transformer doubt

[aletropot]

I have a 230v to 12v 1.5VA transformer and I need 12V dc.
Would be better to use a half-wave rectifier or a full-wave?
The output of the transformer is 17v ac.
Many thanks

 

[duke37]

It is better to use a full wave (bridge) rectifier since this does not put a DC component in the transformer.
Transformer cores can be saturated if DC current pushes the magnetic flux to "one side" and then the primary current is limited only by the wire resistance - boom.
Diodes are very cheap.

 

[Arouse1973]

It is better to use a full wave (bridge) rectifier since this does not put a DC component in the transformer.
Transformer cores can be saturated if DC current pushes the magnetic flux to "one side" and then the primary current is limited only by the wire resistance - boom.
Diodes are very cheap.

Hi Trevor
Please explain how DC would be put into the transformer. I am intrigued!
Thanks
Adam

 

[aletropot]

thanks.
Btw, i'll use a 12v voltage regulator but I also need 5V. Using a 7805 after the 7812 will work well?
or should i use 2 transformers?

 

[Arouse1973]

thanks.
Btw, i'll use a 12v voltage regulator but I also need 5V. Using a 7805 after the 7812 will work well?
or should i use 2 transformers?

No thats fine, we do this all the time.
Adam

 

[aletropot]

cool. many thanks.

 

[duke37]

Adam
The current in an unloaded transformer magnetises the core and in normal use, the current reverses and the core magnetisation reverses giving an average of zero.
If the transformer feeds a half wave rectifier, then the current is offset from the mean and it can be considered as an AC current with a DC component. A big transformer can often provide a small load without problems. A torroidal transformer is quite special and is run closer to its limits and lopsided current could push it into saturation.

A similar situation occurs during the start of a transformer or motor. If they are switched on when the current should be zero, i.e. maximum voltage, then there is no problem but if they are switched on at minimum voltage when current should be maximum, then large currents can occur and the effective DC component will slowly decay due to the winding resistance.

Trevor

 

[Arouse1973]

Adam
The current in an unloaded transformer magnetises the core and in normal use, the current reverses and the core magnetisation reverses giving an average of zero.
If the transformer feeds a half wave rectifier, then the current is offset from the mean and it can be considered as an AC current with a DC component. A big transformer can often provide a small load without problems. A torroidal transformer is quite special and is run closer to its limits and lopsided current could push it into saturation.

A similar situation occurs during the start of a transformer or motor. If they are switched on when the current should be zero, i.e. maximum voltage, then there is no problem but if they are switched on at minimum voltage when current should be maximum, then large currents can occur and the effective DC component will slowly decay due to the winding resistance.

Trevor

Thanks Trevor. I didnt know that. You sure know your transfomers.
Adam