[Amps] Working on my Darwin award
w9ac at arrl.net
Sat Feb 22 21:35:59 EST 2014
> "I would suggest bringing the HV up with a variac while monitoring line
current and DC HV. That's always prudent no matter what you're working on.
High power variacs are expensive but it's a tiny fraction of what you
have invested in ham radio, and you can always sell them for near what
That would probably safer in cases where the HV supply is split from the LV
& filament supplies. With a common transformer and primary, bringing up the
supply on a high power Variac may result in a big clean-up of glass shrapnel
from baked/blown step-start resistors.
Many step-start circuits monitor the LVDC supply to engage a relay for the
shorting contacts around the step resistors. If that LVDC supply does not
reach its target voltage soon after power-on, the resistors can bake and
blow. True, total current is reduced for the duration of the ramp-up period
and that would minimize the impact on the step resistors, but depending on
the design, I could see a situation where if the Variac is left at a point
just below the relay pull-in threshold, the resistors could bake open, even
if they are fused.
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