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Thread: PSU heatsinks electrified?

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    Default PSU heatsinks electrified?

    Just replaced a few caps on a PS for a friend. Tapped the two heatsinks with my fingers to feel for heat and got nailed. Luckily it was the same hand on each so didn't cross my body.

    I had no idea those are electrified.

    Screenshot at Mar 08 20-36-58.jpg

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    They're not supposed to be. Heatsinks are grounded. If they're energized, there's something wrong with the PSU.

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    Quote Originally Posted by jonnyGURU View Post
    They're not supposed to be. Heatsinks are grounded. If they're energized, there's something wrong with the PSU.
    Ähm, I've yet to see a Heatsink that is not charged...
    So I'd rather not touch them for that reason...

    I don't want to open the HXi and prove it but I remember measuring the voltage on some HEC units, wich was up to rectified AC (=325V) on one of the Heatsinks...
    The PFC one was around 240V or so...
    Only the Secondary heatsinkis safe...

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    Quote Originally Posted by Stefan Payne View Post
    Ähm, I've yet to see a Heatsink that is not charged...
    So I'd rather not touch them for that reason...

    I don't want to open the HXi and prove it but I remember measuring the voltage on some HEC units, wich was up to rectified AC (=325V) on one of the Heatsinks...
    The PFC one was around 240V or so...
    Only the Secondary heatsinkis safe...
    You should not feel energy on any heatsink unless something is shorted to the heatsink.

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    Quote Originally Posted by jonnyGURU View Post
    You should not feel energy on any heatsink unless something is shorted to the heatsink.
    https://www.electronicspoint.com/for...is-live.61062/

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    Quote Originally Posted by jonnyGURU View Post
    They're not supposed to be. Heatsinks are grounded. If they're energized, there's something wrong with the PSU.
    Quote Originally Posted by jonnyGURU View Post
    You should not feel energy on any heatsink unless something is shorted to the heatsink.
    This is completely wrong and actually dangerous information on a forum like this.
    Primary side heatsinks are almost always live!

    More info on this if you want to learn:
    https://www.badcaps.net/forum/showth...622#post831622
    "The one who says it cannot be done should never interrupt the one who is doing it."

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    primary heatsink always connected with (-) 380VDC

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    So after capacitor replacement, the power supply is working great. To be clear on getting shocked. With the same hand, two fingers, each finger tapped on a heatsink and that's what did it.

    I would have thought these weren't energized either and that there was something wrong. I almost threw it right in the trash but it's working perfectly after being re-assembled.

    Measured -165v DC and 123v AC across those heatsinks. The secondary side heatsink acts like a ground. Same result when probing black lead to the chassis.
    Attached Images Attached Images
    Last edited by jayjr1105; 03-09-2019 at 10:09 AM.

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    Modern power supplies should not have voltage on the heatsinks. I had older designs that had high voltage on the main heatsink.

    These days most power supplies have mica or other insulators between each to220 (or similar package) and the heatsink, or they use fully insulated ICs (in plastic, epoxy, whatever). With mica insulations rarely they break down, they can get tiny holes in them and the heatsink can become life.
    Or maybe the manufacturer was stupid and used thermal paste AND insulator and maybe somehow the paste became conductive over time due to heat cycles, drying out etc

    The second heatsink should not have high voltages since it's on the secondary side. It's really uncommon but I suppose it may be possible to see some voltage as side effect of that Y class capacitor coupling the high voltage side and the low voltage side but the current should be really low.

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    Quote Originally Posted by Per Hansson View Post
    This is completely wrong and actually dangerous information on a forum like this.
    Primary side heatsinks are almost always live!

    More info on this if you want to learn:
    https://www.badcaps.net/forum/showth...622#post831622

    Also scary is how close they are to the grounded shell

    live.jpg

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