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torroidal transformer vendor

P

Pooh Bear

Jan 1, 1970
0
John said:
I read in sci.electronics.design that John Larkin <jjSNIPlarkin@highTHIS
landPLEASEtechnology.XXX> wrote (in <mu6j01ds31dgvsq8mfa0br3eft55946dbl@
4ax.com>) about 'torroidal transformer vendor', on Tue, 8 Feb 2005:


Which standard are you applying? It sounds to me that you may not be
applying the correct one.

I think you're right John.


Graham
 
J

John Larkin

Jan 1, 1970
0
This sounds very wrong. IEC regs simply apply component failure simulations
to test such stuff.



I think someone's misadvising you re: the CE safety test btw.

I have considerable experience of both safety and EMC CE approvals.

Is it a US lab that thinks you have to test that way ?


Graham


My customer wanted it CE tested, so we told them to do it themselves.
They took it to ITS, a big US test lab, and that's what they did. I'll
try to find out which spec they were working to.

But the transformer smoke test really only forced us to use the TT
fuses and the inrush limiters... it didn't really dominate the
transformer design, which was more of an available-space problem.

John
 
P

Paul Mathews

Jan 1, 1970
0
I'm still looking for a better reference, but I've been led to believe
(by a visiting rep) that this toroidal transformer technology, touted
for its EMI filtering benefits, also has improved inrush
characteristics:

http://powerelectronics.com/mag/power_power_transformer_attenuates/

Having just re-read the article, I find no mention there, so apologize
for possibly sending anyone on a goose chase. However, the EMI aspects
are interesting in their own right.
Paul Mathews
 
I

Ian

Jan 1, 1970
0
John Larkin said:
On 8 Feb 2005 16:51:22 -0800, "Paul Mathews" <[email protected]>
wrote:


As I told Speff, we have NTC surge limiters and TT fuzes, so that
seems to be under control.

John

Hi John:

I hit this problem (too) many years ago on a switcher - no toroid, but
serious
issues on inrush current "burning" switch contacts. Even back then the
NTC thermistor was an often used solution, but had a major problem
with time constant - there always seemed to be some timing of the user
"mis-hitting" the power switch that would bypass the protection.

What I did was to put a high voltage power MOSFET inside a bridge
rectifier in series with the input, arranged to always turn on, with a real
simple
R/transistor current sense to turn it off when the instantaneous current
exceeded a set point (together with gate protection). Worked a treat.
No idea if this would now fall foul of CE harmonics legislation, ask
John Woodgate. There may now be easier ways to do this, but it was
pretty cheap even back then.
 
J

John Larkin

Jan 1, 1970
0
Hi John:

I hit this problem (too) many years ago on a switcher - no toroid, but
serious
issues on inrush current "burning" switch contacts. Even back then the
NTC thermistor was an often used solution, but had a major problem
with time constant - there always seemed to be some timing of the user
"mis-hitting" the power switch that would bypass the protection.

What I did was to put a high voltage power MOSFET inside a bridge
rectifier in series with the input, arranged to always turn on, with a real
simple
R/transistor current sense to turn it off when the instantaneous current
exceeded a set point (together with gate protection). Worked a treat.
No idea if this would now fall foul of CE harmonics legislation, ask
John Woodgate. There may now be easier ways to do this, but it was
pretty cheap even back then.


I did a similar thing on one big amp: AC power series resistor to
limit surge, then shorted by a triac later on. We even cheated and
used it to improve transformer regulation, sort of a 1-step AC
switching regulator.

John
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that John Larkin <jjSNIPlarkin@highTHIS
landPLEASEtechnology.XXX> wrote (in <92dk015neh8nlivbuecd6si92or91135k6@
4ax.com>) about 'torroidal transformer vendor', on Wed, 9 Feb 2005:
But the transformer smoke test really only forced us to use the TT fuses
and the inrush limiters... it didn't really dominate the transformer
design, which was more of an available-space problem.

