Isolating Transformer

P

pfm

Guest
I was working on an CRT TV using an isolating transformer. I happened
to measure the voltage between the chassis 'dag' and a nearby metal
fixture and was surprised to find 75Vac. Without the transformer the
reading was 110Vac.

Something is wrong, right?

BobH
 
In article <7ef56656-549f-4b89-846f-2422d545f6c3@m26g2000yqb.googlegroups.com>, pfm <bobh@pfmbox.com> wrote:
I was working on an CRT TV using an isolating transformer. I happened
to measure the voltage between the chassis 'dag' and a nearby metal
fixture and was surprised to find 75Vac. Without the transformer the
reading was 110Vac.

Something is wrong, right?

BobH
Click to expand...
You shold be able to trace it out. I'm glad you said isolation transformer
rather than isolation transformer. You want ground isolation because the primary
is referenced to ground. You can check out some resistances with the AC
removed.

greg
 
pfm <bobh@pfmbox.com> writes:

On Dec 18, 3:19 pm, zekfr...@zekfrivolous.com (GregS) wrote:
In article <7ef56656-549f-4b89-846f-2422d545f...@m26g2000yqb.googlegroups.com>, pfm <b...@pfmbox.com> wrote:

I was working on an CRT TV using an isolating transformer. I happened
to measure the voltage between the chassis 'dag' and a nearby metal
fixture and was surprised to find 75Vac. Without the transformer the
reading was 110Vac.

Something is wrong, right?

BobH

You shold be able to trace it out. I'm glad you said isolation transformer
rather than isolation transformer. You  want ground isolation because the primary
is referenced to ground. You can check out some resistances with the AC
removed.

greg

On Dec 18, 3:19 pm, zekfr...@zekfrivolous.com (GregS) wrote:
In article <7ef56656-549f-4b89-846f-2422d545f...@m26g2000yqb.googlegroups.com>, pfm <b...@pfmbox.com> wrote:

I was working on an CRT TV using an isolating transformer. I happened
to measure the voltage between the chassis 'dag' and a nearby metal
fixture and was surprised to find 75Vac. Without the transformer the
reading was 110Vac.

Something is wrong, right?

BobH

You shold be able to trace it out. I'm glad you said isolation transformer
rather than isolation transformer. You  want ground isolation because the primary
is referenced to ground. You can check out some resistances with the AC
removed.

greg

Thanks Greg but I'm not sure what you are saying. I use the terms
'isolating' and 'isolation' interchangeably. I thought the purpose of
an isolating/isolation transformer is to ensure that no potential
difference can exist between the device under test and a grounded
object such as a concrete floor or a water pipe. The risk of shock is
restricted to within the device under test.
Click to expand...
There's always capacitive and inudctive coupling. You were probably
measuring with a high-Z DMM. Put any sort of load between the meter
probes and the voltage will go to 0. If it does not, you do have
a serious safety issue.

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On Dec 18, 3:19 pm, zekfr...@zekfrivolous.com (GregS) wrote:
In article <7ef56656-549f-4b89-846f-2422d545f...@m26g2000yqb.googlegroups..com>, pfm <b...@pfmbox.com> wrote:

I was working on an CRT TV using an isolating transformer. I happened
to measure the voltage between the chassis 'dag' and a nearby metal
fixture and was surprised to find 75Vac. Without the transformer the
reading was 110Vac.

Something is wrong, right?

BobH

You shold be able to trace it out. I'm glad you said isolation transformer
rather than isolation transformer. You  want ground isolation because the primary
is referenced to ground. You can check out some resistances with the AC
removed.

greg
Click to expand...
On Dec 18, 3:19 pm, zekfr...@zekfrivolous.com (GregS) wrote:
In article <7ef56656-549f-4b89-846f-2422d545f...@m26g2000yqb.googlegroups..com>, pfm <b...@pfmbox.com> wrote:

I was working on an CRT TV using an isolating transformer. I happened
to measure the voltage between the chassis 'dag' and a nearby metal
fixture and was surprised to find 75Vac. Without the transformer the
reading was 110Vac.

Something is wrong, right?

