I have an transformer from a old power supply.
It has 2 red wires and 2 yellow wires.
I think the red wires are the high voltage (mains) in and the yellow
the low voltage (12v ?) out,
To check this I touched a 6V battery across the yellow wires and got a
high voltage across the red wires (enought to spark across a 3mm gap)
But when I measure the resistance across the red wires I get 31.1ohms
and across the yellow 10.4ohms.
This is not what I expected.
I thought the high voltage winding would be thicker wire and less
turns than the low voltage winding and so would have a lower
resistance.
Could anyone explain where I am going wrong?

"davidt" <dav1d@talktalk.net> wrote in message
news:cc842a62-c98b-44de-a743-4e2b99a6c09c@o77g2000hsf.googlegroups.com...
>I have an transformer from a old power supply.
> It has 2 red wires and 2 yellow wires.
> I think the red wires are the high voltage (mains) in and the yellow
> the low voltage (12v ?) out,
> To check this I touched a 6V battery across the yellow wires and got a
> high voltage across the red wires (enought to spark across a 3mm gap)
> But when I measure the resistance across the red wires I get 31.1ohms
> and across the yellow 10.4ohms.
> This is not what I expected.
> I thought the high voltage winding would be thicker wire and less
> turns than the low voltage winding and so would have a lower
> resistance.
> Could anyone explain where I am going wrong?

You have it backwards. The high voltage side will have more turns
of smaller wire. (it carries less current) The low voltage side will have
less turns if thicker wire. (it carries more current)

On 25 Feb, 12:43, "amdx" <a...@knology.net> wrote:
> "davidt" <da...@talktalk.net> wrote in message
>
> news:cc842a62-c98b-44de-a743-4e2b99a6c09c@o77g2000hsf.googlegroups.com...
>
> >I have an transformer from a old power supply.
> > It has 2 red wires and 2 yellow wires.
> > I think the red wires are the high voltage (mains) in and the yellow
> > the low voltage (12v ?) out,
> > To check this I touched a 6V battery across the yellow wires and got a
> > high voltage across the red wires (enought to spark across a 3mm gap)
> > But when I measure the resistance across the red wires I get 31.1ohms
> > and across the yellow 10.4ohms.
> > This is not what I expected.
> > I thought the high voltage winding would be thicker wire and less
> > turns than the low voltage winding and so would have a lower
> > resistance.
> > Could anyone explain where I am going wrong?
>
> =A0 You have it backwards. The high voltage side will have more turns
> of smaller wire. (it carries less current) The low voltage side will have
> less turns if thicker wire. (it carries more current)

Yes that makes sense now when think about the current.
Thanks for your help.

"amdx" <amdx@knology.net> wrote in message
news:5fe78$47c2b7ac$18d6b40c$13581@KNOLOGY.NET...
>
> "davidt" <dav1d@talktalk.net> wrote in message
> news:cc842a62-c98b-44de-a743-4e2b99a6c09c@o77g2000hsf.googlegroups.com...
>>I have an transformer from a old power supply.
>> It has 2 red wires and 2 yellow wires.
>> I think the red wires are the high voltage (mains) in and the yellow
>> the low voltage (12v ?) out,
>> To check this I touched a 6V battery across the yellow wires and got a
>> high voltage across the red wires (enought to spark across a 3mm gap)
>> But when I measure the resistance across the red wires I get 31.1ohms
>> and across the yellow 10.4ohms.
>> This is not what I expected.
>> I thought the high voltage winding would be thicker wire and less
>> turns than the low voltage winding and so would have a lower
>> resistance.
>> Could anyone explain where I am going wrong?
>
> You have it backwards. The high voltage side will have more turns
> of smaller wire. (it carries less current) The low voltage side will have
> less turns if thicker wire. (it carries more current)
>

Only smaller wire because the current requirements are smaller. There is no
restriction on the wire size for a transformer except that it be able to
handle the current. (so it definitely could have been larger)

On Mon, 25 Feb 2008 08:11:52 -0600, "Jon Slaughter"
<Jon_Slaughter@Hotmail.com> wrote:

>
>"amdx" <amdx@knology.net> wrote in message
>news:5fe78$47c2b7ac$18d6b40c$13581@KNOLOGY.NET...
>>
>> "davidt" <dav1d@talktalk.net> wrote in message
>> news:cc842a62-c98b-44de-a743-4e2b99a6c09c@o77g2000hsf.googlegroups.com...
>>>I have an transformer from a old power supply.
>>> It has 2 red wires and 2 yellow wires.
>>> I think the red wires are the high voltage (mains) in and the yellow
>>> the low voltage (12v ?) out,
>>> To check this I touched a 6V battery across the yellow wires and got a
>>> high voltage across the red wires (enought to spark across a 3mm gap)
>>> But when I measure the resistance across the red wires I get 31.1ohms
>>> and across the yellow 10.4ohms.
>>> This is not what I expected.
>>> I thought the high voltage winding would be thicker wire and less
>>> turns than the low voltage winding and so would have a lower
>>> resistance.
>>> Could anyone explain where I am going wrong?
>>
>> You have it backwards. The high voltage side will have more turns
>> of smaller wire. (it carries less current) The low voltage side will have
>> less turns if thicker wire. (it carries more current)
>>
>
>Only smaller wire because the current requirements are smaller. There is no
>restriction on the wire size for a transformer except that it be able to
>handle the current. (so it definitely could have been larger)

---
Sure, but since what a transformer does is transfer _power_ from one
winding to another, the product of current and voltage have to be
the same for both windings.

