Cheap Isolation Transformer ??

Jim Thompson · Dec 9, 2003

I need an isolation transformer, 1:1, 115VAC input/output, at about
100mA.

Everything I can quickly locate is big and expensive.

Any problems I could get into if I simply used two cheap filament
transformers back to back?

Thanks!

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice

480)460-2350 | |
| E-mail Address at Website Fax

John Fields · Dec 10, 2003

On Tue, 09 Dec 2003 16:52:46 -0700, Jim Thompson
<invalid@invalid.invalid> wrote:

I need an isolation transformer, 1:1, 115VAC input/output, at about
100mA.

Everything I can quickly locate is big and expensive.

Any problems I could get into if I simply used two cheap filament
transformers back to back?

---
Don't use the primaries as backs...

--
John Fields

Jim Thompson · Dec 10, 2003

On Tue, 09 Dec 2003 18:06:53 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Tue, 09 Dec 2003 16:52:46 -0700, Jim Thompson
invalid@invalid.invalid> wrote:

I need an isolation transformer, 1:1, 115VAC input/output, at about
100mA.

Everything I can quickly locate is big and expensive.

Any problems I could get into if I simply used two cheap filament
transformers back to back?

---
Don't use the primaries as backs...

Now would *I* do something like that ?

480)460-2350 | |
| E-mail Address at Website Fax

Michael A. Terrell · Dec 10, 2003

Jim Thompson wrote:

I need an isolation transformer, 1:1, 115VAC input/output, at about
100mA.

Everything I can quickly locate is big and expensive.

Any problems I could get into if I simply used two cheap filament
transformers back to back?

Thanks!

...Jim Thompson

Why waste two transformers? MCM 28-3480 1A 120 VAC Isolation
Transformer $14.23 each. page 169, catalog #48. http://www.mcminone.com
--
16 days!

Michael A. Terrell
Central Florida

artie · Dec 10, 2003

In article <u1octvoj244vv76es9vmovnha97b6f6jfb@4ax.com>, Jim Thompson
<invalid@invalid.invalid> wrote:

I need an isolation transformer, 1:1, 115VAC input/output, at about
100mA.

Everything I can quickly locate is big and expensive.

Any problems I could get into if I simply used two cheap filament
transformers back to back?

Thanks!

...Jim Thompson

A lot of power transformers these days have dual primaries -- I use a
small one to float an audio preamp...

Chris Carlen · Dec 10, 2003

Jim Thompson wrote:

I need an isolation transformer, 1:1, 115VAC input/output, at about
100mA.

Everything I can quickly locate is big and expensive.

Any problems I could get into if I simply used two cheap filament
transformers back to back?

Thanks!

...Jim Thompson

Jim, why are *you* asking *this*? It sounds like a newbie question.

Unless you have some critical requirements, I can't see why it's not
obviously a reasonable thing to do. But you know that.

Strange. Have you been crawling around the insides of chips for too
long? Heck, you could have simulated two trannys and found out it'd
work, with just a somewhat more mediocre regulation than with a
dedicated one, right?! :-D

Anyway, a respectable Stancor P-6411 straight isolation transformer
(with leads), 115VAC, 0.13A, 15VA is $15.37 from Allied, catalog # 928-3033.

If you want one with a cover, plug and socket, they get a little more
expensive, the Stancor GIS-100 0.87A 100VA is $63.05 catalog # 928-0325.

I recently bought a 500VA Stancor, and it's real nice. I returned a
Hammond 500VA because it got too hot.

Good day!

--
_____________________
Christopher R. Carlen
crobc@earthlink.net
Suse 8.1 Linux 2.4.19

Ralph & Diane Barone · Dec 10, 2003

In article <u1octvoj244vv76es9vmovnha97b6f6jfb@4ax.com>,
Jim Thompson <invalid@invalid.invalid> wrote:

I need an isolation transformer, 1:1, 115VAC input/output, at about
100mA.

Everything I can quickly locate is big and expensive.

Any problems I could get into if I simply used two cheap filament
transformers back to back?

Thanks!

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

If you also need low voltage power, try one of the universal dual-primary
120/240 V: whatever transformers.

Or rip the isolation transformer out of the shaver outlet in your bathroom.

Jim Thompson · Dec 10, 2003

On Tue, 09 Dec 2003 19:43:21 -0800, Chris Carlen
<crobc@BOGUS_FIELD.earthlink.net> wrote:

Jim Thompson wrote:
I need an isolation transformer, 1:1, 115VAC input/output, at about
100mA.

Everything I can quickly locate is big and expensive.

Any problems I could get into if I simply used two cheap filament
transformers back to back?

Thanks!

...Jim Thompson

Jim, why are *you* asking *this*? It sounds like a newbie question.

Unless you have some critical requirements, I can't see why it's not
obviously a reasonable thing to do. But you know that.