BobH

You shold be able to trace it out. I'm glad you said isolation transformer
rather than isolation transformer. You  want ground isolation because the primary
is referenced to ground. You can check out some resistances with the AC
removed.

greg
Click to expand...
Thanks Greg but I'm not sure what you are saying. I use the terms
'isolating' and 'isolation' interchangeably. I thought the purpose of
an isolating/isolation transformer is to ensure that no potential
difference can exist between the device under test and a grounded
object such as a concrete floor or a water pipe. The risk of shock is
restricted to within the device under test.
 
"pfm"
I was working on an CRT TV using an isolating transformer. I happened
to measure the voltage between the chassis 'dag' and a nearby metal
fixture and was surprised to find 75Vac. Without the transformer the
reading was 110Vac.

Something is wrong, right?
Click to expand...
** Wrong.

DMMs typically have 10 Mohms impedance across the leads when in any of their
AC volts ranges - so a reading of 75 volts AC indicates there is leakage
of about 7.5 uA of AC current.

Equates to about 600 pF of capacitance ( primary to secondary) in the iso
tranny.

Try connecting a 0.1 uF cap from the TVs chassis to ground - that should
swamp the effect.


..... Phil
 
On Dec 19, 10:29 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"pfm"

I was working on an CRT TV using an isolating transformer. I happened
to measure the voltage between the chassis 'dag' and a nearby metal
fixture and was surprised to find 75Vac. Without the transformer the
reading was 110Vac.

Something is wrong, right?

** Wrong.

DMMs typically have 10 Mohms impedance across the leads when in any of their
AC volts ranges  -  so a reading of 75 volts AC indicates there is leakage
of about 7.5 uA of AC current.

Equates to about 600 pF of capacitance ( primary to secondary) in the iso
tranny.

Try connecting a 0.1 uF cap from the TVs chassis to ground  -  that should
swamp the effect.

....  Phil
Click to expand...
Thanks Sam and Phil, you were right, it was just stray voltage.
 
In article <2f7ae314-fd6e-478b-b13f-bb9241790a3c@k17g2000yqh.googlegroups.com>, pfm <bobh@pfmbox.com> wrote:
On Dec 18, 3:19=A0pm, zekfr...@zekfrivolous.com (GregS) wrote:
In article <7ef56656-549f-4b89-846f-2422d545f...@m26g2000yqb.googlegroups=
..com>, pfm <b...@pfmbox.com> wrote:

I was working on an CRT TV using an isolating transformer. I happened
to measure the voltage between the chassis 'dag' and a nearby metal
fixture and was surprised to find 75Vac. Without the transformer the
reading was 110Vac.

Something is wrong, right?

BobH

You shold be able to trace it out. I'm glad you said isolation transforme=
r
rather than isolation transformer. You =A0want ground isolation because t=
he primary
is referenced to ground. You can check out some resistances with the AC
removed.

greg

On Dec 18, 3:19=A0pm, zekfr...@zekfrivolous.com (GregS) wrote:
In article <7ef56656-549f-4b89-846f-2422d545f...@m26g2000yqb.googlegroups=
..com>, pfm <b...@pfmbox.com> wrote:

I was working on an CRT TV using an isolating transformer. I happened
to measure the voltage between the chassis 'dag' and a nearby metal
fixture and was surprised to find 75Vac. Without the transformer the
reading was 110Vac.

Something is wrong, right?

BobH

You shold be able to trace it out. I'm glad you said isolation transforme=
r
rather than isolation transformer. You =A0want ground isolation because t=
he primary
is referenced to ground. You can check out some resistances with the AC
removed.

gregI
'isolating' and 'isolation' interchangeably. I thought the purpose of
an isolating/isolation transformer is to ensure that no potential
difference can exist between the device under test and a grounded
object such as a concrete floor or a water pipe. The risk of shock is
restricted to within the device under test.
Click to expand...
I was suggesting that you may be using the wrong kind of transformer.
We went all through this recently. Most all isolation transformers sold to the
public are NOT isolated. They are for noise reduction. You
seem to be correctly talking about a transformer that does not have the secondary
tied to ground. It can easily be checked by resistance measurment.
I was just trying to troubleshoot your problem, and wanted all the facts.

greg
 
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