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Old November 14th 08, 04:32 AM posted to rec.radio.amateur.homebrew
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Default HiPot testing my filament xformer

I built my own 7.5VACT 21amp filament xformer. I needed to supply
-500VDC screen voltage to the center tap.So a local ham tells me the
insulation better be able to withstand at least 500VDC.

I built my own hipot tester from a voltage doubler and a bank of eight
400WVDC caps. Each cap had a 330 volt charge so like 2640VDC. I
figured no problem. I looky up the specs and see if this thingy
passes.

I googled and can find no specs for hipot testing. I came across in a
usernet group: 2500VAC at 1ma for 1 minute. That didn't really tell me
much.

My results are 0,2 micro amps thru a 1 meg series resistor at 1980VDC
source voltage. This from either the core or the primary to secondary.
The current didn't change even when I went to a 10meg series resistor.

Is there nowhere that you can get specs without paying big bucks for a
UL or IEEE book on the topic?

73
n8zu
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Old November 14th 08, 08:04 AM posted to rec.radio.amateur.homebrew
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Posts: 18
Default HiPot testing my filament xformer

HI from Ken, G4KIR,

Why not just borrow ( if you don't own one that is ) an Electricians
insulation tester, known over here in the UK as a "MEGGER".
These units can produce around 250-1000volts (if you can get hold of a good
one).
This will test your insulation no problem.

Hope this helps, cheers, ken, G4KIR.

"raypsi" wrote in message
...
I built my own 7.5VACT 21amp filament xformer. I needed to supply
-500VDC
73
n8zu



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Old November 14th 08, 09:57 AM posted to rec.radio.amateur.homebrew
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First recorded activity by RadioBanter: Sep 2006
Posts: 14
Default HiPot testing my filament xformer


"raypsi" wrote in message
...
I built my own 7.5VACT 21amp filament xformer. I needed to supply
-500VDC screen voltage to the center tap.So a local ham tells me the
insulation better be able to withstand at least 500VDC.

I built my own hipot tester from a voltage doubler and a bank of eight
400WVDC caps. Each cap had a 330 volt charge so like 2640VDC. I
figured no problem. I looky up the specs and see if this thingy
passes.

I googled and can find no specs for hipot testing. I came across in a
usernet group: 2500VAC at 1ma for 1 minute. That didn't really tell me
much.

My results are 0,2 micro amps thru a 1 meg series resistor at 1980VDC
source voltage. This from either the core or the primary to secondary.
The current didn't change even when I went to a 10meg series resistor.

Is there nowhere that you can get specs without paying big bucks for a
UL or IEEE book on the topic?


I'm surprised that you couldn't find UL, IEEE or MIL-SPEC (or MIL-STD)
sources on the net through Google or any other search engine.
Just taking the snippet you did come across, I believe that it means that
you must maintain a leakage current less than 1 ma. for 1 minute with an
applied voltage of 2500VAC (about 3500V peak). This is so far above your
typical operating conditions, I'm tempted to say you passed. You do mention
that your results were from the primary OR core to the secondary instead of
primary AND core to the secondary, which is typical for this test, but I'm
not sure it matters for a home-built. I used to have to do a test like this
using 1500VDC with an automatic tester. If there was a problem with
insulation, it showed up quickly. If the room lights were dimmed, you could
see the place where it failed by the arc to ground.

-NM



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Old November 14th 08, 03:10 PM posted to rec.radio.amateur.homebrew
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Posts: 87
Default HiPot testing my filament xformer

Howdy,

A good rule of thumb (derived from many UL specifications)
for equipment with a safety ground is use twice the operating
voltage plus 1KV AC with a test period of one minute.

For reduced test time, one second is allowed for _production_line_
testing with increased test voltage

So for equipment operating at 115VAC you'd qualifying test of
1230VAC from primary to secondary for one minute (use 1250VAC.)