Strange. Have you been crawling around the insides of chips for too
long? Heck, you could have simulated two trannys and found out it'd
work, with just a somewhat more mediocre regulation than with a
dedicated one, right?! :-D

Anyway, a respectable Stancor P-6411 straight isolation transformer
(with leads), 115VAC, 0.13A, 15VA is $15.37 from Allied, catalog # 928-3033.

If you want one with a cover, plug and socket, they get a little more
expensive, the Stancor GIS-100 0.87A 100VA is $63.05 catalog # 928-0325.

I recently bought a 500VA Stancor, and it's real nice. I returned a
Hammond 500VA because it got too hot.

Good day!

480)460-2350 | |
| E-mail Address at Website Fax

Tony Roe · Dec 10, 2003

The most problem with identical filament transformers back-to-back is the
regulation, which can easily be 10-20% for each small transformers, so 20-40%
for the pair. So with 115VAC in, the output starts out with 110-114VAC at no
load, but might drop under 80VAC on load.

At the higher power end, I used a pair of 60+60VAC/500VA toroids (both
conventional step-downs, with the secondaries in series and good secondary
isolation to case) to make a cheap(ish) isolated balanced 230VAC supply for the
studio.

On Tue, 09 Dec 2003 22:34:34 -0700, Jim Thompson <invalid@invalid.invalid>
wrote:

On Tue, 09 Dec 2003 19:43:21 -0800, Chris Carlen
crobc@BOGUS_FIELD.earthlink.net> wrote:

Jim Thompson wrote:
I need an isolation transformer, 1:1, 115VAC input/output, at about
100mA.

Everything I can quickly locate is big and expensive.

Any problems I could get into if I simply used two cheap filament
transformers back to back?

Thanks!

...Jim Thompson

Jim, why are *you* asking *this*? It sounds like a newbie question.

Unless you have some critical requirements, I can't see why it's not
obviously a reasonable thing to do. But you know that.

Strange. Have you been crawling around the insides of chips for too
long? Heck, you could have simulated two trannys and found out it'd
work, with just a somewhat more mediocre regulation than with a
dedicated one, right?! :-D

Anyway, a respectable Stancor P-6411 straight isolation transformer
(with leads), 115VAC, 0.13A, 15VA is $15.37 from Allied, catalog # 928-3033.

If you want one with a cover, plug and socket, they get a little more
expensive, the Stancor GIS-100 0.87A 100VA is $63.05 catalog # 928-0325.

I recently bought a 500VA Stancor, and it's real nice. I returned a
Hammond 500VA because it got too hot.

Good day!

I don't handle transformers every day, thus the question.

Bought two 25.2V/450mA filament transformers for $11.05 including tax.

...Jim Thompson

Tony (remove the "_" to reply by email)

Bill Sloman · Dec 10, 2003

Jim Thompson <invalid@invalid.invalid> wrote in message news:<u1octvoj244vv76es9vmovnha97b6f6jfb@4ax.com>...

I need an isolation transformer, 1:1, 115VAC input/output, at about
100mA.

Everything I can quickly locate is big and expensive.

Any problems I could get into if I simply used two cheap filament
transformers back to back?

The only thing you might have to keep in mind is that the primary
windings are sized to carry both the rated VA product current and the
magnetising current.

The data sheet doesn't usually tell you what the magnetising current
is, and it can be quite big for small transformers. A preliminary
measurement to establish the magnetising (no-load) current through the
primary might be in order to let you work out how much you have to
reduce the VA rating for the pair.

Since I'm on your kill-file, you aren't going to get to read this
unless I've made a mistake and someone posts a correction.

------
Bill Sloman, Nijmegen

N. Thornton · Dec 10, 2003

Tony Roe <tony_roe@tpg.com.au> wrote in message news:<shfdtvsutceocgbbf9v2hqo0s47lrm13jd@4ax.com>...

The most problem with identical filament transformers back-to-back is the
regulation, which can easily be 10-20% for each small transformers, so 20-40%
for the pair. So with 115VAC in, the output starts out with 110-114VAC at no
load, but might drop under 80VAC on load.

Jim Thompson wrote:
I need an isolation transformer, 1:1, 115VAC input/output, at about
100mA.

Everything I can quickly locate is big and expensive.

Any problems I could get into if I simply used two cheap filament
transformers back to back?

There's another probem waiting for you too Jim. Transformers have
copper losses, so your open circuit V_out for the first TF might be
say 10% or 20% high off load. This means the 2nd TF dosent have the
turns ratio one would think, and when driven at nominal V on its
secondary will never produce V_mains at its primary, it will always be
low.

The quick dirty best avoided way is to use TFs with primary taps -
here TFs had primaries with 200,220,240v taps for a long time:
adjusting the tap setting can correct the problem. But to do that one
must ensure the thing is properly fused, thermally cut-out-ed, and
fireproofly housed, since doing so can result in substantial
overcurrents, shorting metlting and fire. So for those that dont
understand how to engineer safely round the implications of that,
dont.