So, if the voltage in one winding is higher than in another, then
the current in the high-voltage winding _must_ be less than in the
low-voltage winding and it would be pointless to use wire with a
diameter more than that required to carry the current.

More precisely, for a lossless transformer:

Np Ep Is
---- = ---- = ----
Ns Es Ip

Where: Np is the number of turns on the primary,
Ns is the number of turns on the secondary,
Ep is the voltage impressed across the primary,
Es is the voltage induced across the secondary,
Ip is the current forced through the primary, and
Is is the current into the load

--
JF

davidt wrote:
> I have an transformer from a old power supply.
> It has 2 red wires and 2 yellow wires.
> I think the red wires are the high voltage (mains) in and the yellow
> the low voltage (12v ?) out,
> To check this I touched a 6V battery across the yellow wires and got a
> high voltage across the red wires (enought to spark across a 3mm gap)
> But when I measure the resistance across the red wires I get 31.1ohms
> and across the yellow 10.4ohms.
> This is not what I expected.
> I thought the high voltage winding would be thicker wire and less
> turns than the low voltage winding and so would have a lower
> resistance.
> Could anyone explain where I am going wrong?
The resistant you're measuring has nothing to do with the operation
of the transformer other than lowering its efficiency and Q. It's simply
a by product of the gauge wire used along with the length of wire used
for each winding.

The number of turns ratio between the Red windings and yellow windings
tells the whole story along with the gap spacing between them..

At this point, maybe you should reference some data from the net on
transformer theories.


--
http://webpages.charter.net/jamie_5"

On Mon, 25 Feb 2008 19:40:06 -0500, Jamie
<jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

>davidt wrote:
>> I have an transformer from a old power supply.
>> It has 2 red wires and 2 yellow wires.
>> I think the red wires are the high voltage (mains) in and the yellow
>> the low voltage (12v ?) out,
>> To check this I touched a 6V battery across the yellow wires and got a
>> high voltage across the red wires (enought to spark across a 3mm gap)
>> But when I measure the resistance across the red wires I get 31.1ohms
>> and across the yellow 10.4ohms.
>> This is not what I expected.
>> I thought the high voltage winding would be thicker wire and less
>> turns than the low voltage winding and so would have a lower
>> resistance.
>> Could anyone explain where I am going wrong?
>The resistant you're measuring has nothing to do with the operation
>of the transformer other than lowering its efficiency and Q. It's simply
> a by product of the gauge wire used along with the length of wire used
>for each winding.
>
> The number of turns ratio between the Red windings and yellow windings
>tells the whole story along with the gap spacing between them..
>
> At this point, maybe you should reference some data from the net on
>transformer theories.

---
Hmm... What theory would explain the "gap spacing" (???) between the
windings as a function of the turns ratio?

--
JF

John Fields wrote:

> On Mon, 25 Feb 2008 19:40:06 -0500, Jamie
> <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:
>
>
>>davidt wrote:
>>
>>>I have an transformer from a old power supply.
>>>It has 2 red wires and 2 yellow wires.
>>>I think the red wires are the high voltage (mains) in and the yellow
>>>the low voltage (12v ?) out,
>>>To check this I touched a 6V battery across the yellow wires and got a
>>>high voltage across the red wires (enought to spark across a 3mm gap)
>>>But when I measure the resistance across the red wires I get 31.1ohms
>>>and across the yellow 10.4ohms.
>>>This is not what I expected.
>>>I thought the high voltage winding would be thicker wire and less
>>>turns than the low voltage winding and so would have a lower
>>>resistance.
>>>Could anyone explain where I am going wrong?
>>
>>The resistant you're measuring has nothing to do with the operation
>>of the transformer other than lowering its efficiency and Q. It's simply
>> a by product of the gauge wire used along with the length of wire used
>>for each winding.
>>
>> The number of turns ratio between the Red windings and yellow windings
>>tells the whole story along with the gap spacing between them..
>>
>> At this point, maybe you should reference some data from the net on
>>transformer theories.
>
>
> ---
> Hmm... What theory would explain the "gap spacing" (???) between the
> windings as a function of the turns ratio?
>
my point was short and simple. difference in current mode xformers and
potential types along with turn ratio's. also the construct of the
cores. With all this put together leads to PF (power factors), current
ratio's, form factor, wet/dry, efficiency etc..


