12 Nov
2009
12 Nov
'09
4:10 p.m.
Tony Duell wrote:
The 555 even has regulated heater supplies. The
heater
transformer is fed
through a saturable reactor. The current in the control
winding of that
comes from a pentode valve in the PSU unit, the grid
voltage of that is
controlled by a bright-emitter diode running off one of the
heater lines.
So the thing actually gets the RMS value of the heater
supplies correct
(which is what you want, of course).
That's fascinating. I'd seen "passive" ballast tubes
for heater regulation, but not an "active" solution like
this. I found a good write-up at:
http://www.radiomuseum.org/forum/2as15_how_it_works.html
Especially interesting to me is the mechanical link in the
diode that shorts it out when the filament burns out, pulling
down the regulated voltage. I wonder if they learned the
necessity of that the hard way.
John Finigan