Using TFs rated at well over what youre drawing helps, since the
deregulation drops to only a percentage of the nominal regulation
figure.

A better way is to use a variac with your iso setup. A better way than
that is to add a few more turns onto your first TF's secondary to get
the right off load V_out on TF2. And an even better option is to add
more primary turns on TF2, long as you understand how to do it without
compromising insulation.

At best back to backing is a rough hack. Is that what you need?

Regards, NT

budgie · Dec 10, 2003

On Tue, 09 Dec 2003 17:32:30 -0700, Jim Thompson
<invalid@invalid.invalid> wrote:

On Tue, 09 Dec 2003 18:06:53 -0600, John Fields
jfields@austininstruments.com> wrote:

On Tue, 09 Dec 2003 16:52:46 -0700, Jim Thompson
invalid@invalid.invalid> wrote:

I need an isolation transformer, 1:1, 115VAC input/output, at about
100mA.

Everything I can quickly locate is big and expensive.

Any problems I could get into if I simply used two cheap filament
transformers back to back?

---
Don't use the primaries as backs...

Now would *I* do something like that ?

....(waits for smoke to clear ....)

Jim? Jim? Is that you Jim?

Tony Williams · Dec 10, 2003

In article <a0cdtvcqs7habq7oe6ff22a8ei3suc6pgl@4ax.com>,
Jim Thompson <invalid@invalid.invalid> wrote:

Bought two 25.2V/450mA filament transformers for $11.05
including tax.

115V*100mA is 11.5VA and 25.2V*450mA is only 11.34VA.

If you genuinely did need 100mA, (or a lower current
dc from a rectifier+capacitor), then you could be
disappointed. To get a reasonably good 115V from
back-back transformers for those loads would require
transformers of around 50VA nominal rating.

If you only need (say) 10mAdc or so for neon tube
experiments then you will probably get away with it.

--
Tony Williams.

Jim Thompson · Dec 10, 2003

On Wed, 10 Dec 2003 13:41:44 GMT, me@privacy.net (budgie) wrote:

On Tue, 09 Dec 2003 17:32:30 -0700, Jim Thompson
invalid@invalid.invalid> wrote:

On Tue, 09 Dec 2003 18:06:53 -0600, John Fields
jfields@austininstruments.com> wrote:
[snip]
Don't use the primaries as backs...

Now would *I* do something like that ?

...(waits for smoke to clear ....)

Jim? Jim? Is that you Jim?

480)460-2350 | |
| E-mail Address at Website Fax

Jim Thompson · Dec 10, 2003

On Wed, 10 Dec 2003 13:23:56 +0000 (GMT), Tony Williams
<tonyw@ledelec.demon.co.uk> wrote:

In article <a0cdtvcqs7habq7oe6ff22a8ei3suc6pgl@4ax.com>,
Jim Thompson <invalid@invalid.invalid> wrote:

Bought two 25.2V/450mA filament transformers for $11.05
including tax.

115V*100mA is 11.5VA and 25.2V*450mA is only 11.34VA.

If you genuinely did need 100mA, (or a lower current
dc from a rectifier+capacitor), then you could be
disappointed. To get a reasonably good 115V from
back-back transformers for those loads would require
transformers of around 50VA nominal rating.

If you only need (say) 10mAdc or so for neon tube
experiments then you will probably get away with it.

480)460-2350 | |
| E-mail Address at Website Fax

N. Thornton · Dec 11, 2003

bill.sloman@ieee.org (Bill Sloman) wrote in message news:<7c584d27.0312100235.5089faf9@posting.google.com>...

Jim Thompson <invalid@invalid.invalid> wrote in message news:<u1octvoj244vv76es9vmovnha97b6f6jfb@4ax.com>...
I need an isolation transformer, 1:1, 115VAC input/output, at about
100mA.

The data sheet doesn't usually tell you what the magnetising current
is, and it can be quite big for small transformers.

yes, which will load up TF1 considerably, further dropping the V_out.

Since I'm on your kill-file, you aren't going to get to read this
unless I've made a mistake and someone posts a correction.

there is that.

Regards, NT

Jim Thompson · Dec 15, 2003

On 11 Dec 2003 03:33:41 -0800, bigcat@meeow.co.uk (N. Thornton) wrote:

bill.sloman@ieee.org (Bill Sloman) wrote in message news:<7c584d27.0312100235.5089faf9@posting.google.com>...
Jim Thompson <invalid@invalid.invalid> wrote in message news:<u1octvoj244vv76es9vmovnha97b6f6jfb@4ax.com>...
I need an isolation transformer, 1:1, 115VAC input/output, at about
100mA.

The data sheet doesn't usually tell you what the magnetising current
is, and it can be quite big for small transformers.

yes, which will load up TF1 considerably, further dropping the V_out.

Since I'm on your kill-file, you aren't going to get to read this
unless I've made a mistake and someone posts a correction.

there is that.

Regards, NT

480)460-2350 | |
| E-mail Address at Website Fax

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