i have an old SMPS power supply from an old external SCSi drive that died. its outputs are +5, +12V both at 2A. i am powering an old laptop off the 12V output. laptop takes apr0x 1.5A. it runns but i have noteced only when the laptop is on every 15-25 SEC, the humm of the PSU chnges. it gets slichtly louder for aprox 1 sec. is this going to cause problems for me? whay would it doo this is a capacitor discharging and this is more load to charge it back up?

 

Yes, it sounds as if there is a load change. Wether or not it exceeds the smps rating you will only know if you measure the current. Are you sure your laptop draws 1.5A at 12V? My laptops work on a higher voltage and current also depends on what they are doing. Did you measure this current?

 

The regular time interval makes me fear that something is wrong with the power supply...unless your lappy is doing something that has the same timing characteristics. If this happens when the computer is just sitting there with nothing going on, I would be suspicious.