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How to figure out the amperage capacity of a transformer

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  • jgeidl@sbcglobal.net
    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 7:19 PM
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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 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
    • Howard Hansen
      ... 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 11:42 AM
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        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]
      • Larry Beaty
        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 11:52 AM
        • 0 Attachment
          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]
        • janrwl
          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 12:59 PM
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            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
            runs loaded 24/7 is 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]
          • Derek
            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 8:49 AM
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              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
              > runs loaded 24/7 is 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]
              >
              >



              [Non-text portions of this message have been removed]
            • James M. Geidl
              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 10:26 AM
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                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
              • James M. Geidl
                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 10:26 AM
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                  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
                • Larry Beaty
                  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 10:27 AM
                  • 0 Attachment
                    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]
                  • James M. Geidl
                    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 10:33 AM
                    • 0 Attachment
                      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
                      > the ONE I made for a commercial application which runs loaded 24/7 is
                      > 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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