Modifying AT or ATX SMPS for 15V instead of 12V.

[Wizard]

One notebook I'm aquiring doesn't have power brick needs 120W on 15V (8A)

One thing I was wondering about the doughnut shaped coil that have multiple voltage windings through it to the capacitors from the transformer. What is it? I forgot that term.

I'm not going to use other windings but leave it alone for feedback and add one or few windings to the 12V winding on the transformer to get 15V output. Possible?

Cheers, Wizard

[linuxguru]

Re: Modifying AT or ATX SMPS for 15V instead of 12V.

It's fairly easy to modify an AT SMPS for +/- 13.2 V, as long as the other rails are left unregulated/unused. It requires only the removal/alteration of a few voltage-divider resistor networks that control regulation and overvoltage protection. You may have to remove the +5v power rectifier, but it's usually not necessary.

If you want +/- 15v, some complications arise because the power for the TL494 controller IC is drawn from the same windings as +12v - and this voltage will increase along with the rail voltage, which can cause certain TL494-clones to fail. The solution is to add a suitable power zener in series to drop the rail voltage for the controller to a convenient voltage, but this needs experimentation on a case by case basis. Something between 5.6v and 8.2v in series is usually about right.

In both cases, it is not necessary to add any windings to either the transformer or the secondary inductor.

[Wizard]

Re: Modifying AT or ATX SMPS for 15V instead of 12V.

Excellent!

I'll pull up the 494 datasheet and any recommendations on tweaking the resistors network to output 3V more? Like a "formula" to figure out the new value?

I recall that brand name like TI and like TL494 is specified to stand up to 18V unregulated input supply pin. Been awhile last time I consulted the datasheet.

Cheers, Wizard