Switching PSU problem


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  1. #1
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    Default Switching PSU problem

    Hi,

    I've got several 240V to 12V 150W switching PSUs running circuits I've made. Most are working fine but today one of my circuits started resetting randomly.
    After a long time checking various things I noticed that there seems to be a huge potential across the ground.

    The PSU case and the 0V ground rail of my PCBs are all connected to earth through the 3-pin mains plug.
    If I put a multimeter probe on the PSU case and the other probe on my PCB ground it measures over 1,300V.
    With the mains disconnected I can confirm that there is definitely continuity between these points and with less resistance than my multimeter will measure.
    Different points on the PCB get different readings. Points that are electrically further away from where the ground wire is connected read higher voltages.

    Does anyone know what might be causing it and if it can be fixed? I've got quite a few of these PSUs now and I'm worried that everything will start failing and even become dangerous.

  2. #2
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    Default Re: Switching PSU problem

    The PSU case and the 0V ground rail of my PCBs are all connected to earth through the 3-pin mains plug.
    If I put a multimeter probe on the PSU case and the other probe on my PCB ground it measures over 1,300V.
    With the mains disconnected I can confirm that there is definitely continuity between these points and with less resistance than my multimeter will measure.
    one or more of those statements cannot be true

    power = voltage squared divided by resisence
    if the case to gnd rail resistence was 0.1 ohms then
    {ridiculous amount of power} 16.9 mega watts = 1300v x 1300v / .1 ohm
    where is all the heat ?
    Warning I'm not a teacher

  3. #3
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    Default Re: Switching PSU problem

    1 of the components on the PCB is getting slightly hotter than usual but there's not a huge amount of heat coming from it and this is likely because one of the chips draws more current every time it resets.

    I know it's not impossible to get voltage that high with all the coils and capacitors in this thing but I find it hard to believe it would happen without being specifically designed to do that.

    I've tested it several times and my multimeter was showing almost 2KV last night. Could there be some capacitance or something that's confusing the multimeter?
    The main PCB has 4 large capacitors between GND and 12V. I've tried the multimeter in DC and AC mode. AC mode shows 0V between ground points.

    In the PSU the secondary is *almost* completely isolated from the primary.
    After the 240V AC is rectified, the negative goes through what looks like a very large blue ceramic disc capacitor to the negative on the secondary. Could this be causing issues as it's essentially linking half cycles from live and neutral to my PCB ground.

    The problem is also strangely intermittent. It doesn't seem to happen for the first few minutes after being powered up. My PCBs work fine and the multimeter reads 0V between all ground points. After a few minutes it goes back into that state.

    While in that state, if I put my multimeter between the 0V and 12V outputs on the PSU it reads 12V and my PCB starts working as normal. As soon as I disconnect it the PCBs start resetting again.

    I've had the same PCBs running overnight using another PSU of the same type and it hasn't reset once.

  4. #4
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    Default Re: Switching PSU problem

    lets examine this one point at a time

    I know it's not impossible to get voltage that high with all the coils and capacitors in this thing but I find it
    hard to believe it would happen without being specifically designed to do that.
    it is impossible to get a dc voltage that high between two points connected by a low impedence unless a massive current is
    flowing , circuit theory 101
    I've tested it several times and my multimeter was showing almost 2KV last night.
    qed the fg[psu frame gnd] is not connected to the 12 gnd
    Could there be some capacitance or something that's confusing the multimeter?
    no
    The main PCB has 4 large capacitors between GND and 12V. I've tried the multimeter in DC and AC mode.
    AC mode shows 0V between ground points.
    as you would expect, the voltage is basically a static build up
    In the PSU the secondary is *almost* completely isolated from the primary.
    as you would expect
    After the 240V AC is rectified, the negative goes through what looks like a very large blue ceramic disc capacitor
    to the negative on the secondary. Could this be causing issues as it's essentially linking half cycles from
    live and neutral to my PCB ground.
    no , the cap is for rfi suppression and usually 2.5kv rated and a few hundred pf in value
    you may also have similar caps on the secondary rails, its also possible to have a MOV between the secondary rail
    and fg to ensure psu isolation voltage rating is not exceeded

    The problem is also strangely intermittent. It doesn't seem to happen for the first few minutes after being powered
    up.
    as you would expect the measured voltage is basically a static build up from switching transients and takes time to
    accumulate
    My PCBs work fine and the multimeter reads 0V between all ground points.
    fg is not connected to dc gnd ,it simply cannot be .
    it is impossible to get a dc voltage that high between two points connected by a low impedence
    without a massive amount of power being consumed .
    While in that state, if I put my multimeter between the 0V and 12V outputs on the PSU it reads 12V
    and my PCB starts working as normal. As soon as I disconnect it the PCBs start resetting again.
    i would guess that the meter impedence probably bleeds off the static buildup in some way
    although "the 0V" mentioned is ambiguous
    Warning I'm not a teacher

  5. #5
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    Default Re: Switching PSU problem

    Quote Originally Posted by richard View Post
    i would guess that the meter impedence probably bleeds off the static buildup in some way
    although "the 0V" mentioned is ambiguous
    Static goes on surface of probe, and to your body to ground. Add 1MegaOhm resistor or bigger, from wall GND to 12V(- or +) and use 5W or bigger resistor, or multiple smaller in series to avoid arcing over body of resistor.

  6. #6
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    Default Re: Switching PSU problem

    The ground of my PCBs (0V negative from the PSU) and the PSU case are both connected to the mains earth.
    They are housed in an ABS box with screws visible on the outside so the whole lot needed to be earthed.

    There is another one of these blue ceramic caps inside the PSU that goes from the negative out to case ground. I'm guessing this is the MOV you mentioned.
    This is essentially bypassed by the earth wire I have running between case ground and PCB ground.

    The "0V" is the negative output of the PSU.

    I've got some 0.25W 1MOhm resistors that I could string together as a test.

    Someone I work with has suggested the blue cap between the rectified mains and negative output may be failing.
    This PSU has been on 24/7 for months without a problem but I know caps will fail eventually.
    I've got several other working supplies and some that have never been turned on yet so I can try swapping out small components like that.

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