Geo,
Sure, enclosed, but I have already modified
the Zener String with a temperature compensation scheme that
will be somewhat familiar to the discerning eye. I am NOT
suggesting this circuit for a build, merely playing around with
ideas!
So, if I measure the voltage of the top of
that Zener String alone, it is a nominal 788V at 25degC, and
821V at 75degC, and 754V at-25degC. That is a +67V change over
100degC or about +0.67V/degC tempco. (parenthetically, the
original full Zener String was about +/-40V.)? If I compensate
that with the negative tempco of a Vbe which is about
-2.2mv/degC,
I need 0.67V/2.2mV = 304 Vbes...... whew!
That's a lot of diodes!
Instead the BJT circuit is multiplying the sum
of Q1&Q3 Vbe by 129 = 258Vbe's. All I can say about the
difference from calculated is that the actual tempco of the
bjt's is in the model somewhere and differs from my Mindsim
approximation. And btw, the pnp BJT Q2 is a local feedback trick
that is "fortifying" Q1...... without Q2 you get weak knee, with
Q2 you get a sharp knee. Q2 and R3 can be removed and the
circuit still functions but regulation suffers.
Anyway, the second enclosure shows the
regulation performance over temperature. When I expand the scale
I can see the regulated voltage at 906V nominal 25degC, 908V at
75degC, and 905V at -25degC.
So, for one thing, this whole exercise begs a
question: Who lets their GM Counter get THAT hot or cold? I
think that is a philosophical question.... If a GM Counter fell
in the forest, in the winter up north, and there was not a woman
there to turn it on, does it really matter? Does it even
exist???
rogerw
On 4/7/2022 3:36 AM, Geo Dowell wrote:
Roger, show schematic of sim?
Geo
From: "Roger Whatley"
<rogwhat53@...>
To: "CDV700CLUB"
<[email protected]>
Sent: Wednesday, April 6, 2022 9:28:41 PM
Subject: Re: [CDV700CLUB] Question and an interesting site for
those that modify CDV700s and more.
Well I just simulated something I had not yet thought to
do...... I simulated a Zener string.
Yeah, it worked...... ha!
Largest Zener I could find in the model library was 36V
so I strung enough of those to make about 890V nominal
with a 3.3Meg dropper resistor. Its regulation is very
flat only rose about 3V from 1000V to 1200V input. That as
good as my Penultimate Way-Good Cascode Shunt Regulator
can do, and far less complex.
But guess what? over temperature it sucks big wind!
+/-40V from -25 to 25 to 75degC! The Penultimate only
varies about +/-3V over that temp range (overkill).
But the Good Enough Cascode Shunt Regulator, with its
temperature compensation scheme, only varies less tha
+/-5V. Regulation varies less than 10V from the knee to
950 to 1200V
On 4/6/2022 10:53 AM, Roger Whatley via groups.io
wrote:
Peter,
Minor change, I noticed after I posted the note below
that I had not specified the second BJT in the
Darlington. The enclosed schematic is corrected and now
the simulation predicts about dead-nuts on for the
tempco of the output. Other than that, same comments as
below. I think this is the best "good enough" cascoded
shunt regulator.
rogerw
On 4/6/2022 10:40 AM, Roger Whatley via groups.io
wrote:
Peter,Enclosed is a schematic of my suggestion below.
That is an 8.2V Rohm Zener chosen for tempco to cancel
the negative tempco of two Vbe's. It is slightly too
positive in the result but quite "good enough." You
willr just get much flatter regulation with the
Darlington pair and that can be had in a single
package.
Of course, depending on the Vcemax spec of your
cascode MOSFET, you might have only one cascode
instead of two..... That will not make a noticeable
difference except to breakdown.
rogerw
On 4/6/2022 8:32 AM, Roger Whatley via groups.io
wrote:
Peter,
I realize that circuit described below might be
"good enough," but realize that making that one BJT
a Darlington pair is a significant improvement to
the regulation. You can even get the Darlington in
one transistor package, of course, cheap.
As UnRegHV increases, the current in the BJT
necessarily increases in order to regulate the loop.
For every extra 0.1uA of extra base current
required, the voltage drop in the feedback resistors
(66M + 66M + 1M) goes up 0.1uA*(66M+66M+1M) = 13.3V
and thus regulation suffers. The BJT current easliy
goes from less than 10uA to over 200uA, thus the
base current might go up by a worst-case of as much
as 1uA, giving a 133V change in the output. So that
is the motivation for the Darlington.
(In case anyone wonders, why not an FET instead of
a BJT, that WAS the original circuit and it suffers
worse circuit errors.....)
The next big error is that as the BJT current
increases with UnRegHV increases, so does Vbe and Vz
increase. Those are the motivations for subsequent
circuit complications and resulting in the
Penultimate Way-Improved version..... it is
overkill, I know, to the requirement, but by
simulating those improvements I gain insights as to
just what the remaining errors were. And it was fun!
My take is that your present circuit, but with a
Darlington Pair, will be "good enough" and if it
were a purpose-built Darlington in one three-lead
package, no more expensive or even board-space than
a single BJT.
Of course, a single Zener voltage may be chosen to
match the negative tempco of two Vbe's. Again, no
extra expense or board space. (Off the top of my
head, I am thinking about 8V Zener for +4mV/degC?)
rogerw
On 4/4/2022 10:33 PM, peter via groups.io wrote:
Roger:
?back to the circuit you posted in message #300
Simple cascode, with only 1 npn tr + 2 mosfets.
Since I found the 2SK3265, I removed the 2nd mosfet
so its very basic circuit.
66Meg, two 1 Meg, 100k+ 2.2M trimmer , 5.1V zener,
2SK3265, 2SC3311
Set trimmer for 900V, Vary input from 950~ 1150,
output only varied 5V or less.
Heated the circuit with a hair dryer, got it up from
25C to about 60C, voltage rose may be 5 or 6 volts.
Good enough for me!
Time to make it small enough to stuff inside my
CDV-700 !
It on the LHS of the attached jpg, circuit in
message #300 is on the RHS
P
--
rogerw
On the Catclaw
¦¬¦¯¦«¦¸¦ ¦«¦¡¦¢¦¥
The PanDemic is over, but the DemPanic goes on.
MisDisMal is a dictator's tool of repression.
--
rogerw
On the Catclaw
¦¬¦¯¦«¦¸¦ ¦«¦¡¦¢¦¥
The PanDemic is over, but the DemPanic goes on.
MisDisMal is a dictator's tool of repression.
--
rogerw
On the Catclaw
¦¬¦¯¦«¦¸¦ ¦«¦¡¦¢¦¥
The PanDemic is over, but the DemPanic goes on.
MisDisMal is a dictator's tool of repression.
--
rogerw
On the Catclaw
¦¬¦¯¦«¦¸¦ ¦«¦¡¦¢¦¥
The PanDemic is over, but the DemPanic goes on.
MisDisMal is a dictator's tool of repression.
--
rogerw
On the Catclaw
¦¬¦¯¦«¦¸¦ ¦«¦¡¦¢¦¥
The PanDemic is over, but the DemPanic goes on.
MisDisMal is a dictator's tool of repression.
