## How to figure out the amperage capacity of a transformer

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• I may have dreamed this after a real spicy meal, but did some one say that you can use Ohm s law to figure out the capacity of a transformer by taking the
Message 1 of 9 , Mar 17, 2013
I may have dreamed this after a real spicy meal, but did some one say that you can use Ohm's law to figure out the capacity of a transformer by taking the output voltage times the coil resistance?

I have a transformer that I salvaged out of a clock that is 120VAC primary with a center tap 12VAC secondary. If my imagination got away with me above, is there a way to figure out the capacity?

Thanks,

Jim, K6JMG
• ... What type of display did the clock have? Unless it was a unusual display most likely the transformer is rated around 5 to 10 volt amps. This based on the
Message 2 of 9 , Mar 18, 2013
On 3/17/2013 9:19 PM, jgeidl@... wrote:
>
> I may have dreamed this after a real spicy meal, but did some one say
> that you can use Ohm's law to figure out the capacity of a transformer
> by taking the output voltage times the coil resistance?
>
> I have a transformer that I salvaged out of a clock that is 120VAC
> primary with a center tap 12VAC secondary. If my imagination got away
> with me above, is there a way to figure out the capacity?
>
> Thanks,
>
> Jim, K6JMG
>
> __

What type of display did the clock have? Unless it was a unusual
display most likely the transformer is rated around 5 to 10 volt amps.
This based on the type of application.

I have never heard of using output voltage ties coil resistance to
calculate capacity of a transformer.

The other Howard

[Non-text portions of this message have been removed]
• I rate power transformers by weight. I put it in my hand and feel the weight, and observe the size. Then guess the Power handling capability by experience. I
Message 3 of 9 , Mar 18, 2013
I rate power transformers by weight. I put it in my hand and feel the
weight, and observe the size. Then guess the Power handling capability by
experience.

I little better approach is to observe listed transformers in a catalog.
Observe the weight and power handling of the transformers in the catalog.
Use ratio to get the power handling of your transformer.

Larry

From: Electronics_101@yahoogroups.com
[mailto:Electronics_101@yahoogroups.com] On Behalf Of Howard Hansen
Sent: Monday, March 18, 2013 2:43 PM
To: Electronics_101@yahoogroups.com
Subject: Re: [Electronics_101] How to figure out the amperage capacity of a
transformer

On 3/17/2013 9:19 PM, jgeidl@... <mailto:jgeidl%40sbcglobal.net>
wrote:
>
> I may have dreamed this after a real spicy meal, but did some one say
> that you can use Ohm's law to figure out the capacity of a transformer
> by taking the output voltage times the coil resistance?
>
> I have a transformer that I salvaged out of a clock that is 120VAC
> primary with a center tap 12VAC secondary. If my imagination got away
> with me above, is there a way to figure out the capacity?
>
> Thanks,
>
> Jim, K6JMG
>
> __

What type of display did the clock have? Unless it was a unusual
display most likely the transformer is rated around 5 to 10 volt amps.
This based on the type of application.

I have never heard of using output voltage ties coil resistance to
calculate capacity of a transformer.

The other Howard

[Non-text portions of this message have been removed]

[Non-text portions of this message have been removed]
• I am surprised no transformer makers have responded to this! VERY simply, if you are designing VERY conservatively, use a factor of 1000 cm/amp . A
Message 4 of 9 , Mar 18, 2013
I am surprised no transformer makers have responded to this! VERY simply,
if you are designing VERY conservatively, use a factor of "1000 cm/amp".
A "circular mil" is wire-dia. in thousandths of an inch squared; no pi
used. I understand from at least ONE professional transformer-maker/rewinder
that "750 cm/amp" is VERY common on commercial "mass-produced"
transformers. I do NOT know if this should vary as transformer-capacity drops below,
say, 100 VA. Or, may it be less for those over 1 KVA. Don't "quote me" on
that, as I have made only a dozen or so transformers for "one-time
applications" but I understand the ONE I made for a commercial application which
workmanship is the main factor in "good" transformer-making.

The "iron core" should be reasonably-nearly "square" rather than
"rectangular". I do NOT know of a FORMULA for calculating core-cross-section from
known VA; I use an old graph which seems fine, for "one-off" use, as I do
such. I must remember to ask my professional transformer-maker friend if HE
uses a formula, or a similar graph. He may-well just do it "by eye" as he
has made THOUSANDS of "one-off" transformers, and has a "feel" for how
thick to make the stack, and what "middle leg width" to use for a given VA job.
— Jan Rowland.

