Ground Loop Problem!!

[Adrian]

Hi,

I have two devices:

1) Power is supplied from a 120VAC to 12VAC Adapter (not grounded)

2) Power is supplied from 120VAC (grounded) to an internal switching power
supply

I would like to have the two devices physically connected and since they
both have metal chassis', this entails electrical connection as well.

The problem is that there is a potential difference between the two chassis
and whenever they touch, they conduct current through a ground loop causing
a multitude of problems, from blowing transistors, to malfunction of the
device.

Is there any way to solve this other than to physically separate the
devices?

THANKS!!!

 

[CFoley1064]

Hi,

I have two devices:

1) Power is supplied from a 120VAC to 12VAC Adapter (not grounded)

2) Power is supplied from 120VAC (grounded) to an internal switching power
supply

I would like to have the two devices physically connected and since they
both have metal chassis', this entails electrical connection as well.

The problem is that there is a potential difference between the two chassis
and whenever they touch, they conduct current through a ground loop causing
a multitude of problems, from blowing transistors, to malfunction of the
device.

Is there any way to solve this other than to physically separate the
devices?

THANKS!!

If your description of the situation is correct, I think you should first look
at replacing the 12VAC wall wart. Before replacing it with an adapter or
transformer that's guaranteed to be isolated, use your DMM to check if one side
of the 112VAC is grounded or has high leakage to ground. Place a 47K 1/4W
resistor from each leg in turn to GND, and measure the voltage across the
resistor with the wall wart plugged in. One leg of the AC from your adapter
may be grounded, or you might just have sufficient leakage current to read a
significant voltage and cause problems. Wall warts are usually made just good
enough, but that might not be good enough isolation for your application.

You haven't given a complete description of your AC adapter, but I would start
by looking at the Stancor 4112 (12VAC@0.5A). Unless you have a "medical
isolation"-type application, that should be sufficient.

Another thing you might want to try is using "cheater plugs" to get the ground
out of the equation, at least for a test to see if the electrical problems stop
or change.

If that doesn't solve the problem, your easiest route would be to contact the
manufacturers of the devices and ask them for assistance. Without knowing more
about the devices or your application, it's difficult to give advice. And
sometimes there's a "gotcha" that isn't obvious.

This type of thing is a "common" problem (sorry ;-), but there's usually some
kind of solution. Luck is the residue of hard work.

Good luck.
Chris

 

[Adrian]

Hi Chris,

Thanks for the reply.

I checked the wallwart with a 100K resistor and measured 2V potential to
ground. I also measured the open circuit potential to ground, and on the
center pin of the adapter it is 12V!

This seems to indicate the leaky adapter as you suggested, correct?

If so, is there any way to check an adapter for isolation before purchasing
it? Using this same method perhaps? What is an acceptable potential for
leakage?

Thanks!!

Adrian

"CFoley1064" <cfoley1064@aol.com> wrote in message
news:20040112002556.11235.00002702@mb-m11.aol.com...

Hi,

I have two devices:

1) Power is supplied from a 120VAC to 12VAC Adapter (not grounded)

2) Power is supplied from 120VAC (grounded) to an internal switching
power
supply

I would like to have the two devices physically connected and since they
both have metal chassis', this entails electrical connection as well.

The problem is that there is a potential difference between the two
chassis
and whenever they touch, they conduct current through a ground loop
causing
a multitude of problems, from blowing transistors, to malfunction of the
device.

Is there any way to solve this other than to physically separate the
devices?

THANKS!!

If your description of the situation is correct, I think you should first
look
at replacing the 12VAC wall wart. Before replacing it with an adapter or
transformer that's guaranteed to be isolated, use your DMM to check if one
side
of the 112VAC is grounded or has high leakage to ground. Place a 47K 1/4W
resistor from each leg in turn to GND, and measure the voltage across the
resistor with the wall wart plugged in. One leg of the AC from your
adapter
may be grounded, or you might just have sufficient leakage current to read
a
significant voltage and cause problems. Wall warts are usually made just
good
enough, but that might not be good enough isolation for your application.

You haven't given a complete description of your AC adapter, but I would
start
by looking at the Stancor 4112 (12VAC@0.5A). Unless you have a "medical
isolation"-type application, that should be sufficient.

Another thing you might want to try is using "cheater plugs" to get the
ground
out of the equation, at least for a test to see if the electrical problems
stop
or change.

If that doesn't solve the problem, your easiest route would be to contact
the
manufacturers of the devices and ask them for assistance. Without knowing
more
about the devices or your application, it's difficult to give advice. And
sometimes there's a "gotcha" that isn't obvious.

This type of thing is a "common" problem (sorry ;-), but there's usually
some
kind of solution. Luck is the residue of hard work.

Good luck.
Chris

 

[CFoley1064]

Hi Chris,

Thanks for the reply.

I checked the wallwart with a 100K resistor and measured 2V potential to
ground. I also measured the open circuit potential to ground, and on the
center pin of the adapter it is 12V!

This seems to indicate the leaky adapter as you suggested, correct?

If so, is there any way to check an adapter for isolation before purchasing
it? Using this same method perhaps? What is an acceptable potential for
leakage?

Thanks!!

Wall warts made by reputable "name brand" manufacturers are tested for
compliance to UL specification for hipot and leakage before shipment. These
specifications can usually be obtained by looking at the manufacturer catalog.

