[Elecraft] 120 vs 240

[Jim Brown]

On 1/25/2011 10:37 AM, Scott Ellington wrote:
> but my AL-82 once blew the 15 A 240 V fuses, which suggests PF was less than about 0.7.

There are now two elements to power factor -- the phase angle at 60 Hz 
and the impulsive (non-sine wave) nature of the AC line current due to 
the input filter capacitor.  With any form of power supply, the 
impulsive current dominates the power factor

  More to the point, the main thing that blows breakers with very large 
power supplies is the inrush of current at turn-on to charge the input 
filter cap. It's a statistically random thing -- if the turn-on occurs 
at or near zero crossing of the sine wave, the inrush current is small. 
If it occurs near the positive or negative peak of the sine wave, the 
inrush current is high and a breaker is more likely to pop. That does 
NOT cause any sort of destructive failure -- it's merely the 
inconvenience of having to get up to reset the breaker -- and it ONLY 
happens when the power supply is first turned on.

We learned about these issues in the pro audio world about 30 years ago 
when audio power amps rated for up to 2kW started coming into wide use. 
Over the years, various methods were used to soften the turn-on 
transient, and/or to spread the turn-on times of a rack full of these 
amps over several seconds so that turning the system on didn't blow the 
main breaker for a building!

The difference between a linear supply and a switching supply in this 
regard is that while they both have input filter caps, the cap in the 
linear supply is much larger (and the transformer is usually more 
robust), so the inrush current will usually be greater.

There's a tutorial view of all of this in the audio section of my website.
http://audiosystemsgroup.com/publish.htm

73, Jim Brown K9YC