PW 1986 High Impedance MOSFET Voltmeter

[Skywave]

Thank you David; thank you Joe: pdf file now down-loaded and stored.

Al. / May 3, 2013 //

[Yorkie]

Thanks for reading the thread Nick, and for your helpful comments.

[Joe]

PCB art-work now added to the Archive.

- Joe

[jimmc101]

As an aside, with the overall unit at nominal input impedance of around 10Mohms and with the CA3140s having an extremely high input impedance of 1.5Tohms, it might be a good idea not to use sockets for the opamps and to use a conformal layer on both sides of the PCB to prevent moisture ingress and leakage...

Another possibility is to use individual poles from a (turned pin) socket and a guard ring on the PCB to minimise leakage.
See http://www.ti.com/ww/en/bobpease/assets/...om_rap.pdf page 46 (!19.7MB download)

Jim

[Nick]

As an aside, with the overall unit at nominal input impedance of around 10Mohms and with the CA3140s having an extremely high input impedance of 1.5Tohms, it might be a good idea not to use sockets for the opamps and to use a conformal layer on both sides of the PCB to prevent moisture ingress and leakage...

Another possibility is to use individual poles from a (turned pin) socket and a guard ring on the PCB to minimise leakage.
See http://www.ti.com/ww/en/bobpease/assets/...om_rap.pdf page 46 (!19.7MB download)

Did you mean http://www.ti.com/lit/an/snoa664/snoa664.pdf (the original app note, AP241 by Bob Widlar, 1980) ?

Guard rings would help if making a new PCB, however as David has already made & populated the PCB, its not an option - if starting again, I'd completely redesign the PCB, for sure

As the board is extant, desiccating the board then conformal coating it as soon as its cooled would be a big help, as would getting rid of the opamp sockets (bad reliability & lots of stray capacitance).

If no suitable spray is available, I suspect that a quick dip (for the board) in Ronseal would do the trick nicely !

Cheap wins !

[Refugee]

I can agree with getting rid of the IC sockets as these can go intermittent.
As far as guard rings go I must point out that the greatest resistance to ground is the 11 meg resistor chain and that would be from pin3 on IC1 to ground on the highest voltage range.
The guard rings may not be worth the trouble.

[Joe]

I've finished mine:-

       

It's more accurate than I was expecting as I hadn't got 1% resistors in the required values.

It's cost more than a DMM which would have been easily as good but wouldn't have given me a week's amusement, which was the whole point of making it.

- Joe

[Refugee]

I have just finished restoring one with a double triode in it.
The restoration story is on the way in the next couple of days.

[Mark Hennessy]

Looks really good Joe!!!

Given the very high impedances on that circuit, it might be worth spacing the Veroboard up from the bottom of the case - looks a bit close on the photo (sorry if that's stating the obvious). Old Bicc biros cut down make good spacers for the cash-poor, time-rich

I have a small collection of moving-coil meters, and keep thinking that I really must do something with them. One day...

[Joe]

Making and fitting stand-offs for the PCB will be easy enough as I want to take it to bits again as I damaged the paint-work putting to together. Now I know it works it'll also be worth putting better resistors in, when I get some.

I would be interested to learn what problems the board being flat on the bottom may cause. Nothing is going to get very hot and I wouldn't have thought that wood would cause problems with unwanted capacitance but then I don't know much about such things.

Cheers

Joe