You could do worse than look at a Stancor product. Their wall warts are
available through standard distributors such as Newark and Allied. Since there
isn't any information available as to the type or size of wall wart, you can
choose one yourself which has a current rating greater than or equal to the one
you're using:

http://www.stancor.com/pdfs/quiksel.pdf

(By the way, if the two devices are permanently affixed together, you probably
don't need a GND pin on the plug).

If you don't mind possibly having a few return trips, just buy one and test it
as above. If it doesn't work, take it back and buy another one somewhere else.

Good luck
Chris

 

[Jim Thompson]

On Mon, 12 Jan 2004 04:11:04 GMT, "Adrian" <afeudale34@rogers.com>
wrote:

Hi,

I have two devices:

1) Power is supplied from a 120VAC to 12VAC Adapter (not grounded)

2) Power is supplied from 120VAC (grounded) to an internal switching power
supply

I would like to have the two devices physically connected and since they
both have metal chassis', this entails electrical connection as well.

The problem is that there is a potential difference between the two chassis
and whenever they touch, they conduct current through a ground loop causing
a multitude of problems, from blowing transistors, to malfunction of the
device.

Is there any way to solve this other than to physically separate the
devices?

THANKS!!!

You have a serious problem. Please get professional help.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Euthymios Kappos]

"Adrian" <afeudale34@rogers.com> wrote in message
news:s7pMb.118758$1g41.56851@twister01.bloor.is.net.cable.rogers.com...

Hi,

I have two devices:

1) Power is supplied from a 120VAC to 12VAC Adapter (not grounded)

2) Power is supplied from 120VAC (grounded) to an internal switching
power
supply

I would like to have the two devices physically connected and since they
both have metal chassis', this entails electrical connection as well.

The problem is that there is a potential difference between the two
chassis
and whenever they touch, they conduct current through a ground loop
causing
a multitude of problems, from blowing transistors, to malfunction of the
device.

Is there any way to solve this other than to physically separate the
devices?

THANKS!!!

I suspect capacitive coupling between the primary and secondary circuits
of the power transformer. If the transformer is not shielded this leakage
capacitance couples common-mode signals to the secondary.

Euthymios Kappos

 

[Walter Harley]

"Euthymios Kappos" <kappos@ath.forthnet.gr> wrote in message
news:1073987897.916724@athnrd02.forthnet.gr...

I suspect capacitive coupling between the primary and secondary circuits
of the power transformer. If the transformer is not shielded this leakage
capacitance couples common-mode signals to the secondary.

Capacitive coupling in a working transformer is only going to be a few pF,
certainly much less than 1nF. At 120V 60Hz, a 100pF capacitor will pass
5uA, if my math is correct. That's not enough to blow transistors (as the
OP claims is happening). If he's getting that kind of current between the
chasses of two mains-powered devices, then as Jim said, he's got a serious
problem and he should seek the help of someone who's got test equipment and
knows how to use it.

 

[Genome]

"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote in message
news:1008aid2e910u25@corp.supernews.com...

"Euthymios Kappos" <kappos@ath.forthnet.gr> wrote in message
news:1073987897.916724@athnrd02.forthnet.gr...
I suspect capacitive coupling between the primary and secondary circuits
of the power transformer. If the transformer is not shielded this
leakage
capacitance couples common-mode signals to the secondary.

Capacitive coupling in a working transformer is only going to be a few pF,
certainly much less than 1nF. At 120V 60Hz, a 100pF capacitor will pass
5uA, if my math is correct. That's not enough to blow transistors (as the
OP claims is happening). If he's getting that kind of current between the
chasses of two mains-powered devices, then as Jim said, he's got a serious
problem and he should seek the help of someone who's got test equipment
and
knows how to use it.

Mr OP doesn't describe his set up too well, oh dear......

I think....

C = Eo.Er.A/S

Take a small switch mode transformer with a width of 25mm and a winding
length of 25mm for an area of 25mm x 25mm.

A = 6.25E-4 M^2

Eo = 8.85E-12 Something or another

Er = oh..... call it one

S = 1.5E-4 M

S is the winding separation. 3 layers of 2mil (mixed units!!) yellow stickey
stuff.

C = 8.85E-12 x 6.25E-4/1.5E-4
C = 37pF

Turn 120VAC into 170VDC for the supply. Switch at 100KHz and ignore the
harmonics. That's about 4mA.

Not totally insignificant...... but, sort of, not enough to blow up
transistors.

Unless;

It's an SMPS with feedback from secondary to primary via some thing,
probably an opto-coupler. If you don't pay attention to a bit of judicious
capacitive/resistive coupling between primary and secondary then the
resulting common mode hash is likely to send the feedback loop to apesville
and things might well go bang.

If you rolled you own then you'd make sure it wouldn't.

Then you'd run it up and see what the nasties were. Couple primary to
secondary wiv a bit of Class Y. Preferably, primary to ground and then back
from ground up to secondary.... using two of them and taking care of
regulatory requirements. Depends on where you live.

See how it rings.... wiv your 20MHz analog(ue) scope, do some sums.... add a
bit of appropriate R in series with the Y to settle things down.

If you've bought an El-Cheapo from wing-tong-tiddle-aye-po then life might
become a bit harder.



There you go. I do know about EMC type stuff, next question.

DNA

 

[GPG]

If you've bought an El-Cheapo from wing-tong-tiddle-aye-po then life might
become a bit harder.
http://www.scbd.connectfree.co.uk/flops/ying-tong.html