed in a plastic box (no ventilation what so ever) does not feel any warmer than the surroundings, even after running for a few hours. mine is using a TO92 transistor, your transistor looks a bit different, perhaps yours is higher power?

if you want to test things, to see if it will overheat, try potting the unit with paraffin wax ... if there's some sort of problem and the unit heats up, it will free itself of the wax fairly easily

I agree with zevon that shrinkage would be a concern, but I don't think in this particular instance you have to worry much about it, these things are pretty rugged and assembled with all through-hole construction. If it were a bunch of tiny SMD components, it would be a different story.

I suppose if shrinkage is a terrible concern, encapsulate the unit first with silicone caulk, and then pot it using a hard epoxy ... the caulk will act as a stress barrier protecting anything from being crushed by the epoxy.

I'm not sure about the jbweld, other than what I can buy locally, but I read an open circuit checking its resistance with a DMM - I suppose it could be a different story with high voltage / high frequency involved.

 

[Exie]

Ok Folks,

I flood-filled it with Silicone, the silicon has set to a soft rubbery texture. So I plugged the unit into my 12v PC power supply and sure enough, it still makes a racket!

I'm not sure if its any quieter than before, but its still too audible for me to leave it running without getting a headache.

I did email the shop I brought it from who responded with the following:
"Unfortunately all EL inverters will emit an electrical noise (we've tried
several manufacturers). The best recommendation is to wrap a piece of cloth
around the unit to dampen the noise, while still allowing some airflow to
the vent holes."

So it looks like all cheapo asian-made EL gear like this is crap essetially. Which is a real bummer, because simple lights (not going overboard) can look really nice and allmost smooth & classy. I guess with most things, you get what you pay for, and I suppose no-one is prepared to pay for good EL lights in their PC's.

Huge thanks to all the respondants for the great ideas. Glad I found these forums, they're great!

 

[Hero999]

My multimeter has a setting for "V~" and "A~" but it seems to register about the same under both, so it maybe AC

Probably because it's AC, and the frequency is too high for your meter to measure.

but I dont really know what I'm doing.

Maybe you should leave it alone then.

I've never measured frequency before but for what its worth, I set the multimeter to "F" which has a scale of "2n" and tested the output and it registered about 0.034nF

That F setting on your meter isn't an F for frequency, it's an F for farrads, it measures capacitance not frequency, the frequency setting is normally marked as Hz, I suggest you try that.

... or this may be a poorly designed circuit .... maybe someone could suggest a new circuit to power the light and I'll get all the components and start from scratch ?

I think all electro-luminecent stuff needs an audio frequency to work.

How old are you?

Do you have good hearing?

It could be producing near-ultrasound (above 15kHz) which only younger people (under 20s) can hear.