PSU resin - why?

[maverick77_uk]

Hi all,

Just been reading some posts about PSUs, and that reminded me about something I've been meaning to ask.

Quite commonly when I look through the grill or fan of PSUs, there's some sort of solid resin almost randomly splatted over the circuit board and / or components. Upon prodding in the past, its solid - almost like someone has got a load of aryldite and splatted it on the board for some reason. When I first saw this, I assumed it was a leaky capacitor or something, but most PSUs (and some other components) I've looked at have it. All makes / grades old and new.

Is this something to do with cooling, where in manufacture they've been a bit too liberal with something? Or to do with integrity, ie if you started changing components yourself within the PSU, you'd have to break this resin stuff off the circuit board, so they would know its been tampered with?

Was just wondering really!

Cheers!

[DDY]

Hot melt glue? It's usually used to keep components stuck down to the PCB and prevent removable components like DIP chips and screws from falling loose or removed by the user. Occasionally I've seen it used to act as electrical insulation.

In many power supplies, even expensive ones the stuff is applied very liberally, if you've ever used a glue gun you'd know the goo continues to dispense even when the trigger is released. Strings of solid glue can be seen strewn over some PSUs as the dispenser is moved across the board as the glue sets.

The more expensive PSUs tend to use a cement like material in addition or instead of hot melt glue for the same purpose, it usually looks neater but I can imagine it being used too generously some times. It doesn't do cooling any favours, but also doesn't impede it too much either.

I guess the stuff does act like a tamper seal if there isn't a sticker on the case already.

[TAKTAK]

It reduces the coil whine of the coils, doesn't reduce it fully in some circumstances, but it does help

You get coil whine due to the mechanical resonance (http://en.wikipedia.org/wiki/Mechanical_resonance) of said coil, due to the frequency of the signal passing through said coil, if said freq' is in resonance (http://en.wikipedia.org/wiki/Resonance) to the coil structure it's self then it can cause the structure to oscillate, which in turn causes the (in the coils case) high pitched noise, usually too high for the human ear to hear >20000Hz, meaning that most of the time we don't hear it, but environmental factors can affect the resonant frequency of the coil, and a good way to reduce the effect of the resonance is by covering the coil in something, i.e. glue or wax, this reduces the effect significantly by altering the resonant frequency of the coil structure.

Obv that is just resonance for the coils, but you get resonance in everything, and it has the be taken into account when building structures, especially when building structures that come into contact with water (there was a good program on [i think] an oil rig that had specially designed sections to drastically alter the resonant frequency of the structure.

It's the same thing that happens with glasses, if you play the correct harmonic then the glass with start to reverberate and also in most cases emit a sound, until it eventually contorts too much and shatters, if however you hold a pencil against the glass, the the resonant frequency is changed, so the same note no longer has the effect (another note would however...)

edit: glass shattering due to resonance: http://www.acoustics.salford.ac.uk/f...wine1video.htm

[Sputnik]

It can also stop vibration of things like toroidal inductors from fracturing the solder joints at the board.

Edit: beaten to it by tak tak (who types like a ninja)

[peterb]

If it what I think it is from your description, it is called a conformal coating and aopart from anchoing components like torroidal transformer to the cct board - and reducing vibration and increasing mechanical strength, it also improves the dielectric strength of the board, reducing the risk of leqakage current and flashover. It is an insulator and provides some degree of protection in that area, and also reduces the risk of corrosion to the tracks.

[maverick77_uk]

Thanks all. All that seems to make sense! I won't worry about it now then! Just seemed weird that all these high-tec micro circuits just had a load of solid slime whacked over them!