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Thread: What is "coil whine" and what is that goopy stuff all over the inside of my PSU

  1. #11
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    On a possibly related note, I sometimes see electrolytic capacitors mounted up high with legs showing, while I tend to solder replacements close to the board so they can't move around much and don't use goo for support.

    One might argue that a capacitor with short legs can get rid of heat to the board more easily, although I do it just to be tidy; by the same reasoning goo will tend to hinder a capacitors ability to cool in the air flow.
    Last edited by ashiekh; 06-09-2019 at 12:02 PM.

  2. #12
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    In a lot of circuits, the leads must be as close as possible because the leads can affect the circuit.
    The extra wire adds resistance to the circuit and can even add inductance (super small value, but nevertheless it is)

    So let's say you have a capacitor with 0.003 ohm ESR and the long leads add around 0.002 ohm of resistance ...

    In switch mode dc-dc converters, especially the ones running at high frequencies, like 1 Mhz and higher, you want the loop as small as possible... so long leads just increase the distance current flows.
    See for example this dc-dc regulator running at only 600kHz and how they stress short distances between components... page 5 (look at the picture in corner) : https://www.analog.com/media/en/tech...eets/1308i.pdf


    If it's not a high frequency circuit, then leads matter less.

    Sometimes a capacitor sits a bit higher because the distance between holes on the circuit board may be different than the distance between leads on capacitor - it leaves room for using whatever's available, for example 47uF 50v , or 47uF 35v ... when you know the maximum voltage's gonna be 20v or so.

    The gray stuff (somewhat elastic material) is to prevent vibrations and reduce coil whine in inductors. It's also sometimes used simply to reduce the number of errors when soldering components to boards ... for example you have a thin and tall capacitor with leads pre-cut and you insert it on circut board and then the board goes on a conveyor belt in a wave soldering machine and a flow of solder hits the bottom wetting everything and soldering the leads to the circuit board.
    When the wave of solder hits the board it can push up the thin and tall capacitor and make it not solder properly... also the conveyor belt can shake a bit and vibrate and make capacitor get out of the holes (due to the pre-cut leads)

    Sometimes it's also used simply to make sure there's some distance between two components (for fan air to move between the parts) or to "glue" some insulation or shielding material to a board or a transformer or component.

  3. The Following 2 Users Say Thank You to mariush For This Useful Post:

    ashiekh (06-09-2019), Jon Gerow (06-09-2019)

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    mariush is correct. I've had issues when there's poor QC and the bulk cap isn't flush with the PCB.

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    Quote Originally Posted by ashiekh View Post
    I agree that higher harmonics make it easier to hear.

    Just wanted to question
    "50 to 60Hz which is way beyond human's scope of hearing"

    http://onlinetonegenerator.com/

    I can hear 20Hz sine-wave using the above (volume cranked all the way up)
    I think we agree. I think I'm just failing to make my point. For example: If your PWM controller is switching at 15kHz, you don't all of the sudden hear the AC. But you do if it passes through a component like a coil. Yes, you can hear "Miami Bass" at 50Hz, but if you simply hold a power cord in your hand with 50Hz AC, you're not going to "hear the power."

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    Quote Originally Posted by Jon Gerow View Post
    but if you simply hold a power cord in your hand with 50Hz AC, you're not going to "hear the power."

    Agreed.

    But agreeing is so boring; listen to a bread toaster buzz (resistive load).
    Last edited by ashiekh; 06-09-2019 at 07:35 PM.

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    Quote Originally Posted by Jon Gerow View Post

    The goo you see is just "RTV", a type of caulk. You can even buy it from Amazon: https://www.amazon.com/Clear-Electro...dp/B0063U2RT8/

    It's often applied to dampen vibration from alternating current, but is also widely used to keep "heavy" components from pulling off the PCB when the product is in transit so that the solder points aren't the only thing keeping the component attached to the PCB.



    Can I use RTV as lube though?

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    Quote Originally Posted by Orion View Post
    Can I use RTV as lube though?
    Why would you use room temperature vulcanizating silicone as a lubricant?

    I think glue is the exact opposite of lubricant.

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    Quote Originally Posted by Jon Gerow View Post
    The goo you see is just "RTV", a type of caulk. You can even buy it from Amazon: https://www.amazon.com/Clear-Electro...dp/B0063U2RT8/

    What is the shelf life given that it is "moisture- curing"

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    Quote Originally Posted by ashiekh View Post
    What is the shelf life given that it is "moisture- curing"
    Hope that it's on the label?

    Yes... I too have see the video of the guy cleaning his PCB because the RTV had expired and was seeping moisture back out.

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    It's in a sealed package... it can last months, maybe more. But once you open it, it should be used soon.

    Once applied it needs about 15-20 minutes at ambient temperature to skin over (form a hardened layer on top of applied stuff) ... it needs up to 7 days or even more for the process to be complete.

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