On Jul 8, 3:58*pm, wrote:
On Jul 7, 11:15*pm, "Phil Allison" wrote:
"Patrick Turner"
Even the simplest thing you may want to do with 500Vdc needs careful
analysis and thought or else you'll end up with smoke and regrets. If
there is a choke involved in the 500Vdc rail, it makes matters worse
to design around it, and a suitable diode, say IN5408 may need to be
placed across the choke to prevent back EMF causing voltage across the
relay to go very high when contacts oen and DC flow is interupted.
** Putting a diode across filter choke defeats the choke's operation.
..... *Phil
He must have meant a ' coil ' instead of choke . The diode solution is
quite common in relays .
Putting a diode across a relay coil to allow current to flow in
opposite direction to normal coil DC stops a back emf rising when coil
current cuts off so that SS devices powering the coil don't cop
excessive voltage which exceeds their V rating.
Putting a diode across a filter choke in C+L filter in PSU does the
same thing. But the diode would be a complete nuisance if the voltage
drop across choke winding resistance was low and Vac across choke was
high. But where Vac at choke input was say 1Vrms, and choke output Vac
was say 0.01Vrms, or negligible, and where Vdc drop was say 10Vdc,
then Vout would not rise above Vin and diode would never conduct
unless someone rapidly switched off the current in the choke at its
output, when choke Vo would try to rise hugely. I recall enduring the
failures of some high voltage power transistors in a regulated power
supply because of this phenomena and I then used a couple of 6AS7G
instead of the BU208 I'd tried to use.
http://www.datasheetcatalog.com/data.../0/BU208.shtml
When farnarkling with 500Vdc B+ PSUs, there's a long list of horrible
things one has to learn about to avoid smoke.
Not much is in old textbooks about simple solid state stuff dealing
with PSUs for tubes. So each time a puff of smoke happens, you are
forced to discover what should have been written in the text books.
Patrick Turner.