very low power line isolation transformer

[Norm Dresner]

Many decades ago at my then favorite surplus electronics shop I found a
transformer that was about 1"x1"x1" which turned out to be a 1:1 transformer
with about 10 ma max output with a 110V 60Hz AC line input. I quickly
installed it in my shop-built bench power supply as a "safe" means of
testing unknown transformers and such and have used it since for just that
purpose. Now I find myself needing a few more -- say 2-4. I can't find any
10ma 110V transformers in the catalogs that I have. I'm hoping that someone
either knows of a manufacturer who makes a similar unit or can suggest a
practical alternative. Could I safely use a 1:1 audio output transformer?

TIA
Norm

 

[mike]

Many decades ago at my then favorite surplus electronics shop I found a
transformer that was about 1"x1"x1" which turned out to be a 1:1 transformer
with about 10 ma max output with a 110V 60Hz AC line input. I quickly
installed it in my shop-built bench power supply as a "safe" means of
testing unknown transformers and such and have used it since for just that
purpose. Now I find myself needing a few more -- say 2-4. I can't find any
10ma 110V transformers in the catalogs that I have. I'm hoping that someone
either knows of a manufacturer who makes a similar unit or can suggest a
practical alternative. Could I safely use a 1:1 audio output transformer?

Except that audio output transformers usually aren't 1:1 Nor are they
insulated or safety rated for line use. If your house burns down or
anyone gets hurt, you're gonna have some 'splainin to do.

How about a couple of wall warts back to back? They ARE rated for line use.

mike

TIA
Norm

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[Arshad Shaikh]

Hello Norm,

One alternative that you can use is to couple two (110 to 12V 100mA)
transformers in series as follows:

| |
| 110V |
| |
wwwwwwwww
========= T1
mmmmmmmmm
| |
| 12 V |
| |
x x
| |
| 12 V |
| |
wwwwwwwww
========= T2
mmmmmmmmm
| |
| 110V |
| |

The net effect will be like a single 1:1 transformer for 110V at
10.9mA approx.

Please note that you have to couple the 12V sides the two transfomers.

The above suggestion is what I would have done if posed to this
problem but u try it at your own risk, I will not be responsible for
any damages.

Good Luck.

-Arshad Shaikh.

 

[.:: Evanescence ::.]

Norm Dresner escreveu no sci.electronics.components:

Could I safely use a 1:1 audio output transformer?

No. They're not made for this

[]s

 

[Watson A.Name \Watt Sun - the Dark Remover\]

Many decades ago at my then favorite surplus electronics shop I found a
transformer that was about 1"x1"x1" which turned out to be a 1:1 transformer
with about 10 ma max output with a 110V 60Hz AC line input. I quickly
installed it in my shop-built bench power supply as a "safe" means of
testing unknown transformers and such and have used it since for just that
purpose. Now I find myself needing a few more -- say 2-4. I can't find any
10ma 110V transformers in the catalogs that I have. I'm hoping that someone
either knows of a manufacturer who makes a similar unit or can suggest a
practical alternative. Could I safely use a 1:1 audio output transformer?

TIA
Norm

Audio output transformers were not made for 120VAC 60 Hz. or whatever.
Use two regular low current filament transformers back-to-back. Connect
the secondaries together, and put 120VAC on one, and the other will have
120VAC on its primary. You can put a low value resistor in series with
one lead of the secondaries, and it will limit the max output current.
Adjust the resistor to get the current you want. But make sure that
it's capable of handling the power dissipation. A wirewound resistor
would be appropriate.

 

[Watson A.Name \Watt Sun - the Dark Remover\]

Except that audio output transformers usually aren't 1:1 Nor are they
insulated or safety rated for line use. If your house burns down or
anyone gets hurt, you're gonna have some 'splainin to do.

How about a couple of wall warts back to back? They ARE rated for line
use.

That's the same as my suggestion to use a pair of filament transformers
back-to-back. However, make sure you use AC wall warts, not DC!! And
one other thing: wall warts have an internal fuse that is nearly
impossible to replace if it burns out. So use a resistor in series with
the secondaries to limit the current as I suggested in my other followup.

 

[Norm Dresner]

[Okay, I goofed.] I should have made it clear when I wrote the original
question that I already knew how to connect similar transformers back to
back to get the isolation I wanted. [Twice] But I also failed to make it
clear that size was an important consideration and that the 1"x1"x1" outline
of the original transformer -- while not absolute -- was a very desirable
goal.

I think that the best course, failing to (get lucky and) find a similar
transformer again, synthesizing the suggestions from several people would be
to cut apart some low-power, inexpensive wall-warts and to connect those
transformers as suggested. I could -- if I get really ambitious -- put a
replaceable fuse in the intermediate secondary to provide even more
protection.

I just placed an order with Jameco [another sob-story, I won't be doing that
any time soon again] which included an empty wall-wart shell and plug. I
may try to see if I can stuff two transformers from the smallest wall warts
I can find into this one and then close it up securely. I suppose if I
really got really ambitious, I could provide a sliding cover to a
compartment that held a (possibly automotive-type blade) fuse to make the
whole thing easier to service ... Hmmm .. I wonder if there's a market for
anything like this ;-)

Thanks to all who answered.

