transformer resistances

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?

Reply to
davidt
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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)

Reply to
amdx

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

Reply to
davidt

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)

Reply to
Jon Slaughter

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Sure, but since what a transformer does is transfer _power_ from one
winding to another, the product of current and voltage have to be
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Reply to
John Fields

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.

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http://webpages.charter.net/jamie_5"
Reply to
Jamie

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

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.

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http://webpages.charter.net/jamie_5"
Reply to
Jamie

"davidt"

** 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

Reply to
Phil Allison

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)

Reply to
Jon Slaughter

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You mean those of your ilk have _all_ plonked me?
Reply to
John Fields

So your transformer has #10 wire on the primary and #18 wire on the secondary? I think your secondary will have about 60% or less of the resistance of the primary. Please post the resistances of the primary and secondary windings, that way I can see how I misled the questioner. What is the use for this transformer? It's not like anything I'm used to seeing. Thanks, Mike

Reply to
amdx

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