Switching PSU problem

[richard]

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

[pedja089]

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.

[The Master]

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.

[richard]

The ground of my PCBs (0V negative from the PSU) and the PSU case are both connected to the mains earth.

yet you claim a 2kv potential difference .

one more try

it is impossible to get a dc voltage that high between two points connected by a low impedance unless a massive current is flowing , circuit theory 101

[The Master]

My multimeter says 2KV DC. I'm not sure I can trust it.

I understand that Ohm's law doesn't allow this condition. 2KV across even 1Ohm would give 2,000A of current flowing through the PSU case and some wires that only handle around 4A.

I'm sure all of the ground points I've checked are connected by low impedance.
There's no excess heat or melted wires etc.
My components haven't fried despite apparently having hundreds of volts difference between their grounds.

The only conclusion can be that there isn't really 2KV. There has to be something though as my circuits keep resetting and the multimeter is showing such high voltages.

Is there anything I can do to help narrow it down? I will try the 1MOhm resistor idea to see if it makes any difference.

[Dave]

First thing I would have done when I saw such LARGE potential is to, Get another meter and double check it...

[richard]

My multimeter says 2KV DC. I'm not sure I can trust it.

I don't doubt the possibility that the voltage is real, as a tv serviceman I have experienced those sorts of voltages between supposedly earthed TV's and properly grounded antennas many times.

maybe a photo of the psu indicating where the measurement is being made could shed some light

ps its not a meter auto_scaling measurement error