Norm

 

[Watson A.Name \Watt Sun - the Dark Remover\]

mike wrote:


That's the same as my suggestion to use a pair of filament transformers
back-to-back. However, make sure you use AC wall warts, not DC!! And
one other thing: wall warts have an internal fuse that is nearly
impossible to replace if it burns out. So use a resistor in series with
the secondaries to limit the current as I suggested in my other followup.

One other detail I forgot to point out is that when you plug one of the
wall warts in, the other then has two power prongs sticking out of it
that are _very_live_ and pose a shock hazard. So don't leave it laying
around.

 

[Watson A.Name \Watt Sun - the Dark Remover\]

[Okay, I goofed.] I should have made it clear when I wrote the original
question that I already knew how to connect similar transformers back to
back to get the isolation I wanted. [Twice] But I also failed to make it
clear that size was an important consideration and that the 1"x1"x1" outline
of the original transformer -- while not absolute -- was a very desirable
goal.

I think that the best course, failing to (get lucky and) find a similar
transformer again, synthesizing the suggestions from several people would be
to cut apart some low-power, inexpensive wall-warts and to connect those
transformers as suggested. I could -- if I get really ambitious -- put a
replaceable fuse in the intermediate secondary to provide even more
protection.

I just placed an order with Jameco [another sob-story, I won't be doing that
any time soon again] which included an empty wall-wart shell and plug. I
may try to see if I can stuff two transformers from the smallest wall warts
I can find into this one and then close it up securely. I suppose if I
really got really ambitious, I could provide a sliding cover to a
compartment that held a (possibly automotive-type blade) fuse to make the
whole thing easier to service ... Hmmm .. I wonder if there's a market for
anything like this ;-)

Yeah, a big market... There are these soldiers in this Abu something or
other prison over there that could use quite a few, for administering
'mild stimulation' to prisoners on various parts of their anatomy...

 

[Roger Hamlett]

[Okay, I goofed.] I should have made it clear when I wrote the original
question that I already knew how to connect similar transformers back to
back to get the isolation I wanted. [Twice] But I also failed to make it
clear that size was an important consideration and that the 1"x1"x1" outline
of the original transformer -- while not absolute -- was a very desirable
goal.

I think that the best course, failing to (get lucky and) find a similar
transformer again, synthesizing the suggestions from several people would be
to cut apart some low-power, inexpensive wall-warts and to connect those
transformers as suggested. I could -- if I get really ambitious -- put a
replaceable fuse in the intermediate secondary to provide even more
protection.

I just placed an order with Jameco [another sob-story, I won't be doing that
any time soon again] which included an empty wall-wart shell and plug. I
may try to see if I can stuff two transformers from the smallest wall warts
I can find into this one and then close it up securely. I suppose if I
really got really ambitious, I could provide a sliding cover to a
compartment that held a (possibly automotive-type blade) fuse to make the
whole thing easier to service ... Hmmm .. I wonder if there's a market for
anything like this ;-)

Thanks to all who answered.

Norm

Thinking about this, the smallest 1:1 transformers, in 'common use', are
the units inside the bathroom 'strip lights', designed to isolate the
shaver point. These are typically about 1.5*1.5*1, and would be smaller
than the back-to-back transformer solutions proposed. One of these should
fit in a wall-wart shell.

Best Wishes

 

[Norm Dresner]

[Okay, I goofed.] I should have made it clear when I wrote the original
question that I already knew how to connect similar transformers back to
back to get the isolation I wanted. [Twice] But I also failed to make it
clear that size was an important consideration and that the 1"x1"x1" outline
of the original transformer -- while not absolute -- was a very desirable
goal.

I think that the best course, failing to (get lucky and) find a similar
transformer again, synthesizing the suggestions from several people would be
to cut apart some low-power, inexpensive wall-warts and to connect those
transformers as suggested. I could -- if I get really ambitious -- put a
replaceable fuse in the intermediate secondary to provide even more
protection.

I just placed an order with Jameco [another sob-story, I won't be doing that
any time soon again] which included an empty wall-wart shell and plug. I
may try to see if I can stuff two transformers from the smallest wall warts
I can find into this one and then close it up securely. I suppose if I
really got really ambitious, I could provide a sliding cover to a
compartment that held a (possibly automotive-type blade) fuse to make the
whole thing easier to service ... Hmmm .. I wonder if there's a market for
anything like this ;-)

Thanks to all who answered.

Norm

Thinking about this, the smallest 1:1 transformers, in 'common use', are
the units inside the bathroom 'strip lights', designed to isolate the
shaver point. These are typically about 1.5*1.5*1, and would be smaller
than the back-to-back transformer solutions proposed. One of these should
fit in a wall-wart shell.

Best Wishes

That's great. I'll see if I can't scrounge an old light strip to check this
out.

Thank
Norm