Hello,

Back in university, I did a robot arm as my senior project. I spent more time in the engineering machine shop than I should have -- it was a lot more fun to design and machine valve bodies and bespoke position encoders (I counted gearteeth in a multiplying geartrain I handmade) than studying thermo.

Part of my design included solenoids which got hot if held for very long. I remembered from electromagnetics class that reluctance changes meant that the mag field (current) needed to pull the solenoid in was much higher than that needed to hold the solenoid in its actuated position. I came up with an RC timing circuit that would back off the current 6x, iiirc, and that design impressed my faculty advisors as well as getting me an IEEE best student paper award regionally. 

Fast forward to July 2025 and I got an Electronic Design reader's request for missing figures in a piece on driving solenoids, which is what brought all this nostalgia about - a subject near and dear to my formerly young, and less calcified, heart..."economizer" circuits.

So, I am including that now-updated economizer article for your review, which uses op amps, as well as some more integrated and modern approaches I found for you to look over, including a piece where automotive and analog editor/guru, and Bob Pease accomplice, Paul Rako recalls discussing solenoid drivers with Bob.

Somewhat related in terms of low-side driver circuits is an octal low-side driver that has the ability to remap its outputs to provide a degree of system robustness through redundancy, aimed at Automotive Integration ("AI").

Also in this issue, digital twins bring a faster, more comprehensive, approach to wringing out new designs, as well as reducing the chances of errors in complex vehicular system design and validation.

And, to wrap up this issue's content, Bill Wong hosts an Inside Electronics podcast discussion about Robot OS, which is middleware that can be used for almost any type of robotic platform, including self-driving cars.

Back in school, we cut our teeth on Intel's 8008 (complete with toggle-switch-entry of the bootloader that pulled our programs in from the TTY's paper tape reader), with the 68k and 8085 just emerging back when I was designing my robot. None of this fancy Robot OS stuff....we only got 64k of memory space on a good day, programs were loaded into 2708 EPROMs, and we wrote code in assembler. Yeah...that was a while ago.

enjoy,

-AndyT