CUPS Resonance – a dark mystery

[jpanhalt]

I spent the past several weeks working on a variable-voltage charger for a capacitor bank. The initial concept was to use a relay controlled by a feedback circuit with an adjustable voltage divider and comparator. That worked, so I decided to go modern and use a high-side P-channel mosfet to replace the relay. The parts worked on breadboard, but when assembled on a PCB there were lots of problems. My amplifier oscillated. There were lots of nasty transients. Those problems were solved by a sprinkling of capacitors. I got good voltage and stable switching of the mosfet, until I tried to pull any real power. At that point, the mosfet almost glowed. I added an inductor – after all, it was basically a crude switch-mode supply. Things got a lot better. So then it was time to redesign, order parts, and wait. In the meantime, I continued to work on it with the parts I had on hand and a little scavenging. Well, I finally got it working today. I get more than 6A to a resistive load and the mosfet barely gets warm. UPS will come tomorrow with the new parts.

I suddenly realized that this episode followed a consistent and familiar pattern. I have a design problem, order parts, continue to work on the problem in the interim, and the UPS man arrives at almost the precise instant when the problem is resolved with the parts I had on hand.

Has anyone else noticed this strange resonance between cerebration and United Parcel Service delivery? Just how does UPS know the precise instant when you no longer need the parts?

I propose that this newly described phenomenon be called Cerebral UPS resonance (C.U.P.S). Best regards. John

 

[Hank Fletcher]

I kind of know what you mean. I ordered a variety of pushbuttons a few weeks ago, with the intent to see what would work best for my application. A chance drop-in at (what was formerly Radio Shack and is now the source) let me discover that they had something so close to what I was looking for, that I didn't mind spending the extra money. When the parts I'd mail-ordered arrived, I was able to confirm that the parts I'd bought locally were superior for the purposes of my prototype design. I wouldn't have bought the extra (local) pushbuttons, if I hadn't been so impatient waiting for the others. I bought them, and sure enough, the mail-order came the next day.

In a somewhat related story, I've been ordering sheet music for my band program, and all the recent selections came fine, except for one. They only sent the conductor's score (no parts for the instrumentalists). I gave the store (www.sheetmusicplus.com) a call, and asked them what the deal was. After a bit of a run-around, they said no problem and they'd send the rest on right away. Two weeks later, the package came: another conductor's score, nothing else. So now I have two conductor's scores, which are useless because I don't have music for the student's to play! I left a message with the latest problem, but I haven't heard from them about it yet. I kind of resent being in a position where I feel like I'm being a scammer - I have no problems returning the conductor's scores, I just want the full score and parts (that I've already paid for)!