[sdiy] Transformer question

[Stewart Pye]

Decent quality transformers will output the rated voltage at the rated 
current, but not all transformers are decent quality.

It will be safer if you go for the 18V secondary transformer, but beware 
that you will need larger heatsinks for the regulators that would have 
for the 15Vac transformer.



Cheers,
Stew.




Simon Brouwer wrote:
> Hi Paul,
>
> Paul Perry schreef:
>   
>> I'm sure the 15AC would be fine.
>> 15v AC gives peak 22v rectified, plenty of room for the rectifier
>> diode drops & the regulator headroom.
>>     
>
> It would, with an ideal transformer. But practical transformers in a PSU
> behave quite differently than one might expect.
>
> A useful rule of thumb for regulated power supply with LM78xx and similar:
> take secondary voltage 3 volts higher than the desired output voltage.
> 2x18V is exactly right for a regulated 2x15V PSU.
>
>