More on Pease’s precision resistor article

-October 07, 2013

An excellent question came up on the EDN article “Bob Pease: His last challenge, Part three--The precision resistor” by “Kevin35” an EDN audience member, “This was a great series, which I was glad I followed and wish would have been able to go on. The statement "I showed Bob how he could rig up a (low resistance) test setup fairly easily if he had access to a good voltage DVM and a good reference resistor(s)" caught my attention. How DO you do that?”

I thought I would put Edwin Pettis’ comments and a good image that you can enlarge on this blog instead in the article comments. Here goes Pettis’ reply:

“Thank you Kevin, this would have been a huge project, their projections for the number of required PS-6 units was in the thousands per month. I would still like to know what happened to the project. As to your question about the bridge, Steve is going to try and post the schematic of Bob's 'stop sign' bridge here if possible plus some additional comments on it for you from me.”

And here are the details as explained by Pettis with the schematic image:

Basically it was a standard Wheatstone bridge configuration which was 'modified' to accommodate Bob's lack of a 'proper' reference resistor.  A null detector would work just as well as a DVM here.  A copy of Bob's schematic (which is for the STOP sign bridge), shows the large number of power resistors and the various adjustment points.  The intention of this setup was to measure 'drift' of the 0.2 ohm resistor which is what Bob was primarily interested in, not the absolute value.  This setup would not be considered an accurate bridge for measuring actual resistance values.  The last I heard from Bob, he was still having some problems with stability, nothing major but bad enough to irritate him.

The Wheatstone was the first resistance bridge designed to accurately measure resistance, I'll give you a brief description here, it basically consists of four resistors in two legs, the top pair of resistors are the same value and are preferably well matched in value and stability, their value depends on the resistance value being measured.  In the left leg, the lower resistor is usually a variable standard resistor.  In the lower right leg is the 'unknown' resistor.  The null detector is connected at the 'bottom of the two matched top resistors and the 'standard' resistor is varied until a null is obtained, this is the value of the unknown resistor.  Just do a search for Wheatstone bridge on the Internet and you will find detailed information on it.

I use an ESI 242D primary resistance bridge with various external reference resistors which allows me to not only measure resistance very accurately but also any drift that may occur.

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