Is this transformer driving a motor or something? You should never need
TT fuses for an AC electronic load or a normal rectifier. Unless there
is something very unusual going on, the inrush phase is, for fusing
purposes, over in one cycle of the supply, and T fuses are OK for this
application.
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Ian <[email protected]>
What I did was to put a high voltage power MOSFET inside a bridge
rectifier in series with the input, arranged to always turn on, with a
real simple R/transistor current sense to turn it off when the
instantaneous current exceeded a set point (together with gate
protection). Worked a treat. No idea if this would now fall foul of CE
harmonics legislation, ask John Woodgate.

No. First 10 s after switch-on is not measured for harmonics. Clause
6.2.3.2 of IEC/EN 61000-3-2.
There may now be easier ways
to do this, but it was pretty cheap even back then.

This is an interesting idea for a high-power toroid, where NTCs and
other fixes are unattractive. You'd have to watch for spikes caused by
leakage reactance when the current is cut off suddenly.
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Paul Mathews
googlegroups.com>) about 'torroidal transformer vendor', on Wed, 9 Feb
2005:
I'm still looking for a better reference, but I've been led to believe
(by a visiting rep) that this toroidal transformer technology, touted
for its EMI filtering benefits, also has improved inrush
characteristics:

http://powerelectronics.com/mag/power_power_transformer_attenuates/

Having just re-read the article, I find no mention there, so apologize
for possibly sending anyone on a goose chase. However, the EMI aspects
are interesting in their own right.
Paul Mathews
I recommend a good deal of caution when considering this paper.
 
J

John Larkin

Jan 1, 1970
0
I read in sci.electronics.design that John Larkin <jjSNIPlarkin@highTHIS
landPLEASEtechnology.XXX> wrote (in <92dk015neh8nlivbuecd6si92or91135k6@
4ax.com>) about 'torroidal transformer vendor', on Wed, 9 Feb 2005:


Is this transformer driving a motor or something? You should never need
TT fuses for an AC electronic load or a normal rectifier. Unless there
is something very unusual going on, the inrush phase is, for fusing
purposes, over in one cycle of the supply, and T fuses are OK for this
application.


It's a pulsed-field gradient amplifier for NMR. The TTs were to allow
us to use the smallest possible fuse rating (to pass the transformer
overload test) and still not blow on high peak pulse loads or
toroid+capacitor powerup surges. I didn't do this personally, but I do
recall that the choice of the relatively hard-to-get TTs wasn't
casual.

John
 
J

John Larkin

Jan 1, 1970
0
I read in sci.electronics.design that John Larkin <jjSNIPlarkin@highTHIS
landPLEASEtechnology.XXX> wrote (in <mu6j01ds31dgvsq8mfa0br3eft55946dbl@
4ax.com>) about 'torroidal transformer vendor', on Tue, 8 Feb 2005:


Which standard are you applying? It sounds to me that you may not be
applying the correct one.

Apparently it's IEC/EN 61010 2001 Edition, Clause 4.4.2.6.2
and UL 3101, 1993 Edition, Clause 14.7.2

John
 
P

Pooh Bear

Jan 1, 1970
0
John said:
I read in sci.electronics.design that Paul Mathews
googlegroups.com>) about 'torroidal transformer vendor', on Wed, 9 Feb
2005:

I recommend a good deal of caution when considering this paper.

The fact that it attemps to encompass the issues of harmonics and EMI in one
overall broad sweep shows the author's basic lack of understanding.


Graham
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that John Larkin <jjlarkin@highSNIPland
THIStechPLEASEnology.com> wrote (in <3sgl01h9kh5dst54uacb30sk2t3eua2dfm@
4ax.com>) about 'torroidal transformer vendor', on Wed, 9 Feb 2005:
Apparently it's IEC/EN 61010 2001 Edition, Clause 4.4.2.6.2
and UL 3101, 1993 Edition, Clause 14.7.2
Unfortunately, I don't have either of those standards. Maybe someone
else can comment. You are sure that the product falls within the scope
of IEC/EN 61010?
 
