Of course, now that I"ve got that working perfectly on the bench, my brother has decided that he really likes Eric's suggestion from 15 November....
Is a CD4047 IC the right place to start for that 'Option B' approach? Something like **broken link removed** ?
That said, I did notice that the LM2907 'Option A' circuit exhibits a transition region wherein the load is neither on nor off, but in fact pulsewidth-modulated. If I can tune the circuit to be in this transition region when the fan is running at its low speed setting, I could theoretically achieve the 'Option B' functionality with the same basic circuit from 'Option A,' if I swap some kind of semiconductor switching device for the relay that's in the current plan. Can anyone recommend a transistor that's good for switching 5 A at 48 V? (Related question--when I see something advertised as a "solid state relay," is that basically a MOSFET [or ???] packaged to look like a relay? In other words, can I buy a "solid state relay" and switch it hundreds of times a second, PWM-style?)