[Non-text portions of this message have been removed]
• Been only able to view lately and can provide some input. When I design power transformers, I use 500 circular mils per amp for the wire cross-section. Some do
Message 5 of 9 , Mar 19, 2013
Been only able to view lately and can provide some input.

When I design power transformers, I use 500 circular mils per amp for
the wire cross-section. Some do use 750. It depends on the current and
the resistance of the winding - or power dissipation in the coil.

A quick check would be to determine the winding wire size for both the
primary and secondary. Figure out the circular mils and then, using
500cm/A, for the conservative side, calculate the current. Then, based
on the input voltage, you would get the VA for winding. Do this for both
sides and take the lower of the two. This would give the conservative
side of things. Then go look at specs on others and see how they compare
for both size and weight.

There are formulas for looking at the core cross-section and I can see
about posting my old notes on this. Transformer design is a combination
of iteration and experience (of burning them out). Knowing the core
material is also important. From this, one can test to see where
saturation occurs and figure out the VA as well.

Post the transformer core dimensions, and some pics (email me the pics
also to ensure I can pull them). Is the core a cut-core, or is it
laminate, or is it powder? If cut or laminate, what is the thickness of
the material? How old is the core? Since this question will help
determine the technology. What was the core originally used for? Since
this could also help reduce the guessing to the material type if a
laminate or cut?

My history... I have designed many transformers over the past 20+ years.
My designs have been from milliwatt to over 25kW supplies of various
usage. Some of my transformer designs have pushed 50 pounds using
MetGlass (a very expensive material, but high flux capacity).

I should be able to provide some guidance on this.

Derek Koonce
DDK Interactive Consulting Services

On 3/18/2013 12:59 PM, JanRwl@... wrote:
>
> I am surprised no transformer makers have responded to this! VERY simply,
> if you are designing VERY conservatively, use a factor of "1000 cm/amp".
> A "circular mil" is wire-dia. in thousandths of an inch squared; no pi
> used. I understand from at least ONE professional
> transformer-maker/rewinder
> that "750 cm/amp" is VERY common on commercial "mass-produced"
> transformers. I do NOT know if this should vary as
> transformer-capacity drops below,
> say, 100 VA. Or, may it be less for those over 1 KVA. Don't "quote me" on
> that, as I have made only a dozen or so transformers for "one-time
> applications" but I understand the ONE I made for a commercial
> application which
> very-careful
> workmanship is the main factor in "good" transformer-making.
>
> The "iron core" should be reasonably-nearly "square" rather than
> "rectangular". I do NOT know of a FORMULA for calculating
> core-cross-section from
> known VA; I use an old graph which seems fine, for "one-off" use, as I do
> such. I must remember to ask my professional transformer-maker friend
> if HE
> uses a formula, or a similar graph. He may-well just do it "by eye" as he
> has made THOUSANDS of "one-off" transformers, and has a "feel" for how
> thick to make the stack, and what "middle leg width" to use for a
> given VA job.
> — Jan Rowland.
>
> [Non-text portions of this message have been removed]
>
>

[Non-text portions of this message have been removed]
• Larry, I have done that too! James M. Geidl, K6JMG D.B. Cooper, you have a message. ... From: Electronics_101@yahoogroups.com
Message 6 of 9 , Mar 19, 2013
Larry,

I have done that too!

James M. Geidl, K6JMG
D.B. Cooper, you have a message.

-----Original Message-----
From: Electronics_101@yahoogroups.com
[mailto:Electronics_101@yahoogroups.com] On Behalf Of Larry Beaty
Sent: Monday, March 18, 2013 11:52 AM
To: Electronics_101@yahoogroups.com
Subject: RE: [Electronics_101] How to figure out the amperage capacity of a
transformer

I rate power transformers by weight. I put it in my hand and feel the
weight, and observe the size. Then guess the Power handling capability by
experience.

I little better approach is to observe listed transformers in a catalog.
Observe the weight and power handling of the transformers in the catalog.
Use ratio to get the power handling of your transformer.

