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Transformer Output Current

rinchan6

Aug 27, 2013
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Hey guys, I'm hoping I'd make sense for you here. I have this question in mind how much current can I really take from a Transformer? The transformer I have is a center tap 18 volts 3 amperes. I know I can't take more than 3 amperes that's pretty obvious but on which voltage, 18 volts or 36 volts? I saw a guy posting his transformer for sale rating 15 volts at 6 amps OR 30 volts at 3 amps. Now this confuses me a lot, anyone want to clear this up for me?

Also, If I had a load of 4Ω connected to 18 volts, normally it would draw 4.5 amperes, now if I just have a 3 ampere transformer, would it just draw 3 amperes? (violating ohms law) :/ any thoughts?
 

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BobK

Jan 5, 2010
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The 3A should be the rating at 36V. If a transformer is rated at twice the current at half the voltage, that is because it is limited by the magnetic field in the core. The magnetic field is related to the current times the number of turns. If you use only one side of the coil, for 18V you have 1/2 the turns so 1/2 the magnetic field at 3A or the same magnetic field at 6A.

That is not to say that you can necessarily draw twice the current if you use only 18V, The size of the wires might also be a limiting factor.

Bob
 

Bluejets

Oct 5, 2014
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In your above example, it is clear the transformer has 2 secondary windings and to get the 15v @ 6 amp, the 2 windings are connected in parallel.
i.e. the two "starts" and the two "finish" of each winding connected together.
To get 30v @ 3 amp, the two windings are seriesed together, i.e finish of the first winding connected to the start of the second winding. The power then comes from the remaining two ends.

In your second example, if a transformer is overloaded by connecting too great a load, the voltage of the secondary will start to drop.
With ac, if accuracy is required, you cannot use just the E=IR rule as there are other factors involved but it would, in your case be accurate enough.

Your other transformer (18v @ 3 Amp) with a centre tap cannot be changed as you only have access to 3 ends of the windings so your 36v will be @ 3 Amp also.
 

duke37

Jan 9, 2011
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A transformer has a long thermal time constant so can be overloaded for a short time if left to cool between bursts.
The current limit will be determined by the heating in the wires, not only the secondaries but also the primary. The magnetic field will change little with current, that will be set by the primary voltage.
 

rinchan6

Aug 27, 2013
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@Bob thank you for clearing that up I now can relate my Electromagnetics subject
@Bluejets I now understand the instance of overloading.
@duke37 thanks for the information I now think torroidal transformers over laminated ones for my amplifier since torroidal transformers heat less. (right?) :)
 

duke37

Jan 9, 2011
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I do not see why a torroidal transformer should heat less. They are lighter but the winding is more difficult and it is unlikely to find a high voltage one. The core is laminated but with only one lamination, wound from a long strip of grain orientated silicon iron (how many grooves on a record?). The magnetic properties are good but there is a more abrupt transition between proper operation and overload. They can therefore have problems on switch on when the core can saturate for a few cycles.
 
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