Testing an Isolation Transformer

[Cursitor Doom]

Hi all,

Well, I picked up a 500VA mains isolation transformer today: 240v in;
240v out. It's not brand new so I thought I'd best check it over. I
hooked up live and neutral to the two connectors on the primary side of
it, and two probes from a voltmeter on the secondary.
Without anything else attached and no current drawn yet, I applied the
tip of a neon screwdriver to the four connections with the following
results:

Primary live connection: strong yellow light from the neon tester

Primary neutral connection: no response from the neon tester

2x Secondary connections: both equally light up the tester by about half
as much as the live primary did.

I was under the impression I would see no response at all from the neon
tester on the secondary side since it's supposedly floating; yet I *did*
get a reaction of sorts from the tester.
What's going on here? Is the transformer faulty?

cheers,
cd.

 

There is almost always some leakage in anytithng, it's a matter of how much.. I would test it with an incandescent lamp. Each side of the seconadry to each side of the primary. An incandescent should not light, even dimly when connected to either side of the seconday and anythong else except the other side of the secondary. If it does, the transformer is shorted.

You're up in the UK huh. There is a specified hot line leakage test in the US wwhich is supposed to be ran on every piece of cunsumer electronics serviced if it runs off house power. Most people just use an ohmmeter and do the cold check but the hot check is alot more revealing. It actually appplies the voltage across.

I am sure they have someting similar there. Just run it on that just like a TV set or something, but also the output. Here they want you to probe each exposed metal part as well, which isn't a bad idea really. You don't want any shorts or heavy leakage to the core either.

I looked it up real quick and didn't even hit the US method, which you probably could use. However, if you can get a service manual for a TV set there, there is probably a section on that. If not, really, you could just do the US one but consider that it is double the line voltage.

Another thing you might do is plug the thing into a GFCI and then ground each side of the output. If the GFCI doesn't trip it is probably safe.

 

[Cursitor Doom]

On Mon, 30 Mar 2015 18:27:11 -0400, Ralph Mowery wrote:

Is this in the US or somewhere else ? In the US there is no neutral on
a 240 volt circuit. Could be differant in other countries.

This is in the UK where it's getting rather late so I'll take it up again
in the morning and check for any further replies. I have precisely zero
experience in this area, I'm afraid.

 

[Ralph Mowery]

"Cursitor Doom" <curd@notformail.com> wrote in message
news:mfcgj4$j54$2@dont-email.me...

Hi all,

Well, I picked up a 500VA mains isolation transformer today: 240v in;
240v out. It's not brand new so I thought I'd best check it over. I
hooked up live and neutral to the two connectors on the primary side of
it, and two probes from a voltmeter on the secondary.
Without anything else attached and no current drawn yet, I applied the
tip of a neon screwdriver to the four connections with the following
results:

Primary live connection: strong yellow light from the neon tester

Primary neutral connection: no response from the neon tester

2x Secondary connections: both equally light up the tester by about half
as much as the live primary did.

I was under the impression I would see no response at all from the neon
tester on the secondary side since it's supposedly floating; yet I *did*
get a reaction of sorts from the tester.
What's going on here? Is the transformer faulty?

Think about it for a while. The transformer seems to be acting normally..
YOu are probably getting some capacitive coupling form the primary to
secondary windings.

Is this in the US or somewhere else ? In the US there is no neutral on a
240 volt circuit. Could be differant in other countries.

 

[Dave Platt]

In article <mfcgj4$j54$2@dont-email.me>,
Cursitor Doom <curd@notformail.com> wrote:

2x Secondary connections: both equally light up the tester by about half
as much as the live primary did.

I was under the impression I would see no response at all from the neon
tester on the secondary side since it's supposedly floating; yet I *did*
get a reaction of sorts from the tester.
What's going on here? Is the transformer faulty?

There may be enough capacitive coupling between primary and secondary
windings to deliver enough current to light up a neon bulb. They
don't take much.

Is this a *true* isolation transformer, sold as such?

Medical-grade isolation transformers would (I believe) include a
conductive shield between the primary and secondary windings. If the
shield is grounded (as it is supposed to be, in this sort of service)
it will prevent capacitive coupling to the secondary.

