Moscaps? Any ideas?

Hi, Anyone care to explain what the function is of the 2 FETS Q1 and Q2 in this Fluke calibrator and what might be the required parameters for a replacement?

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It fails diagnostics with leakage caused by one of these. At the moment I have them replaced with J210 FETS which I had to hand but I think there's maybe more to it than that. I cant find any data on them, they are marked IFC 07703 TIA JC

Reply to
JC
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Protection diodes.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

Yes, jfets used as diodes. Terrible idea.

The c-b junction of a small transistor would be better all-around. BFT25 has fA leakage... if you can get them. I could spare a few.

There's probably a common TO92 transistor with sub-pA leakage.

Reply to
John Larkin

torsdag den 13. oktober 2022 kl. 22.52.31 UTC+2 skrev John Larkin:

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Reply to
Lasse Langwadt Christensen

Oh no! It's Squeaky Dave!

Reply to
Phil Hobbs

he doesn't drink, so his voice never gets lubricated ;)

Reply to
Lasse Langwadt Christensen

A big of brake cleaner might help. ;)

Reply to
Phil Hobbs

Thanks for the replies. JC

Reply to
JC

There are special low-leakage diodes, just Google for 'picoampere diode'.

The best-known is called PAD-1, from several manufacturers.

Reply to
Tauno Voipio

I would think that if this was a Fluke designed circuit, that it wouldn't be something that wouldn't work.

The people I know that work in the Fluke calibration engineering department are pretty smart.

boB

Reply to
boB

The PADS are actually jfets. They are expensive, have crazy high capacitance and series resistance, and leakage is huge compared to a decent transistor c-b junction.

Reply to
John Larkin

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1 pA, C_r 0.8 pF was state of the art at one time :-) and maybe still "OK" for this application

node impedance ~ 1M so

1nA becomes 1mV offset. = RS
Reply to
Rich S

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1 pA, C_r 0.8 pF was state of the art at one time :-) and maybe still "OK" for this application

node impedance ~ 1M so

1 pA becomes 1 uV offset.

= RS

Reply to
Rich S

I tend to agree and as this is an expensive repair/piece of gear I think I might go with the PAD solution suggested by TV. They are easily available and not that expensive in the world of Fluke parts :) Thanks again for all the replies, much appreciated. JC

Reply to
JC

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