Precise voltages from imprecise parts..

Robert Baer · 2015-04-17T08:32:57+00:00

Take a random LM285Z 1.24V 1% part. Select a 124K 1% resistor and select a 2.37M 1% resistor from nominal population by measurement, each one to 5 digits. Goal is to get 25V as close to 0.1% as possible. ASS-u-ME one measures the LM285Z reference voltage in the 20uA to 500uA region,to 5 digits. What is the best strategy to determine that resistor selection? Pick the 124K first or the 2.37Meg first? I calculate 24.703V if all are at the low end of spec, 25.177V if all are at the high end of the spec, and 24.940V if all are at nominal value. I have found that these daze, that the film 1% 2Meg resistors are within 0.3% and nominally high by about 0.2%. I assume other values behave the same way; this makes a target of 25V 0.1% harder to achieve. Suggestions?

Apr 17, 2015 86 Replies

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Tom Miller 11 years ago

Even better, heat it a bit and pull a vacuum while immersed in the varnish.

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Phil Hobbs 11 years ago

Way too much like work. I could build the equivalent from a CCFL supply and a voltage doubler. I have a bunch of HV resistors in the drawer.

Cheers

Phil Hobbs

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M Philbrook 11 years ago

neon sign xformer on a autotransformer :)

Jamie

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Tom Miller 11 years ago

Do you still have it (the HP 6110A) or did it really go to the dump?

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Phil Hobbs 11 years ago

Nah, I heaved it. My lab is failing the cat test too badly atm to keep a lot of broken stuff.

I did fix my two HP 3325As a few weeks ago.

Cheers

Phil Hobbs

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Phil Hobbs 11 years ago

Nah, I heaved it. My lab is failing the cat test too badly atm to keep a lot of broken stuff.

I did fix my two HP 3325As a few weeks ago.

Cheers

Phil Hobbs

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Robert Baer 11 years ago

YES!!! I had no access to a vacuum pump so had to wait longer.

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