Larry

From: Electronics_101@yahoogroups.com
[mailto:Electronics_101@yahoogroups.com] On Behalf Of Howard Hansen
Sent: Monday, March 18, 2013 2:43 PM
To: Electronics_101@yahoogroups.com
Subject: Re: [Electronics_101] How to figure out the amperage capacity of a
transformer

On 3/17/2013 9:19 PM, jgeidl@... <mailto:jgeidl%40sbcglobal.net>
wrote:
>
> I may have dreamed this after a real spicy meal, but did some one say
> that you can use Ohm's law to figure out the capacity of a transformer
> by taking the output voltage times the coil resistance?
>
> I have a transformer that I salvaged out of a clock that is 120VAC
> primary with a center tap 12VAC secondary. If my imagination got away
> with me above, is there a way to figure out the capacity?
>
> Thanks,
>
> Jim, K6JMG
>
> __

What type of display did the clock have? Unless it was a unusual display
most likely the transformer is rated around 5 to 10 volt amps.
This based on the type of application.

I have never heard of using output voltage ties coil resistance to calculate
capacity of a transformer.

The other Howard
• Thanks Other Howard. :-) James M. Geidl, K6JMG D.B. Cooper, you have a message. ... From: Electronics_101@yahoogroups.com
Message 7 of 9 , Mar 19, 2013
Thanks Other Howard. :-)

James M. Geidl, K6JMG
D.B. Cooper, you have a message.

-----Original Message-----
From: Electronics_101@yahoogroups.com
[mailto:Electronics_101@yahoogroups.com] On Behalf Of Howard Hansen
Sent: Monday, March 18, 2013 11:43 AM
To: Electronics_101@yahoogroups.com
Subject: Re: [Electronics_101] How to figure out the amperage capacity of a
transformer

On 3/17/2013 9:19 PM, jgeidl@... wrote:
>
> I may have dreamed this after a real spicy meal, but did some one say
> that you can use Ohm's law to figure out the capacity of a transformer
> by taking the output voltage times the coil resistance?
>
> I have a transformer that I salvaged out of a clock that is 120VAC
> primary with a center tap 12VAC secondary. If my imagination got away
> with me above, is there a way to figure out the capacity?
>
> Thanks,
>
> Jim, K6JMG
>
> __

What type of display did the clock have? Unless it was a unusual
display most likely the transformer is rated around 5 to 10 volt amps.
This based on the type of application.

I have never heard of using output voltage ties coil resistance to calculate
capacity of a transformer.

The other Howard
• You are most brilliant! IMHO! Larry From: Electronics_101@yahoogroups.com [mailto:Electronics_101@yahoogroups.com] On Behalf Of James M. Geidl Sent: Tuesday,
Message 8 of 9 , Mar 19, 2013
You are most brilliant!

IMHO!

Larry

From: Electronics_101@yahoogroups.com
[mailto:Electronics_101@yahoogroups.com] On Behalf Of James M. Geidl
Sent: Tuesday, March 19, 2013 1:26 PM
To: Electronics_101@yahoogroups.com
Subject: RE: [Electronics_101] How to figure out the amperage capacity of a
transformer

Larry,

I have done that too!

James M. Geidl, K6JMG
D.B. Cooper, you have a message.

-----Original Message-----
From: Electronics_101@yahoogroups.com
<mailto:Electronics_101%40yahoogroups.com>
[mailto:Electronics_101@yahoogroups.com
<mailto:Electronics_101%40yahoogroups.com> ] On Behalf Of Larry Beaty
Sent: Monday, March 18, 2013 11:52 AM
To: Electronics_101@yahoogroups.com
<mailto:Electronics_101%40yahoogroups.com>
Subject: RE: [Electronics_101] How to figure out the amperage capacity of a
transformer

I rate power transformers by weight. I put it in my hand and feel the
weight, and observe the size. Then guess the Power handling capability by
experience.

I little better approach is to observe listed transformers in a catalog.
Observe the weight and power handling of the transformers in the catalog.
Use ratio to get the power handling of your transformer.

Larry

From: Electronics_101@yahoogroups.com
<mailto:Electronics_101%40yahoogroups.com>
[mailto:Electronics_101@yahoogroups.com
<mailto:Electronics_101%40yahoogroups.com> ] On Behalf Of Howard Hansen
Sent: Monday, March 18, 2013 2:43 PM
To: Electronics_101@yahoogroups.com
<mailto:Electronics_101%40yahoogroups.com>
Subject: Re: [Electronics_101] How to figure out the amperage capacity of a
transformer

On 3/17/2013 9:19 PM, jgeidl@... <mailto:jgeidl%40sbcglobal.net>
<mailto:jgeidl%40sbcglobal.net>
wrote:
>
> I may have dreamed this after a real spicy meal, but did some one say
> that you can use Ohm's law to figure out the capacity of a transformer
> by taking the output voltage times the coil resistance?
>
> I have a transformer that I salvaged out of a clock that is 120VAC
> primary with a center tap 12VAC secondary. If my imagination got away
> with me above, is there a way to figure out the capacity?
>
> Thanks,
>
> Jim, K6JMG
>
> __

What type of display did the clock have? Unless it was a unusual display
most likely the transformer is rated around 5 to 10 volt amps.
This based on the type of application.

