[volt-nuts] Matched resistors

[Dave M]

Do you have easy, low cost access to an old Fluke 801 or 803 differential 
voltmeter?  These meters have a Kelvin-Varley divider inside that is 
composed of strings of resistors that are highly matched in value and 
tempco.  If I remember correctly, the highest decade is filled with a string 
of 40K resistors, each succeeding smaller decade value is 1/5 of the next 
higher decade, but all very well matched. They should all be very well aged 
by now too. (Just like wine... they get better with age).
Perhaps you could mix & match values to come up with suitable resistors for 
your project.  If your project is a one-off deal, then perhaps that approach 
could prove viable.

Cheers,
Dave M


> From: Tony <vnuts at toneh.demon.co.uk>

> Randy,
>
> Have you considered using multiple identical resistors to reduce the
> variance? Depending on who you believe, you can reduce the variance of
> the overall resistance by SQRT(N) where N is the number of resistors
> in series/parallel. Its not that easy to create a good search query
> for
> this but here is one such explanation:
>
> http://paulorenato.com/joomla/index.php?option=com_content&view=article&id=109:combining-resistors-to-improve-tolerance&catid=4:projects&Itemid=4
>
> Ideally they should all come from the same batch - ie. manufactured by
> the same machine from the same batch of materials. Obviously there's
> no
> way to guarantee that without close liaison with the manufacturer (you
> did want 10 million parts at $.10 each didn't you!) but hopefully a
> set
> of resistors which come off the same reel would come close.
>
> The absolute value isn't important however, but 'statistical gain'
> will also apply to the TCR and stability of the overall divider. The
> following assumes that both factors are similarly improved by SQRT(N),
> but in fact they may be rather better than that.
>
> That80? or $108 for one sealed Vishay foil divider will buy a lot of
> lower spec parts: