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Thread: PSU heatsinks electrified?

  1. #51
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    Quote Originally Posted by jonnyGURU View Post
    There's a reason why people say "kills 99.9% of germs" and not "kills 100% of germs".

    That always freaks me out; so we leave the 0.1% resistant germs to take over... and then...

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    Quote Originally Posted by Per Hansson View Post
    I don't want to open my Corsair AX760 and void it's warranty

    That is where the idea of testing the resistance to ground comes in, as it can be done without opening up the unit (and with the unit unplugged). If one modifies the probe, it could even be used to measure voltage, but I would strongly recommend against it; it is enough to know if the heatsink is grounded or not.
    Last edited by ashiekh; 03-17-2019 at 06:13 PM.

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    Not a Corsair PSU, but in this Aerocool PSU, both the PFC and primary switcher heatsinks are live with ~70V.

    20190322_153428.jpg20190322_153652.jpg

    Even with the power switch OFF, the heatsinks are still live with about .5V (half a volt) until I unplug the PSU completely.

    This is even though I can probe the AC inlet while plugged in with the switch off and get 0V. So this is stored energy.

  4. #54
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    I wonder if the heatsink to heatsink voltage is 0 or 140V.
    Last edited by ashiekh; 03-22-2019 at 10:38 PM.

  5. #55
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    First an advisory before you read this post: NEVER EVER plug an oscilloscope up to a switch mode power supply unless you REALLY know what you are doing.
    You WILL short out the power supply because the scope is almost always* (ref 0:48 in the video) referenced to ground, but the power supply is not, see this video for more in-depth details: EEVblog #279.

    With that out of the way and to answer ashiekh's question the voltage depends on the incoming line voltage and primary side topology of the PSU.
    Here in Sweden it's 230VAC, as can be seen in the picture also this PSU, a Seasonic SS-300ES APFC F3 exhibits similar readings to my other supplies.
    But here I'm showing the results on a oscilloscope that has fully isolated inputs:
    With the PSU off on the switch at the back I still have 235VAC on the primary side heatsink because the switch only interrupts one line.
    When I switch the PSU on I read 114VDC on both the scope and the multimeter.
    However the AC reading differs greatly: The scope reads 174VAC however the meter reads 120VAC.
    This can easily be understood by looking at the waveform: it's not a symmetrical waveform that a TRMS meter needs, so therefore it reads off.
    It's also interesting to see how switching the meter to AC distorts the waveform: I guess the cables connected to the heatsink upsets the feedback circuit.

    *I'm not interested in arguing semantics because marketing does not interest me in the slightest.
    My reply to jonnyGURU's very strange reply to my previous post is simply that when you work on electronics that can kill you.
    Then you ALWAYS explain things in the most safe way possible.
    Saying heatsinks are "almost always" live is allot more correct than saying they are "tied to primary ground or earth ground" which is a direct quote by JG.
    As it is with my sample size of PSU's they all have live heatsinks, however if I had said that "all PSU's always" are live I can understand being attacked for it.
    Therefore I wrote that they are "almost always" to account for the few off the norm supplies that may not be at mains potential.
    Attached Images Attached Images
    "The one who says it cannot be done should never interrupt the one who is doing it."

  6. The Following User Says Thank You to Per Hansson For This Useful Post:

    ashiekh (03-23-2019)

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    Quote Originally Posted by Per Hansson View Post
    NEVER EVER plug an oscilloscope up to a switch mode power supply unless you REALLY know what you are doing.

    Very much appreciated; perhaps reason enough to stick with a battery powered (floating) digital multi-meter.

    I like those Fluke oscilloscopes, not just because they can be run on their own internal batteries (and so have no ground) but also because one can download the screen picture into a computer.

    But I agree that these are lethal voltages and the point of this discussion is not to have people to take risks.

    I seem to recall somewhere on these forums a discussion of a new standard whereby things must be safe for someone probing from outside, and for that the heatsinks would need to be grounded.

    Quote Originally Posted by Per Hansson View Post
    With the PSU off on the switch at the back I still have 235VAC on the primary side heatsink because the switch only interrupts one line.
    In theory it is the live line that should be interrupted, then there should be little voltage on the primary side (the neutral wire is not exactly at ground)
    Last edited by ashiekh; 03-23-2019 at 11:47 AM.

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    Quote Originally Posted by ashiekh View Post
    I wonder if the heatsink to heatsink voltage is 0 or 140V.
    Heatsink to heatsink is 0.

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    Quote Originally Posted by jonnyGURU View Post
    Even with the power switch OFF, the heatsinks are still live with about .5V (half a volt) until I unplug the PSU completely.

    This is even though I can probe the AC inlet while plugged in with the switch off and get 0V. So this is stored energy.

    It might be the small deviation of the neutral from ground; I just checked where I am and it is one tenth of a volt, but we have hardly anything running at the moment.

  10. #59
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    Quote Originally Posted by ashiekh View Post
    It might be the small deviation of the neutral from ground; I just checked where I am and it is one tenth of a volt, but we have hardly anything running at the moment.
    Yeah.. But I can measure right at the switch and get zero. I'm letting it sit "off" with a load over the weekend to see if it's zero on Monday morning.

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    Are you measuring neutral/hot at the switch or neutral/ground?

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