Isolating components like transformers often have to meet the
requirements for double insulation so they are tested at twice
the basic test voltage which is where the 2500VAC test value
comes from.

Often the basic test voltage 1500VAC is applied from input to
ground (chassis) and the double insulation test voltage of
2500VAC is applied from input to output. Because the output
is not tied to the safety ground.

The criteria for failure is non-linear rise of applied voltage or
a sustained arc or measured current above some value. The current
limit varies but 3mA is a good value for equipment that is safety
grounded (not double insulated.)

UL also requires the hi-pot machine to be capable of 500VA output.

Double insulated transformers tested to IEC standards for the EU
with it's higher nominal line voltage may be subjected to hi-pot
voltages as high as 1500V primary to ground and 4000VAC primary
to secondary for one minute.


Caveats

Hi-pot only a test. There are are dimensional requirements
(safety from conductor spacings) and requirements for durability
of insulating materials. You could still grossly violate the
standards, producing a dangerous component, and pass hi-pot.
Two pieces of heavy build magnet wire twisted together will
usually pass 5KVAC hipot, would you trust that with your life?

A person should to follow and understand the applicable standards
under fear of fire, injury or death.


And as always, all of the above may be horribly wrong.


73,
Grumpy


raypsi wrote in news:57a0b89a-3205-4ca1-b5e9-
:

I built my own 7.5VACT 21amp filament xformer. I needed to supply
-500VDC screen voltage to the center tap.So a local ham tells me the
insulation better be able to withstand at least 500VDC.

I built my own hipot tester from a voltage doubler and a bank of eight
400WVDC caps. Each cap had a 330 volt charge so like 2640VDC. I
figured no problem. I looky up the specs and see if this thingy
passes.

I googled and can find no specs for hipot testing. I came across in a
usernet group: 2500VAC at 1ma for 1 minute. That didn't really tell me
much.

My results are 0,2 micro amps thru a 1 meg series resistor at 1980VDC
source voltage. This from either the core or the primary to secondary.
The current didn't change even when I went to a 10meg series resistor.

Is there nowhere that you can get specs without paying big bucks for a
UL or IEEE book on the topic?

73
n8zu


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Old November 14th 08, 03:17 PM posted to rec.radio.amateur.homebrew
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First recorded activity by RadioBanter: Sep 2008
Posts: 87
Default HiPot testing my filament xformer

Grumpy The Mule wrote in
:

Howdy,

A good rule of thumb (derived from many UL specifications)
for equipment with a safety ground is use twice the operating
voltage plus 1KV AC with a test period of one minute.

For reduced test time, one second is allowed for _production_line_
testing with increased test voltage

So for equipment operating at 115VAC you'd qualifying test of
1230VAC from primary to secondary for one minute (use 1250VAC.)

Isolating components like transformers often have to meet the
requirements for double insulation so they are tested at twice
the basic test voltage which is where the 2500VAC test value
comes from.

Often the basic test voltage 1500VAC is applied from input to
ground (chassis) and the double insulation test voltage of
2500VAC is applied from input to output. Because the output
is not tied to the safety ground.

The criteria for failure is non-linear rise of applied voltage or
a sustained arc or measured current above some value. The current
limit varies but 3mA is a good value for equipment that is safety
grounded (not double insulated.)

UL also requires the hi-pot machine to be capable of 500VA output.

Double insulated transformers tested to IEC standards for the EU
with it's higher nominal line voltage may be subjected to hi-pot
voltages as high as 1500V primary to ground and 4000VAC primary
to secondary for one minute.


Caveats

Hi-pot only a test. There are are dimensional requirements
(safety from conductor spacings) and requirements for durability
of insulating materials. You could still grossly violate the
standards, producing a dangerous component, and pass hi-pot.
Two pieces of heavy build magnet wire twisted together will
usually pass 5KVAC hipot, would you trust that with your life?

A person should to follow and understand the applicable standards
under fear of fire, injury or death.