 

[mike]

On 3/30/2015 2:52 PM, Cursitor Doom wrote:

Hi all,

Well, I picked up a 500VA mains isolation transformer today: 240v in;
240v out. It's not brand new so I thought I'd best check it over. I
hooked up live and neutral to the two connectors on the primary side of
it, and two probes from a voltmeter on the secondary.
Without anything else attached and no current drawn yet, I applied the
tip of a neon screwdriver to the four connections with the following
results:

Primary live connection: strong yellow light from the neon tester

Primary neutral connection: no response from the neon tester

2x Secondary connections: both equally light up the tester by about half
as much as the live primary did.

I was under the impression I would see no response at all from the neon
tester on the secondary side since it's supposedly floating; yet I *did*
get a reaction of sorts from the tester.
What's going on here? Is the transformer faulty?

cheers,
cd.

You chose your moniker well.
You have analysis paralysis.

You have the ability to use absolutely the worst, least precise,
tool to make a measurement under conditions outside the normal
specification range of the devices, then fret over the result.

Measure the AC current between the output and ground with
a real AC current meter. Put a resistor in series with it to limit
the current justincase.

It would not be unusual to measure lethal currents for the worst-case
connections to your body and ground. More to fret over.

Working on line-attached stuff is very risky.
There is no such thing as "floating". There are always AC paths.
When I do it, I drag out the TEKTRONIX A6902a.
Depending on the voltages involved and what I'm doing, I might
also use an isolation transformer.
A GFI in the mix is a good idea.

I like caution, but you could have fixed the scope by inspecting
the schematic in less time than this thread has consumed.

 

[Cursitor Doom]

On Mon, 30 Mar 2015 17:08:18 -0700, mike wrote:

You chose your moniker well.
You have analysis paralysis.

You're right there!
I've slept on it and have come up with a better idea. No one here will
like it and I'll probably get abuse for it, but I'm comfortable with it
and it will get the job done with reasonable safety.
I'm going to slowly step up the volts via the variac on the primary side
whilst sticking my finger into each of the secondary terminations in
turn. If I don't feel a tingle by 100V it's probably working as it should
and I'll keep winding up to 230V. I shall of course, keep one hand behind
my back.
I'll let you know how it goes later. Hopefully.

 

[Cursitor Doom]

On Mon, 30 Mar 2015 17:08:18 -0700, mike wrote:

A GFI in the mix is a good idea.

I like caution, but you could have fixed the scope by inspecting the
schematic in less time than this thread has consumed.

If there were no lethal voltages present to freak out about, yes. What's
a "GFI"?

 

[mike]

On 3/31/2015 3:16 AM, Cursitor Doom wrote:

On Mon, 30 Mar 2015 17:08:18 -0700, mike wrote:

A GFI in the mix is a good idea.

I like caution, but you could have fixed the scope by inspecting the
schematic in less time than this thread has consumed.

If there were no lethal voltages present to freak out about, yes. What's
a "GFI"?

Ground Fault Interrupter senses imbalance in the hot/neutral and switches
off if there is imbalance, which likely goes from hot to ground
by external means...possibly through you.

There are no lethal voltages on a schematic diagram you're
reverse engineering...no voltages on the paper at all.

Diagnosis is topology dependent, but, in general, I wouldn't
expect a switcher to behave predictably outside its design input voltage.

Varying the input voltage while testing the output with your finger
is so ludicrous that you're beginning to sound like a troll.

If your representations are truthful, you probably shouldn't be using
a scope, much less trying to fix one.

 

[Cursitor Doom]

On Tue, 31 Mar 2015 03:43:14 -0700, mike wrote:

On 3/31/2015 3:16 AM, Cursitor Doom wrote:
On Mon, 30 Mar 2015 17:08:18 -0700, mike wrote:

A GFI in the mix is a good idea.

I like caution, but you could have fixed the scope by inspecting the
schematic in less time than this thread has consumed.

If there were no lethal voltages present to freak out about, yes.
What's a "GFI"?

Ground Fault Interrupter senses imbalance in the hot/neutral and
switches off if there is imbalance, which likely goes from hot to ground
by external means...possibly through you.

