Re: Power supply puzzle
- From: Don Foreman <dforeman@xxxxxxxxxxxxxxxxxxxx>
- Date: Wed, 13 Sep 2006 11:24:52 -0500
On Wed, 13 Sep 2006 05:08:25 -0000, Wakarusa
<aga@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
Curious if the assembled gang can offer some suggestions for a
power supply design.
The existing design includes a power transformer with outputs at
around 460VAC RMS. This feeds into a vacuum tube rectifier and on
into a pi (CLC) filter and on into a few RC sections. The power
transformer mains are center tapped with the tap grounded.
Now I need -100VDC and +200VDC supplies as well. They'll feed to
the cathode and plate of a vacuum tube cathode follower circuit.
The key is to get the -100VDC onto the tube's cathode without
messing anything else up.
The first thought was to pick the +200VDC off of the end of the
main B+ supply (just add another RC section to drop down to
200VDC) and to develop the -100VDC with a simple full wave
rectifier made from a pair of diodes mounted cathode toward the
high voltage supply (connected before the tube rectifier). Turns
out that, in the models at least, this fails miserably because of
interaction between the + and - supplies.
The solution at present uses a second diode pair (anodes to the
HV supply) to generate the +200V independent of the main B+. This
seems to work well in the models. Down stream of the diode pair
is a simple CRC setup -- the resistor to drop voltage down to the
required level and a pair of caps to smooth things out a bit.
The current demand on both -100 and +200 supplies is expected to
average 15mA (so we'll design for, say, 20mA).
There are all kinds of problems with the design as it stands --
not the least of which are that the series resistor in the
100/200 supplies has to drop hundreds of volts (I'm looking at
10W parts here) and if you operate the circuit with the tube
removed, the current drops to essentially zero so the series
dropping resistor doesn't drop and the filter caps have to be
rated to withstand a huge no-load voltage -- so I'm looking at
pairs of 350V caps in series and a huge amount of circuit real
estate.
I've considered some other setups (combinations of series/shunt
resistance to keep the no-load voltage low -- smaller/cheaper
caps, but still big wattage resistors, a stack of zeners in
series with a tap close to the required voltage, a second power
transformer, a custom wound power transformer), but none seem
elegant or cost effective.
So...
If you had 460VAC already feeding a tube rectified DC supply and
needed to add a +200VDC and -100VDC at 20mA, how would you do it?
Any ideas would be greatly appreciated.
-Todd
This would be easy for a little switchmode suppply running from
rectified filament voltage, but perhaps you don't want the switching
noise or would rather stay completely with tubes. Tube-based buck PWM
is certainly possible but would take too much space.
A simple off-the-shelf solution: get two 241-4-xx transformers from
Signal Transformer, each with dual 117VAC primaries. Ignore the
secondaries. Drive one primary of each from line. Take 117VAC off
one second primary for your -100-volt supply, connect second primaries
in series to get your 200 volt supply. These xfmrs are 1.37" x 1.25" x
2.375".
You could use beam power pentodes as series pass regulators, but that
would take more space than the two little xfmrs. You could use
800-volt transistors as series pass regulators, but they'd need
several watts worth of heatsink -- again more space than two small
xfmrs.
.
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