I have never heard of using output voltage ties coil resistance to calculate
capacity of a transformer.

The other Howard

[Non-text portions of this message have been removed]
• I didn t do a very good job of explaining what I am trying to do. I don t want to build a transformer nor do I want to take the one I have apart. I was just
Message 9 of 9 , Mar 19, 2013
I didn't do a very good job of explaining what I am trying to do. I don't
want to build a transformer nor do I want to take the one I have apart. I
was just hoping that there was some simple way to figure out the amperage
capacity of this little transformer that I removed from a digital clock.
Not a big deal. And I really appreciate all the responses.

James M. Geidl, K6JMG
D.B. Cooper, you have a message.

-----Original Message-----
From: Electronics_101@yahoogroups.com
[mailto:Electronics_101@yahoogroups.com] On Behalf Of Derek
Sent: Tuesday, March 19, 2013 8:50 AM
To: Electronics_101@yahoogroups.com
Subject: Re: [Electronics_101] How to figure out the amperage capacity of a
transformer

Been only able to view lately and can provide some input.

When I design power transformers, I use 500 circular mils per amp for the
wire cross-section. Some do use 750. It depends on the current and the
resistance of the winding - or power dissipation in the coil.

A quick check would be to determine the winding wire size for both the
primary and secondary. Figure out the circular mils and then, using 500cm/A,
for the conservative side, calculate the current. Then, based on the input
voltage, you would get the VA for winding. Do this for both sides and take
the lower of the two. This would give the conservative side of things. Then
go look at specs on others and see how they compare for both size and
weight.

There are formulas for looking at the core cross-section and I can see about
posting my old notes on this. Transformer design is a combination of
iteration and experience (of burning them out). Knowing the core material is
also important. From this, one can test to see where saturation occurs and
figure out the VA as well.

Post the transformer core dimensions, and some pics (email me the pics also
to ensure I can pull them). Is the core a cut-core, or is it laminate, or is
it powder? If cut or laminate, what is the thickness of the material? How
old is the core? Since this question will help determine the technology.
What was the core originally used for? Since this could also help reduce the
guessing to the material type if a laminate or cut?

My history... I have designed many transformers over the past 20+ years.
My designs have been from milliwatt to over 25kW supplies of various usage.
Some of my transformer designs have pushed 50 pounds using MetGlass (a very
expensive material, but high flux capacity).

I should be able to provide some guidance on this.

Derek Koonce
DDK Interactive Consulting Services

On 3/18/2013 12:59 PM, JanRwl@... wrote:
>
> I am surprised no transformer makers have responded to this! VERY
> simply, if you are designing VERY conservatively, use a factor of "1000
cm/amp".
> A "circular mil" is wire-dia. in thousandths of an inch squared; no pi
> used. I understand from at least ONE professional
> transformer-maker/rewinder that "750 cm/amp" is VERY common on
> commercial "mass-produced"
> transformers. I do NOT know if this should vary as
> transformer-capacity drops below, say, 100 VA. Or, may it be less for
> those over 1 KVA. Don't "quote me" on that, as I have made only a
> dozen or so transformers for "one-time applications" but I understand
> still doing fine after years. Apparently, very-careful workmanship is
> the main factor in "good" transformer-making.
>
> The "iron core" should be reasonably-nearly "square" rather than
> "rectangular". I do NOT know of a FORMULA for calculating
> core-cross-section from known VA; I use an old graph which seems fine,
> for "one-off" use, as I do such. I must remember to ask my
> professional transformer-maker friend if HE uses a formula, or a
> similar graph. He may-well just do it "by eye" as he has made
> THOUSANDS of "one-off" transformers, and has a "feel" for how thick to
> make the stack, and what "middle leg width" to use for a given VA job.
> - Jan Rowland.
>
> [Non-text portions of this message have been removed]
>
>
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