There are no lethal voltages on a schematic diagram you're reverse
engineering...no voltages on the paper at all.

Diagnosis is topology dependent, but, in general, I wouldn't expect a
switcher to behave predictably outside its design input voltage.

Varying the input voltage while testing the output with your finger is
so ludicrous that you're beginning to sound like a troll.

If your representations are truthful, you probably shouldn't be using a
scope, much less trying to fix one.

Yes, well I expected such a response. Anyway, my method didn't actually
prove anything; I got no tingle at all even on around 160V on the primary
side so I guess I'm not adequately earthed. And I'm not about to stand in
a bowl of water to improve things, or whack the volts up any further so
I'll defer to someone's earlier suggestion and measure it properly with a
mains meter. I admit it was a bit daft but it seemed a useful expedient
at the time.
BTW, we call those things RCDs here and I do already use one for extra
safety.

 

[Mike]

On Tue, 31 Mar 2015 03:43:14 -0700, mike wrote:

Ground Fault Interrupter senses imbalance in the hot/neutral and
switches off if there is imbalance, which likely goes from hot to ground
by external means...possibly through you.

I guessed it must be what we call a "residual current device" = RCD.

Mike.

 

[Cursitor Doom]

On Wed, 01 Apr 2015 06:43:38 +1100, Trevor Wilson wrote:

On 31/03/2015 8:52 AM, Cursitor Doom wrote:
Hi all,

Well, I picked up a 500VA mains isolation transformer today: 240v in;
240v out. It's not brand new so I thought I'd best check it over. I
hooked up live and neutral to the two connectors on the primary side of
it, and two probes from a voltmeter on the secondary.
Without anything else attached and no current drawn yet, I applied the
tip of a neon screwdriver to the four connections with the following
results:

Primary live connection: strong yellow light from the neon tester

Primary neutral connection: no response from the neon tester

2x Secondary connections: both equally light up the tester by about
half as much as the live primary did.

I was under the impression I would see no response at all from the neon
tester on the secondary side since it's supposedly floating; yet I
*did*
get a reaction of sorts from the tester.
What's going on here? Is the transformer faulty?

cheers,
cd.


**Just disconnect the transformer from the mains supply. Connect a
multimeter from primary to secondary and measure the resistance. It
should read infinity.

If it were that simple I'm sure someone would have already suggested it.
AFAIK - and I could be wrong I admit; it's not my field - most
transformers, isolation type or not, will pass this test so it doesn't
really tell us anything.

 

[Phil Allison]

Cursitor Doom wrote:

Trevor Wilson wrote:

**Just disconnect the transformer from the mains supply. Connect a
multimeter from primary to secondary and measure the resistance. It
should read infinity.

If it were that simple I'm sure someone would have already suggested it.

** It is that simple and someone just did.

AFAIK - and I could be wrong I admit; it's not my field - most
transformers, isolation type or not, will pass this test so it doesn't
really tell us anything.

** Only isolation transformers will pass such a test.

The other kind ( aka auto-transformers) will not.


..... Phil

 

[Trevor Wilson]

On 31/03/2015 8:52 AM, Cursitor Doom wrote:

Hi all,

Well, I picked up a 500VA mains isolation transformer today: 240v in;
240v out. It's not brand new so I thought I'd best check it over. I
hooked up live and neutral to the two connectors on the primary side of
it, and two probes from a voltmeter on the secondary.
Without anything else attached and no current drawn yet, I applied the
tip of a neon screwdriver to the four connections with the following
results:

Primary live connection: strong yellow light from the neon tester

Primary neutral connection: no response from the neon tester

2x Secondary connections: both equally light up the tester by about half
as much as the live primary did.

I was under the impression I would see no response at all from the neon
tester on the secondary side since it's supposedly floating; yet I *did*
get a reaction of sorts from the tester.
What's going on here? Is the transformer faulty?

cheers,
cd.

**Just disconnect the transformer from the mains supply. Connect a
multimeter from primary to secondary and measure the resistance. It
should read infinity.

--
Trevor Wilson
www.rageaudio.com.au

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