Geo,
Now, for your last question below,
the answer is yes, you can replace the BJT with just the HV
Mosfet and the circuit will function. In fact that is exactly
Peter's original circuit, well almost...... It evolved to the
subsequent forms for specific reasons though, in this thread, to
improve performance.
For example, in Peter's simplified
schematic the reference voltage that appears at the negative
feedback terminal of the "opamp" is Vz + Vbe (crude opamp which
is really a BJT gainstage with a load resistor = 1M and gain =
gm*Rload). Since a 6.2V Zener has a positive tempco of about
2mV/degC and a Vbe has a negative tempco of about -2mV/degC,
Wallah! (voila!).
So the BJT was a pretty big
improvement in that regard, There are more comments I could make
about the circuit errors with the MosFets as the gainstage
instead of as the cascodes, but it suffices to say that these
are what motivated the odyssey to more complicated circuitry.
However engineering economics and
"good enough" motivated Peter to step back to the simpler
circuit.
(If you had a 1000V BJT you would not
need the cascodes for mere functionality.)
rogerw
Take a look at Peter's simpler
schematic he just posted yesterday, and ignore the cascode
MosFet, since its main function is to pass the BJT collector
current up to the high voltage levels. Imagine the BJT was a
1000V Vce device and did not need the cascode for high voltage
reasons.
Now, take a look at my enclosure
here....... Look familiar?
rogerw
On 4/5/2022 4:21 AM, Geo Dowell
wrote:
Really the question is, skip being variable right now for
a moment, what function does the transistor play and
ultimately, could you theoretically use a zener on the
gate(? or where?) of the FET without the transistor to
achieve a regulated output, and what value would the Zener
need to be to make say, 900V?
Geo
From: "Geo Dowell"
<GEOelectronics@...>
To: "CDV700CLUB"
<[email protected]>
Sent: Tuesday, April 5, 2022 1:04:13 AM
Subject: Re: [CDV700CLUB] Question and an
interesting site for those that modify CDV700s and more.
Peter and Roger,?
P. your hand drawn schematic helps me understand more
about this.? Here's my question, with either or all of
these circuits, what happens if the divider string is
fixed, even if it has the added adjustments available,
then the ZENER voltage is changed? Would that give the
same effect??
If either one could model that, show change of HV
with a range of Zener V from say 3 to 30V. And while
doing that change the load resistor on P's schematic to
1.8M.
George?
From: "peter via groups.io"
<epkoncept@...>
To: "CDV700CLUB"
<[email protected]>
Sent: Monday, April 4, 2022 10:33:55 PM
Subject: Re: [CDV700CLUB] Question and an
interesting site for those that modify CDV700s and more.
[Edited Message Follows]
[Reason: typo's]
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 a very basic circuit.
66Meg, two 1 Meg, 100k+ 2.2M trimmer , 5.1V zener,
2SK3265, 2SC3311
Set trimmer for 900V, Vary input from 950~ 1150V, output
only varied 5V or less.
I 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!
Its time to make it small enough to stuff inside my
CDV-700 . I think the quiescent current draw while
shunting was about 35uA when set to 900V with? about a
950V input.
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.
