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40v offset to ground?

S

ScottM

Jan 1, 1970
0
I'm using a small, switching power supply for a project. (It's a Kaga
sx15u-05s). When hooking it up, I found terminals for line and neutral,
but no place for a ground wire. It ran fine without one, but after a
bit I decided I'd be happier running grounded, and looked a bit more
carefully.

I found a floating ground (FG), but after reading up on this I decided
it probably wasn't meant to be hooked to the ground prong of a plug.
Then I found that the metal cage of the power supply was marked with a
ground symbol, and had a screw hole to attach a wire to. Ah ha, I
thought.

But being nervous, before I hooked up the ground wire, I checked the AC
voltage between the cage and the outlet's ground - and it came up about
40vac.

Is that normal? I'm not going to attach the outlet ground there until
someone tells me that it is. :)
 
T

Tom Biasi

Jan 1, 1970
0
ScottM said:
I'm using a small, switching power supply for a project. (It's a Kaga
sx15u-05s). When hooking it up, I found terminals for line and neutral,
but no place for a ground wire. It ran fine without one, but after a
bit I decided I'd be happier running grounded, and looked a bit more
carefully.

I found a floating ground (FG), but after reading up on this I decided
it probably wasn't meant to be hooked to the ground prong of a plug.
Then I found that the metal cage of the power supply was marked with a
ground symbol, and had a screw hole to attach a wire to. Ah ha, I
thought.

But being nervous, before I hooked up the ground wire, I checked the AC
voltage between the cage and the outlet's ground - and it came up about
40vac.

Is that normal? I'm not going to attach the outlet ground there until
someone tells me that it is. :)

No its not normal.
The chassis with the screw and marked with the ground symbol is where you
are supposed to connect earth ground.
You could be getting stray readings because of the high impedance of your
meter or something could be wrong. If it were me I would hook the ground to
the chassis and see.
Regards,
Tom
 
C

Chris

Jan 1, 1970
0
ScottM said:
I'm using a small, switching power supply for a project. (It's a Kaga
sx15u-05s). When hooking it up, I found terminals for line and neutral,
but no place for a ground wire. It ran fine without one, but after a
bit I decided I'd be happier running grounded, and looked a bit more
carefully.

I found a floating ground (FG), but after reading up on this I decided
it probably wasn't meant to be hooked to the ground prong of a plug.
Then I found that the metal cage of the power supply was marked with a
ground symbol, and had a screw hole to attach a wire to. Ah ha, I
thought.

But being nervous, before I hooked up the ground wire, I checked the AC
voltage between the cage and the outlet's ground - and it came up about
40vac.

Is that normal? I'm not going to attach the outlet ground there until
someone tells me that it is. :)

Hi, Scott. First, FG is usually interpreted to mean Field Ground.
It's there for safety -- to ensure that, if something goes wrong, a
fuse will blow instead of putting the chassis at line potential.
Safety first -- use it.

Your floating power supply will have a small leakage current between
the line and the chassis, and also between the line and output. That's
primarily capacitive, and is nothing to worry about for most
applications. One exception is medical, where tenths of a mA of
leakage current are critical, for instance with EKG readings -- you can
get medical-rated power supplies where this problem is greatly reduced.
The thing is, a DVM can have up to 10 megohms of impedance, so it's
possibly too good at picking this up.

Try placing a 47K resistor between the cage and GND, and then measure
it with your DVM. I'll bet the voltage reading goes way down. If not,
your power supply is defective.

Good luck
Chris
 
S

ScottM

Jan 1, 1970
0
I'm using a crappy, old analog meter, and I doubt the impedance is
megaohms. I'll try the 47k resistor and retest.

I have two of these supplies, a -12s and a -05s. They both report this
40v difference, so if it's a bug, it's an infestation. Anyone know if
Kaga supplies are prone to this kind of thing?

I'm still not clear on the difference between an FG and a Gnd, but if
it's ok to tie the outlet's ground to FG, I'm happy to do that. There's
no actual screw in the screw hole marked Gnd, but there is one in the
FG position.
 
R

Ralph Mowery

Jan 1, 1970
0
ScottM said:
I'm using a crappy, old analog meter, and I doubt the impedance is
megaohms. I'll try the 47k resistor and retest.

I have two of these supplies, a -12s and a -05s. They both report this
40v difference, so if it's a bug, it's an infestation. Anyone know if
Kaga supplies are prone to this kind of thing?

I'm still not clear on the difference between an FG and a Gnd, but if
it's ok to tie the outlet's ground to FG, I'm happy to do that. There's
no actual screw in the screw hole marked Gnd, but there is one in the
FG position.
While not always correct, I have found that if you have the old 20,000
ohm/volt meter or similar that if you change the range to another scale and
the meter pointer (not the voltage) stayes in about the same position then
the meter is picking up some high impedance leakage.

Sometimes a piece of equipment will have the signal ground isolated from the
actual chassie (frame) . That gives you two grounds. It is similar to the
way the power company and your house is wired. You have a neutral wire
going from the braker box and also a ground wire. They are seperate as far
as the power transmission goes, but are connected at one point back at the
braker box.
 
C

Chris

Jan 1, 1970
0
ScottM said:
I'm using a crappy, old analog meter, and I doubt the impedance is
megaohms. I'll try the 47k resistor and retest.

I have two of these supplies, a -12s and a -05s. They both report this
40v difference, so if it's a bug, it's an infestation. Anyone know if
Kaga supplies are prone to this kind of thing?

I'm still not clear on the difference between an FG and a Gnd, but if
it's ok to tie the outlet's ground to FG, I'm happy to do that. There's
no actual screw in the screw hole marked Gnd, but there is one in the
FG position.

Hi, Scott. Real men use analog meters. ;-)

Most all switches are made to have the cage connected to GND. You need
to find the actual current, and to do that, you'll need a realistic
value resistor. A 47K 1/4 watt resistor is about the smallest value
that can handle line voltage for a few seconds (.3W), so I'll typically
use one of those. Use your meter to read the voltage across the
resistor, and you'll have current. You shouldn't be reading more than
a couple of volts.

Power supplies are supposed to be tested for hipot, so this shouldn't
be a problem unless the power supply is defective. Do the test, and if
it's OK, connect the line cord GND to the chassis (cage) of the
switcher, like the manufacturer recommends.

Good luck
Chris
 
P

Peter Bennett

Jan 1, 1970
0
Hi, Scott. First, FG is usually interpreted to mean Field Ground.
It's there for safety -- to ensure that, if something goes wrong, a
fuse will blow instead of putting the chassis at line potential.
Safety first -- use it.

I consider "FG" to mean "Frame Ground" - it is a terminal used to
connect the frame of the device to ground.
 
P

Pooh Bear

Jan 1, 1970
0
ScottM said:
I'm using a crappy, old analog meter, and I doubt the impedance is
megaohms. I'll try the 47k resistor and retest.

I have two of these supplies, a -12s and a -05s. They both report this
40v difference, so if it's a bug, it's an infestation. Anyone know if
Kaga supplies are prone to this kind of thing?

I'm still not clear on the difference between an FG and a Gnd, but if
it's ok to tie the outlet's ground to FG, I'm happy to do that. There's
no actual screw in the screw hole marked Gnd, but there is one in the
FG position.

I understand FG to mean 'frame ground' as in chassis i.e. the metalwork.
Connecting this to the supply ground wire should not affect anything most
likely.

Graham
 
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