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a 2uA opamp...... wow.

rogerw

not a problem P. Just curious.

Already got more projects than space-time

Well, 1 ma min isn't going to do me any good with 1kV or 2kV bias supplies.. Back to the ol' op-amp + 1G resistor.

Geo

"peter via groups.io" <epkoncept@...>
To: "CDV700CLUB" <[email protected]>
Sent: Tuesday, April 5, 2022 6:50:14 PM
Subject: Re: [CDV700CLUB] Question and an interesting site for those that modify CDV700s and more.IF th

Geo:
?Sorry I don't remember the part numbers. I saw an announcement by Microchip a while ago, but I can't find it, must have been vapor.
TI has a TL783 ( 125V) but its a series and not a shunt regulator. Sorry
P

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Yes, I see that and I should have thought of it..... MindSim failed!

That 1mA must be the minimum to power the onchip reference and opamp.

Oh well, back to the drawing board!

I suppose it could be done with a very low power CMOS opamp, plus a 5.1V zero TC Zener, plus a discrete npn BJT to replace the TL-431..... but I will have to noodle on that

I used to design such opamps but they were not available as discrete opamps........ they were buried in a VLSI processor, etc.

rogerw

I will look into that.

rogerw

On 4/5/2022 3:28 PM, peter via groups.io wrote:

Roger:
?This is what Geo was asking in lieu of a zener.?
Well, there may be a limitation due to minimum current requirements of the IC shunt regulator chip. I think the spec sheet notes about 1ma for lower limit (?)
Its adjustable up to ~+30V
(On a different not there some newer ones that go up to 125 volts or so,)

P

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rogerw
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I will look into that.

rogerw

Roger:
?This is what Geo was asking in lieu of a zener.?
Well, there may be a limitation due to minimum current requirements of the IC shunt regulator chip. I think the spec sheet notes about 1ma for lower limit (?)
Its adjustable up to ~+30V
(On a different not there some newer ones that go up to 125 volts or so,)

P

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Geo, Peter,

Well how about this enclosed circuit(s)?

The TL431 2,5V reference voltage is going to be an onchip bandgap generated voltage reference and the TI spec gives 3 grades: 0.5%, 1%, and 2% unit to unit accuracy.

I used to design analog integrated circuits, but not board level circuits. I was unaware of the TL-431 until you brought it up.... Interesting!

However I would prefer a larger voltage reference than 2.5 so that the multiplication resistor ratio to 900+V is not so extreme.

I know one simple way would be to put a zero TC 5.1V Zener beneath the TL-431 in the path to ground. Then readjust the 360K resistor to about 3x. I like this idea, and given the TL-431 I think this circuit will perform about as well as my Penultimate with it's discrete transistor optimizations. Perhaps even better because of the degree of integration and resulting thermal contact of all the important components (on chip).

Only big circuit error I see left is that the load current (ie, the dropper resistor current) flows on down the cascodes, thru the BJT on the TL-431, and thru the 5.1V Zener. As the load current changes with UnRegHV, so will the Zener voltage...... not too bad though....

rogerw

OK on 1.25, that's even better.? I wonder how precision that reference Voltage is?? That's a good Voltage to have, if it's accessible.

No never used it except for the primary shunt HV reg. and never measured the current, might put that task on the bucket list too.

Here's today's CDV-700 project finished,

Si-3BG into a CDV-700 probe housing:

----- Original Message -----
From: peter via groups.io <epkoncept@...>
To: [email protected]
Sent: Tue, 05 Apr 2022 13:17:01 -0400 (EDT)
Subject: Re: [CDV700CLUB] Question and an interesting site for those that modify CDV700s and more.IF th

geo:
?the xx431 starts at 1.25V( or at least some of them do)
Most of the other shunt regs start at 2.5V
But in either case, I think they are only spec'd for 1mA and up of current. Don't know how they work at uA?

P

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I also like the "zener referenced adjustable regulation in the LV primary side of the transformer."

In that case, the transformer does the translation to HV with some fixed winding ratio, rather than redundantly using HV cascode devices in the secondary HV circuit and controlled by feedback through fixed resistor ratio.

rogerw

Geo:
?You can adjust the HV shunting voltage by either changing the resistor between base and ground or by changing the value of the zener and or both.
In theory, the Vbe temperature coef can be cancelled with a zener with a voltage around 6.2V or so. However, there is also the temp coef of the resistors which can vary +/-

last night I hit the circuit with a hair dryer, with cheapo carbon comp resistors for the 66M and a 5.1V zener the voltage when up about 5V? when raised from 25C? to about 50~60C. But the 1M? resistors were metal film. The trimmer and the 100k were cheapo carbon resistors.
I was thinking of replacing the original regulator tube in my CDV-700 6a with this circuit. Good enough.

Yes, you could also use a TLC431 or was it a 1431 shunt regulator IC inlieu of the zener, but not everyone has one.
I may also try your shunt regulator across the primary side idea!

P

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