[AMRadio] Need Variac

[Donald Chester]

>they work well,
>but there is loss in the variacs, more sag under load
>than without one.


I think part of the reason is the carbon brush contact.  You cannot use a 
metal contact because it would short out adjacent turns as the variac is 
rotated and overheat the coil and burn up the contacts as well.  The 
graphite brush acts as a current limiting resistor to protect the winding, 
but also introduces losses.  I also found the voltage regulation to be 
noticeably better without the variac.  The best tapped autotransformer I 
ever had was the voltage control from an old x-ray machine.  It had huge 
contacts on a rotary switch, and there was no detectable increase in voltage 
sag with the transformer in line.  Unfortunately, I swapped it for some 
other gear after I had acquired a nice new G-R 20 amp variac.

Don K4KYV

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