J

John Larkin

Jan 1, 1970
0
I read in sci.electronics.design that John Larkin <jjlarkin@highSNIPland
THIStechPLEASEnology.com> wrote (in <3sgl01h9kh5dst54uacb30sk2t3eua2dfm@
4ax.com>) about 'torroidal transformer vendor', on Wed, 9 Feb 2005:
Unfortunately, I don't have either of those standards. Maybe someone
else can comment. You are sure that the product falls within the scope
of IEC/EN 61010?


I'm not sure of anything. My customer wanted our boxes tested to CE
and UL, so we told them to test them themselves, and they did. They
told us that the loaded-transformer-smoke-test was required.

But if the only function of a fuse is to clear shorts, why can't I
just slap in 10 amp fuses and be done with it?

And if the transformer overload test isn't required, then I should be
able to use a cheap transformer that will smoke and catch fire if,
say, a fet in the output stage failed.

Right?

John
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that John Larkin <jjSNIPlarkin@highTHIS
landPLEASEtechnology.XXX> wrote (in <ua1n01lg3kh7ms6fgldndt1kg4i2cjf1sp@
4ax.com>) about 'torroidal transformer vendor', on Thu, 10 Feb 2005:
I'm not sure of anything. My customer wanted our boxes tested to CE and
UL, so we told them to test them themselves, and they did. They told us
that the loaded-transformer-smoke-test was required.

But if the only function of a fuse is to clear shorts, why can't I just
slap in 10 amp fuses and be done with it?

The 'smoke test' is indeed designed to prevent that.
And if the transformer overload test isn't required, then I should be
able to use a cheap transformer that will smoke and catch fire if, say,
a fet in the output stage failed.

That's not quite the point. IEC/EN 61010-1 is unusual in requiring that
test, which some consider too stringent, insofar as it assumes a fault
with just the critical worst-case resistance. Other safety standards use
a different test.
 
P

Pooh Bear

Jan 1, 1970
0
John said:
It's a pulsed-field gradient amplifier for NMR. The TTs were to allow
us to use the smallest possible fuse rating (to pass the transformer
overload test) and still not blow on high peak pulse loads or
toroid+capacitor powerup surges. I didn't do this personally, but I do
recall that the choice of the relatively hard-to-get TTs wasn't
casual.

I'm not familiar with the specific EN you mentioned. Seems to be rather more
application specific than the ones I'm used to.

Can you explain the 'transformer overload test ' ? Sounds most unusual to me.


Graham
 
P

Pooh Bear

Jan 1, 1970
0
John said:
I'm not sure of anything. My customer wanted our boxes tested to CE
and UL, so we told them to test them themselves, and they did. They
told us that the loaded-transformer-smoke-test was required.

But if the only function of a fuse is to clear shorts, why can't I
just slap in 10 amp fuses and be done with it?

A short on the secondary side will happily draw maybe 5A from the a.c. supply
and the fuse will never blow.

Just try it and see.

A colleague of mine was once convinced that he could protect a toroidal
transformer of around 30VA with a 1A fuse on the primary. I warned him but he
insisted. We put a short on the secondary and we rapidly had one transformer
with nicely melted insulation after a few secs ! Didn't wait to see the final
outcome.

And if the transformer overload test isn't required, then I should be
able to use a cheap transformer that will smoke and catch fire if,
say, a fet in the output stage failed.

Nope - nothing is allowed to smoke and catch fire.


Graham
 
J

John Larkin

Jan 1, 1970
0
A short on the secondary side will happily draw maybe 5A from the a.c. supply
and the fuse will never blow.

Just try it and see.

No need; I can do the math. More like 50 amps.
A colleague of mine was once convinced that he could protect a toroidal
transformer of around 30VA with a 1A fuse on the primary. I warned him but he
insisted. We put a short on the secondary and we rapidly had one transformer
with nicely melted insulation after a few secs ! Didn't wait to see the final
outcome.



Nope - nothing is allowed to smoke and catch fire.

Which is why, presumably, they did the test.

John
 
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