Transformer

[bernardgreen]

Just out of interest what was the rating in watt or VA of the transformer ?

In a 60 watt ( or VA ) transformer working on half voltage to feed a 60 watt load the currents in both windings will be double the design current so the heating effect due to resistive impedence in the coils will be 4 times the design value

Watts = V x I

V = R x I

Watts = R x I x I

double the current through a resistance and the wattage increases to 4 times

 

[MrTinker]

In a 60 watt ( or VA ) transformer working on half voltage to feed a 60 watt load the currents in both windings will be double the design current so the heating effect due to resistive impedence in the coils will be 4 times the design value

Watts = V x I

V = R x I

Watts = R x I x I

double the current through a resistance and the wattage increases to 4 times

It dosen't work like that though.
say with a 2870 ohm resistance.

at 415v
415/2870=0.1446A giving 60.0W

at230v
230/2870=0.0801A giving 18.4W

 

[bernardgreen]

It dosen't work like that though.
say with a 2870 ohm resistance.

at 415v
415/2870=0.1446A giving 60.0W

at230v
230/2870=0.0801A giving 18.4W

What you are describing is a purely resistive impedance such as a heating element.

The ideal transformer has windings of zero resistive impedance so there is no heating of the wires. The current into the input of the transformer is limited by the reactive ( inductive ) impedence of the primary winding. In an ideal transformer this impedance is infinite at the supply frequency so no curent flows. It is reduced by reactive effects of any current flowing in the secondary.

 

[MrTinker]

It dosen't work like that though.
say with a 2870 ohm resistance.

at 415v
415/2870=0.1446A giving 60.0W

at230v
230/2870=0.0801A giving 18.4W

What you are describing is a purely resistive impedance such as a heating element.

.

Granted yes, and I agree completely with your theory of a transformer, however in practice small transformers are very far from ideal, the true answer will be somewhere between our two arguments.

 

[bernardgreen]

Granted yes, and I agree completely with your theory of a transformer, however in practice small transformers are very far from ideal, the true answer will be somewhere between our two arguments.

The "answer" is a combination of both, the "ideal" transformer with zero resistance windings has no losses due to heating of the windings but as you say most transformers have a design trade off between resistive losses and energy usefully transferred to loads on the outputs.

The theory is very simple. the design of a practical transformer is not so simple.

 

[aptsys]

Sadly the transformers are no longer made...

You could just stick in a transformer for 12V halogen lamps.