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The 8.25K Resistor

September 7, 2010

One day I was in the lab with another engineer working on a signal conditioning circuit for a current sensor. The sensor was on a 25 Amp inverter board which had a large socket for a control board that plugged into it. The current sensor output was a voltage rather than a current, being a smallish sensor. We had been having random current trips occur on a few units and we suspected it was due to that particular circuit.

We spent hours poking and prodding the boards but didn't find anything significant. Then I tried pushing on the control board and rocking it back and forth in its socket. The bad behavior that had been reported almost immediately appeared before our eyes on the o-scope.

The other engineer came up with a solution which was to load down the sensor output voltage driver, which was on the power board, with a resistor placed at the signal conditioning circuit, which was on the control board. This would make the sensor drive more current through the socketed connection so the signal wouldn't flake out as easy.

Anyway that solution worked but that is not the point of this story. What was interesting was how I picked the resistor: 8.25k Ohms. When we decided to try adding the resistor I looked up from the lab bench and the first thing I saw was a random thru-hole resistor laying there. Any well use lab bench gets littered with random parts as work gets done and nobody ever has enough time to clean up as much as they should. I am not very good at reading resistor color codes so I really didn't even know what the value was but I did know it was in the range between 1.00k and 9.99k Ohms. That looked close enough to me so I soldered it into the circuit and it seemed to fix the problem well enough.

We reported what we came up with to the rest of the team and said we would eventually go back and pick a better value for the resistor, given we picked the original somewhat randomly. The exact value of the resistor wasn't critical, it simply needed to be a low enough resistance to load down the sensor. With a base level of milliamps flowing across the socket connection the circuit seemed to be more reliable. A more commonly used 1k or 4.99k resistor would have worked just as well.

The next day some big new emergency came up and I never got back to picking a better resistor. About a year later we were updating that control board (again) and I noticed that 8.25k resistor still on the schematic. Worse than that I found out that part was the only 8.25k resistor we used in any of our designs. That meant production was stocking and managing that resistor just for this one board.

I sent out an email to let people know I was going to start a change order to swap out that part for another value that we were already using on the board. To my surprise nobody wanted to change it. Production already had the parts bought in bulk and they were still skittish about me touching that circuit due to all the problems we had before adding the fix.

I few years back I took a job with another company, but to this day that 8.25k resistor is still shipping in production units. I guess the moral of the story is that as an engineer your small daily decisions can be multiplied to have effects for years and years.