Glue on the vents...

[Tom41]

I know that many PSU manufacturers put glue on the components inside, apparently to minimise vibration as the electricity flows through them. Most of the time this glue is around the base of the components - however, I've seen several with the glue on TOP of the capacitors! Often it's like a string of glue from a glue-gun over all the caps, but I've also seen caps completely covered in glue, and even once a small plastic sheet glued to the top of the vents!

Is this anything to worry about in PSUs? Will the glue have sealed the vents and make any failure more violent? Will it make the caps more prone to overheating and other failures?

Here's a picture of the kind of thing I mean in one of my PSUs:

[Tom41]

Re: Glue on the vents...

Sorry for double post, edit time limit expired. Here's the glued plastic sheet vents that I mentioned earlier:

[willawake]

Re: Glue on the vents...

well the inductors could need it. the caps you could definitely remove it. i guess they just tell the guy with the glue gun to get anything standing up

yeah i think the caps more likely to blow their bungs out like that. heating i dunno, if its only partially covered then ok, ripple is less your friend than heat anyway.

[Tom41]

Re: Glue on the vents...

I was talking to some friends on AIM recently, who are into electronics. Apparently the plastic sheet that's glued over the big caps is to provide insulation. The tops of those caps will be at mains voltage, and you wouldn't want to touch them while the PSU is on! Likewise, if something conductive falls into the PSU it may conduct the mains voltage to the chassis or something else, causing a shock hazard.

That's exactly why the caps used in the input section usually have a plastic-covered top, to provide insulation. Using normal caps with exposed metal tops, then sticking a piece of plastic over them is a sign of shoddy PSU design/manufacture.

That at least explains the glued-on plastic on the input caps, but I still can't understand why they put the glue on the caps in the output section on that HP PSU. Those caps are more likely to be at a lower DC voltage (12v or 5v), so shouldn't cause severe shocks if touched.

[Krankshaft]

Re: Glue on the vents...

I think that the reason they glue the secondary caps is because after they are inserted by machine the glue prevents the caps from falling over or out before the wave soldering process.

The glue solidifies them as a unit.

And don't forget some parts on certain boards still need to be inserted by hand before the wave soldering process and if the assembler turns the board over too much there goes all the caps.

Also I have a supply with that plastic cap over the line filters too and its a Bestec power supply that HP uses regularly so I guess its not only cheapies that use that trick.

The problem is they use high voltage radial caps for the input filters if they used radial snap-ins the metal vents of the caps would be covered and this plastic piece wouldn't be needed.

An example of these caps is at this link:

http://www.41hz.com/main.aspx?pageID=153

I always use the Panasonic radial snap-ins in place of the radial line filters that come with the supply I simply bend the snap in leads down clip the old leads off the old cap and solder them onto the radial and the old caps legs back onto the PCB.

[Krankshaft]

Re: Glue on the vents...

Sorry about the second post that 10 minute edit thing is annoying.

Anyways the above trick works great on LCD inverter supplies where space is limited and the line filters are ALWAYS bent on their side.

I also have not seen a single LCD inverter where the line filter cap hasn't had glue under it to hold it down.

Which strengthens my theory that its purpose is to keep the caps from falling off.

As for the glue reducing the caps ability to vent that all depends on the melting point and the strength of the glue to resist stretching and pressure.

[gonzo0815]

Re: Glue on the vents...

Most of this kind of glue is not very hard or strong. So i would guess, that the material at the vent will certainly still rupture.
The glue can?t seal the cap, if even the tiniest whole is in there. Hydrogen isn`t easily sealed away.

May be the pressure would become a little higher, but probably not that significant.

I see only a problem with very strong glue like epoxy or similar.