Hi,If I keep the primary voltage constant

[strokebow]

Hi,

If I keep the primary voltage constant, then is it correct for me to assume that the secondary voltage will increase as I increase the load (resisitive load, that is)?

Thanks

 

[Pumblechook]

No, decrease.

 

[strokebow]

Why will this happen?

I thought the increased load would amount to a greater voltage because of the secondary current times the increased resistance.

 

[KnowPhysics]

Yes, The output voltage of a transformer varies by changing the load resistances, with a constant voltage input. study about voltage regulation it may help you to understand this.

 

[Pumblechook]

A transformer is actually quite complex if you look at an equivalent circuit.

The classic n1:n2 turns ratio giving a v1:v2 voltage ratio only applies with no load connected.

As far as loading is concerned a transformer behaves almost like a battery or the idealised voltage source with series resistor. The output voltage falls with increasing current because of the voltage drop across the internal resistance. The internal impedance may be a more accurate term as inductance comes into it but it mainly due to the resistance of both windings (copper loss) and eddy current losses in the core (iron loss).

 

[TVP45]

Why will this happen?

I thought the increased load would amount to a greater voltage because of the secondary current times the increased resistance.

If you have an increased load, that implies a decreased load resistance. The larger current times the constant winding resistance of the secondary coil will lead to a greater Voltage drop inside the secondary coil and that will leave less Voltage at the output terminals.

 

[Pumblechook]

http://claymore.engineer.gvsu.edu/~johnsodw/egr325mine/paper2/paper2.html

 

[stewartcs]

The classic n1:n2 turns ratio giving a v1:v2 voltage ratio only applies with no load connected.

No it doesn't. It works for loaded transformers as well. It is derived from the conservation of energy law so ideally the power in = power out. Which is why the voltage and current change proportionately (i.e. to conserve energy and keep the power the same).

CS

 

[Pumblechook]

No.

power in = power out + losses.

You run a REAL 240 to 12 Volt transformer with no load. You will find it kicks out more like 17 Volts rms.

The rated ouput voltage is that when the transfomer is loaded at its VA rating.

 

[stewartcs]

No.

power in = power out + losses.

That's why I said "ideally" (i.e. ignoring any losses).

You run a REAL 240 to 12 Volt transformer with no load. You will find it kicks out more like 17 Volts rms.

The rated ouput voltage is that when the transfomer is loaded at its VA rating.

This is due to the transformer design (i.e. it is not an ideal transformer). The turns ration vs. voltage ratio vs. current ratio is still applicable with or without a load. Although in "real" life there are some voltage regulation problems due to the practical design of the transformer.

Nevertheless, the ratio's still hold.

CS

 

[Pumblechook]

That is contradictory.

The turns ratio applies but there are voltage regulation problems.??

Obviously you have to modify the turns ratio in a real transformer to produce a higher voltage than a lossless transformer (doesn't exist) would produce.

Those cheap little transformers they use a lot in small PSUs with no electronic regulation for radios etc are particularly poor. The no-load DC output Voltage can be up to double the 'rated' output.

If the ratio always applied then the output voltage would be constant irrespective of load.

I have built plenty of PSUs in my time.

 

[stewartcs]

That is contradictory.

The turns ratio applies but there are voltage regulation problems.??

Obviously you have to modify the turns ratio in a real transformer to produce a higher voltage than a lossless transformer (doesn't exist) would produce.

Those cheap little transformers they use a lot in small PSUs with no electronic regulation for radios etc are particularly poor. The DC ouput can be up to double the 'rated' output.

If the ratio always applied then the output voltage would be constant irrespective of load.

I have built plenty of PSUs in my time.

Perhaps I wasn't clear. The theoretical turns ratio applies for ideal transformers with or without a load. In real transformers it also applies (essentially), however, the output voltage is dependent on the voltage regulation capabilities of the transformer. If a real transformer is built such that the regulation is say 1%, then the ratio is essentially the same. In other words the output voltage would be essentially the same as expected from the turns ratio. If the voltage regulation is high, then obviously there will be a discrepancy with the exact value predicted by the turns ratio.

CS

 

[mrajeebkhan]

what is core form transformer

 

[stewartcs]

what is core form transformer

In a core-form transformer, the primary and secondary windings are wrapped around the core.

CS