Why are these inductors covered with white stuff?
- Is this to reduce EMI/EMC?
- To reduce of unwanted noise?
Whatever it is, please explain it.
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4 Answers
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The windings on the inductors can move just a tiny bit, especially if there's a lot of current flowing through the coil. The magnetic field generated by the current flowing in the coil makes the wires move. If the current varies at a frequency that the ear can hear, then the coils will "whine" - they make an audible noise.
The white stuff is to deaden the mechanical vibration from the inductors.
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It is to prevent the coils from oscillating (mechanically). You may know that from GPUs "coil whine"
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\$\begingroup\$ Is it just glue? \$\endgroup\$ Commented Oct 27, 2023 at 8:23
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1\$\begingroup\$ Did you mean "PSU"? A GPU is a microchip, it has no coils (though coils may be present on a graphics card). \$\endgroup\$– SneftelCommented Oct 27, 2023 at 15:59
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1\$\begingroup\$ @EndAnti-SemiticHate some sort of non-conductive silicone or resin normally \$\endgroup\$ Commented Oct 27, 2023 at 16:19
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Check how the coils are fastened to the board. They can be too heavy to be held only by a couple of thin soldered wires. I have seen how parts vibrate themselves out of the circuit when there's nothing else keeping them fastened than soldered straight wires in the holes of the PCB. The vibrations were caused by moving the device daily on a car seat, not even in the trunk.
In audio speakers, vehicles and machines the normal operation can be too violent for poorly fastened parts. Coils can vibrate also due the magnetic forces caused by the pulse currents.
You probably see in the future many devices which have parts which are glued to the circuit board with a big splash of white glue.
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Officially, the white stuff is called "Potting Compound" and is intended to physically prevent things moving, and/or protect them from being tampered with.
Other answers have explained this specific case, but you will also come across it on many other (mainly quite old) circuit boards with discrete components, particularly where large/heavy items might get shaken loose, eg capacitors on a portable radio.
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2\$\begingroup\$ Is that really the same as potting compound? My understanding is that potting compound is quite thin before curing (so it can thoroughly fill in the gaps being potted), but that would make it less useful as a spot-applied adhesive. \$\endgroup\$– SneftelCommented Oct 27, 2023 at 16:02
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\$\begingroup\$ @Sneftel AFTER curing, potting compound gets fairly solid, and this stops things moving - more 'filler' than 'adhesive'. \$\endgroup\$– MikeBCommented Oct 30, 2023 at 9:50
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1\$\begingroup\$ Thanks, I’m aware of how curing works. I was referring to the pre-curing viscosity, which (I would think) would be too low with potting compound to partially ‘bury’ the component on the flat board surface without flowing away. \$\endgroup\$– SneftelCommented Oct 30, 2023 at 11:35
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1\$\begingroup\$ Wouldn’t the only expected circumstance be “potting a circuit”? I can’t really see why certain PCBs would require or benefit from something other than a low viscosity while being potted. And I can’t see why that viscosity would also be optimal or even reasonable for things other than potting. \$\endgroup\$– SneftelCommented Oct 30, 2023 at 20:11
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1\$\begingroup\$ I've always thought that potting compound was the stuff that they poured into potted elements. I would call that white stuff Silastic. \$\endgroup\$– EyalCommented Oct 31, 2023 at 13:25