Keeping it simple since the poster obviously needs to research some
transformer basics.


--
http://webpages.charter.net/jamie_5"

"davidt"
>
> I have an transformer from a old power supply.
> It has 2 red wires and 2 yellow wires.
> I think the red wires are the high voltage (mains) in and the yellow
> the low voltage (12v ?) out,
> To check this I touched a 6V battery across the yellow wires and got a
> high voltage across the red wires (enought to spark across a 3mm gap)
> But when I measure the resistance across the red wires I get 31.1ohms
> and across the yellow 10.4ohms.


** If it is from a 240 volt mains power supply, then the red wires are for
240 volts AC and the yellow ones are for 140 volts AC. The unit is rated
for about 75 VA.

If from a 120 volts supply, the yellow wires are for 70 volts AC and the
unit is rated for about 25 VA.


........ Phil

"John Fields" <jfields@austininstruments.com> wrote in message
news:ssq5s3t2trdn67m2uosqt9ic55hn49b5hl@4ax.com...
> On Mon, 25 Feb 2008 08:11:52 -0600, "Jon Slaughter"
> <Jon_Slaughter@Hotmail.com> wrote:
>
>>
>>"amdx" <amdx@knology.net> wrote in message
>>news:5fe78$47c2b7ac$18d6b40c$13581@KNOLOGY.NET...
>>>
>>> "davidt" <dav1d@talktalk.net> wrote in message
>>> news:cc842a62-c98b-44de-a743-4e2b99a6c09c@o77g2000hsf.googlegroups.com...
>>>>I have an transformer from a old power supply.
>>>> It has 2 red wires and 2 yellow wires.
>>>> I think the red wires are the high voltage (mains) in and the yellow
>>>> the low voltage (12v ?) out,
>>>> To check this I touched a 6V battery across the yellow wires and got a
>>>> high voltage across the red wires (enought to spark across a 3mm gap)
>>>> But when I measure the resistance across the red wires I get 31.1ohms
>>>> and across the yellow 10.4ohms.
>>>> This is not what I expected.
>>>> I thought the high voltage winding would be thicker wire and less
>>>> turns than the low voltage winding and so would have a lower
>>>> resistance.
>>>> Could anyone explain where I am going wrong?
>>>
>>> You have it backwards. The high voltage side will have more turns
>>> of smaller wire. (it carries less current) The low voltage side will
>>> have
>>> less turns if thicker wire. (it carries more current)
>>>
>>
>>Only smaller wire because the current requirements are smaller. There is
>>no
>>restriction on the wire size for a transformer except that it be able to
>>handle the current. (so it definitely could have been larger)
>
> ---
> Sure, but since what a transformer does is transfer _power_ from one
> winding to another, the product of current and voltage have to be
> the same for both windings.
>
> So, if the voltage in one winding is higher than in another, then
> the current in the high-voltage winding _must_ be less than in the
> low-voltage winding and it would be pointless to use wire with a
> diameter more than that required to carry the current.
>
> More precisely, for a lossless transformer:
>
> Np Ep Is
> ---- = ---- = ----
> Ns Es Ip
>
> Where: Np is the number of turns on the primary,
> Ns is the number of turns on the secondary,
> Ep is the voltage impressed across the primary,
> Es is the voltage induced across the secondary,
> Ip is the current forced through the primary, and
> Is is the current into the load
>


And why not Np/Ns = D_Rs/D_Rp where D_Rx is the diameter? or even Np/Ns =
Rs/Rp?

The fact of the matter is that it doesn't matter as long as the as the wire
can carry the current(which essentially means heat dissipation).

You can use any wire size in any circumstance and its not *pointless*. It
depends on the application. True that you pretty never need a larger
diameter on the secondary than the primary in a step up... but potentially
there is still the possibility in certain applications. (such as if the
secondary happens to be in a much much hotter environment)

The idea transformer equations say nothing about the diameter's of the wire
used and for good reason. (not to mention that most wire sizes are
standardized)

So going off the restance or the diameter of the wire is by no means correct
and technically has nothing to do with a abstract ideal transformer. (I'm
not saying you can't use that relationship to find the optimal diameter for
a given number of turns but it is not necessary)

On Wed, 27 Feb 2008 01:35:59 GMT, "Jon Slaughter"
<Jon_Slaughter@Hotmail.com> wrote:


>Damn, I really thought I put you on my ignore list... along with the other
>morons.

---
You mean those of your ilk have _all_ plonked me?
---

>Well, you won't escape this time.

---
Escape, hell... I'd volunteer!

But, if you've plonked me, how do you expect to be able to read the
answers to your dumbass questions?

--
JF