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Geo,

Ok, I got a bit curious...... If a Zener String of 900+V can change +/-40V over -25degC to +75degC, what did the original Corotrons do?

IE, a Victoreen GV3A Corotron?

It turns out that this Vic catalog does not give a spec on the basic GV3A used, but does give temp info on the "High Temperature" version:

? page 15

So, you can see an original 900V High Temp Corotron only varied 16V from -65degC to 150degC...... pretty impressive I think.

We are replacing these things with Zener Strings with relatively high positive tempco......

I think the ultimate economic answer is to regulate in the primary circuit, just as you suggested, and as ENi did, and depend on the fixed transformer secondary ratio to get the regulated HV, and to temperature compensate the primary along with an adjustable trim capability.

rogerw

1) yes, could be multiple junctions.

2) You have an exray machine?

3) We used to "de-pot" plastic packaged IC chips with acid to expose the die in the package, to examine under a microscope, photograph, etc. trace circuits that way....... might be hard to do with an axial lead diode.... I don't know.

rogerw

On 4/7/2022 9:49 AM, Geo Dowell wrote:

I've always assumed the multiple junction idea, because in practice the higher voltage rectifier diodes get longer and longer. Ones for a 100kV stack are a good 6 inches long each. Don't know, just guessing. Will have to X-Ray one, but when it's out of the X-Ray machine how to do that?

Geo

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From: "Roger Whatley" <rogwhat53@...>
To: "CDV700CLUB" <[email protected]>
Sent: Thursday, April 7, 2022 9:43:53 AM
Subject: Re: [CDV700CLUB] Question and an interesting site for those that modify CDV700s and more.

Well, the answer to your first question is: I am not sure, but I think it is probably just very lightly doped silicon pn junction. The light doping (and perhaps playing around with the doping profile - Its just fizzicks, ya' know!) gives higher reverse breakdown voltage. And the lighter doping also gives a higher bulk resistivity such that the extrinsic series resistance is higher than a LV diode, so higher forward drop at least is due to higher series resistance.

This is a "no-cost" contract. It is going to take an infinity of diodes!

rogerw

On 4/7/2022 9:32 AM, Geo Dowell wrote:

Question Roger, are HV (like 8kV) low current diodes a stack of normal PN junctions inside? or are they a single special PN junction? What I do measure is higher and higher Fwd V drop as the V (sub) max rating increases.

By the way, if you only need that many PN junctions to achieve thermal cancelling, try switching to Schottkey diodes. That will double or tripple the parts count. This is a "cost-plus-contract", right?

Geo

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From: "Roger Whatley" <rogwhat53@...>
To: "CDV700CLUB" <[email protected]>
Sent: Thursday, April 7, 2022 9:06:12 AM
Subject: Re: [CDV700CLUB] Question and an interesting site for those that modify CDV700s and more.

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

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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
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rogerw
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The PanDemic is over, but the DemPanic goes on.
MisDisMal is a dictator's tool of repression.

-- 
rogerw
On the Catclaw
ΜΟΛΩΝ ΛΑΒΕ

The PanDemic is over, but the DemPanic goes on.
MisDisMal is a dictator's tool of repression.

-- 
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On the Catclaw
ΜΟΛΩΝ ΛΑΒΕ

The PanDemic is over, but the DemPanic goes on.
MisDisMal is a dictator's tool of repression.

-- 
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On the Catclaw
ΜΟΛΩΝ ΛΑΒΕ

The PanDemic is over, but the DemPanic goes on.
MisDisMal is a dictator's tool of repression.

-- 
rogerw
On the Catclaw
ΜΟΛΩΝ ΛΑΒΕ

The PanDemic is over, but the DemPanic goes on.
MisDisMal is a dictator's tool of repression.

-- 
rogerw
On the Catclaw
ΜΟΛΩΝ ΛΑΒΕ

The PanDemic is over, but the DemPanic goes on.
MisDisMal is a dictator's tool of repression.