Sorry, no coffee yet. I should have wrote...

Often the basic test voltage 1250VAC is applied from input to
ground (chassis) and the double insulation test voltage of
2500VAC is applied from input to output. Because the output
is not tied to the safety ground.


Grumpy The Mule wrote in
:

Often the basic test voltage 1500VAC is applied from input to
ground (chassis) and the double insulation test voltage of
2500VAC is applied from input to output. Because the output
is not tied to the safety ground.



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Old November 14th 08, 03:17 PM posted to rec.radio.amateur.homebrew
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First recorded activity by RadioBanter: Sep 2008
Posts: 87
Default HiPot testing my filament xformer

Sorry, no coffee yet. I should have wrote...

Often the basic test voltage 1250VAC is applied from input to
ground (chassis) and the double insulation test voltage of
2500VAC is applied from input to output. Because the output
is not tied to the safety ground.


Grumpy The Mule wrote in
:

Often the basic test voltage 1500VAC is applied from input to
ground (chassis) and the double insulation test voltage of
2500VAC is applied from input to output. Because the output
is not tied to the safety ground.


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Old November 14th 08, 07:05 PM posted to rec.radio.amateur.homebrew
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First recorded activity by RadioBanter: Sep 2008
Posts: 87
Default HiPot testing my filament xformer


Getting side tracked by the mention of agency requirements,
I didn't answer your question.

To answer your question, there is no agency requirement for
what you're trying to test. Both input and output are not
covered by the low voltage directive because they're both
unsafe to touch. So the only requirement is operational
insulation which your test proved.

I also failed to mention that Most standards allow for DC
testing at 1.414 times the AC voltage required.

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Old November 15th 08, 03:51 AM posted to rec.radio.amateur.homebrew
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First recorded activity by RadioBanter: Jul 2008
Posts: 242
Default HiPot testing my filament xformer

On Nov 14, 1:05*pm, Grumpy The Mule wrote:
Getting side tracked by the mention of agency requirements,
I didn't answer your question.

To answer your question, there is no agency requirement for
what you're trying to test. *Both input and output are not
covered by the low voltage directive because they're both
unsafe to touch. *So the only requirement is operational
insulation which your test proved.

I also failed to mention that Most standards allow for DC
testing at 1.414 times the AC voltage required.


Thank you OM my transformer will not pass the 1500VAC test but it does
ok at 1350VAC, leakage down to 0,2 nano amps after I got rid of the
stray leakage caused by my test set up, always best to zero out the
hipot tester first. Thus eliminating the stray leakage. Must have
nicked the kapton insulation for it to breakdown at 1500VAC Now I'll
have to pot the transformer in epoxy.

73 OM

n8zu
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Old November 15th 08, 08:48 AM posted to rec.radio.amateur.homebrew
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First recorded activity by RadioBanter: Sep 2008
Posts: 87
Default HiPot testing my filament xformer

Howdy,


Kapton tape is great stuff. It's best to use at least
two full wraps. Some standards for reenforced or double
insulation require three wraps where any two can
withstand the hi-pot. Even where it's not required
the two full wraps minimum is generally a good practice.

The breakdown could be due to insufficient creepage distance
between the windings. If you can build up the bobbin with
tape at both ends to form a barrier which keeps the winding
centered it will help.

Another thing to do is sleeve the wire where it leaves the
winding. A tape "pad" under the lead-out where it crosses
the barrier tape is another option.

Although Kapton is resistant to temperature you're not building
with an insulation system where the components have been aged
with each other at high temperature in oxygen to see if there's
any degredation due to chemical reactions. So the temperature
rise of the transformer should be limited to 65C above ambient.
If it's too hot to hold your finger on for about three seconds
it's too hot. I'd call it a rule of thumb but I'd use the
index finger for that test.

A person should to follow the applicable standards under fear
of fire, injury or death. And as always, all of the above may
be horribly wrong.


73,
Grumpy
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