Konfirmacija (Zen v4)
2006 05 02 22:04Žodž, ne aš vienas apie zeno maisto tobulinimą sapalioju
I don’t think the real reason has been identified. The real reason is the peak currents in the transformer are much larger than you may think.
I have posted some graphs to identify this phenomenon:
http://www.diyaudio.com/forums/show…8536#post318536
Here is a summary: if you use a diode bridge and some capacitors for a power supply that sources 2 Amperes DC, then the currents in the transformer are over 10 Amperes peak. Since heat (and the magnetic field) is a squared term of current (P = R * i^2), the peak currents generate tremendous amounts of heat in the transformer. As the transformer windings heat up, their resistance increases with temperature. Therefore, the transformer heats up even more – positive feedback.
There is a technical term to describe this effect: Power Factor (PF). If the peak currents are 10A and the power supply delivers 2A, then the PF is very poor. Low power factors also tell you that the filter capacitors have to handle the ripple currents (hum), too.
High power factors mean the peak currents are lower (proportionally to the delivered current). The efficiency inside the transformer is better, the diodes don’t have to conduct the peak currents, and the filter capacitors will have less ripple currents.
The only way to solve this problem is use a massively over-rated transformer that can handle the peak currents, or to use a more complex rectifier. Choke-loading the rectifier can increase the power factor. Another solution is using a tube rectifier, but that’s something I’d rather not mess with, and it also requires a choke. Beware, using a choke will reduce your transformer’s voltage output by a couple volts.
For a 2 Ampere DC power supply for a Class-A amplifier, only a small choke is required. Digikey sells high-current chokes in the 1-5mH range for about $5 from JW Milller. Radio Shack also sells a high-current choke for mobile CB radios for about $5. I’ve successfully used the Radio Shack choke in my Zen power supply. I’m using a 120VA transformer, and with the choke installed, I don’t get any transformer heating.
Štai jums ir prašom! 120VA trafas kanalui 
šalitinis: http://www.diyaudio.com/forums/showthread.php?postid=325343#post325343