In reading back over my sarcastic description of the 5061A battery charger, I was pretty hard on the unnamed designer of that circuit. I did find a schematic of an older generation batter charger for the 5061A, and it had the same basic implementation.
As far as I know, the circuit, in all it’s complexity, was reliable, and causing no field problems. This really is the most important aspect. For low volume products, another $5 of component cost, on an instrument that sells for $20,000+, isn’t material. A guess on what happened, to cut this dude some slack: As all of you know, the most important aspect of any design is the original definition, or objective for the circuit. Many times, the circuit designer is not the “architect” of the system, and is just implementing what they are told to do. Maybe the lead system designer wanted a fast recovery circuit for the batter charger, because marketing wanted to push the feature about how the system would be back to full capacity quickly. Who knows? While the implementation had some excessively complicated details, the bigger issue was the requirement for a fast/slow charge, which I felt was unnecessary. This first designer may not have had the freedom to make that decision. While working on the circuit, I first started developing my now very strong bias towards “elegant simplicity” in my designs, that I have carried forward to this day. I’m sure I got a lot of encouragement from some of the older HP engineers to think this way. I hate complexity that doesn’t add value. I don’t find it cool or exciting, just a waste. I am not a good engineer on programs that want to bling out a system with lots of fluff. Somewhere over the years I picked up this line: “A good engineer is a lazy engineer. They are always looking for the easiest way to do things.” The designer of the 5061A battery charger was definitely not a lazy engineer. Cheers, Hugh Rice _____________________________________________ From: Rice, Hugh (IPH Writing Systems) Sent: Saturday, February 9, 2019 10:31 PM To: '[email protected]' <[email protected]> Cc: '[email protected]' <[email protected]> Subject: HP Stories: Battery Chargers, and a fading idolization of HP I grew up on Silicon Valley (Santa Clara Valley, California), graduating from High School in 1980. My home town is Cupertino, also the home of Apple Computer. During my formative teen years in the 1970s, HP was THE company in the valley, the elder statesmen of Hi-Tech. (Apple was just a small upstart business for computer geeks, and their headquarters were just down Bubb Road from my HS.) The reputation of HP as a great company was untouchable. Both in the quality of the products, and as a the best place to work. Being from Silicon Valley, I choose to study electrical engineering (rather than my natural inclination towards mechanical engineering, having done a lot of work on cars and bicycles), hoping to come back to the area to work when I graduated. I had fantasies of working for HP in my college summers as a “SEED” student, but was never able to make the right connections. A HP job upon graduation with a BSEE was like winning the lottery, especially for a Silicon Valley Kid. Home for Christmas during my senior year, I was visiting a friend from church, and her dad (Charles Adams) asked me how my job search was going, and if I had considered working for HP. “I’d LOVE to work for HP. I just can’t crack in and get an interview.” He said they had an opening for a new grad EE, and asked if I would be interested in considering it. “Uh, yes!” A few days later, I was in the Precision Frequency Sources Production Engineering area, doing the all day rounds of a classic HP interview. I did well enough that they offered me a job the next day, and I went back to school for my final semester with a HP job in hand. A certifiable miracle. It didn’t get any better than this. I asked Charles what I could do to prepare for the job, and he mailed me a 5061A Operating and Service Manual to review. It was incomprehensible. But I could tell that the 5061A was something pretty special, because all the circuit diagrams and theory of operation descriptions had things in them that even the grad students I knew couldn’t understand. And I had a job at HP! Working on Atomic Clocks! (Whatever they were.) As you know from past postings, my job was part of the 5061A to 5061B development team. The first task I was assigned to was to freshen up A2 Battery Charger Assembly. The purpose of this circuit was to keep the 20 cell NiCad backup battery ready to supply power in emergencies. The must fix issue was a gigantic mica capacitor used in a RC timing circuit, which was both expensive and unprocurable. I think my mentor, Roberto, encouraged me to look over the whole circuit, and sift out all the other old parts that would a problem in the near future. (All that code 4 part stuff.) So I studied the circuit like only a new grad can do. Brand new engineers are nearly worthless, and no one has any reason to talk to them or distract them. This was before the internet, cell phones, email (at my location), and any other distraction. I had hours and hours of uninterrupted time. The circuit was strikingly complex for a battery charger. But you have to remember by frame of reference. HP was the best electrical engineering company in the world. Their products were awesome. Surely every circuit in every product was the result of deep expertise from brilliant engineers, and every resistor, capacitor and transistor had a sacred purpose. So I kept digging until I understood every aspect of the circuit. The “brilliant” designer had decided that the battery charger needed to be a two-level system. A fast charge to replenish the battery quickly after a power failure, and then a lower “trickle” charge to keep it topped off long term. But how long to do the fast charge? Well, if we time the discharge time, we can then use that to set the fast charge time. Battery power for 20 minutes? Fast charge 20 minutes at a similar rate. In addition, the charger assembly had a drop-out relay circuit, so if the battery voltage got too low, the relay would trip to disconnect the battery protect it from over discharge. (Oh yes, the relay used was also impossible to procure, being some ridiculous double pole, double throw, part unique to the 5061A). Add another circuit to flash the front panel light when the back-up battery was supplying power. And then the charge current regulator itself, which of course was a transistor level fully discrete design with dozens of components. But it gets better: The discharge, recharge timing circuit used the most complex counter IC available from the TI TTL catalog. A pre-settable, programable, up-down counter. There may have been two counters, for 8 bits of resolution. To make it better, the counters were pre-set to a specific but obscure number, that had them exactly count down to zero after about 90 minutes, when using the timing rate set by the RC circuit with the aforementioned giant unprocurable Mica capacitor. I wish I could find a circuit diagram of the 5061A, to fully relive the experience and share it with you all. OK, now that I finally had the circuit figured out, what do I do? I think Roberto recommended that I read the “GE Battery Book”, since GE was the supplier of the Ni-CAD battery pack, to give me background on rechargeable batteries. I bought a new copy, and read the whole thing cover to cover in a couple of days. All kinds of information on charging at the “C” rate (a 1Amp Hour battery “C” factor is 1 Amp) for fast charging, and 0.1C for trickle charging (full charge in 10 hours), and how trickle charging is best for the batteries, and so forth. As I’m reading the GE book, I’m slowing realizing that the 5061A battery charger is pretty complicated, for what appears to be a fairly simple task. As I chat with Roberto more about it, it becomes clear that the circuit is perhaps a bit over complicated. But wait, aren’t HP engineers the best in the world, and every circuit they design the best possible implementation known to man? The idolization I had developed from my Silicon Valley roots, and all the praise from my university professors and peers is starting to loose its luster. This battery charger wasn’t a great design. I wasn’t even a good design. It was a ridiculously complicated, expensive, idiotic design that never should have made it through any sensible design review. I never did learn who did the work. I don’t think they worked at Santa Clara Division any more. Probably a good thing. This was actually a blessing in disguise. Only a few months into my HP career I learned the priceless lesson that inherited work isn’t always good work. I also learned to really understand the task that needed to be done, before you set out to do it. Like learn how batteries work before designing chargers. And then I had the opportunity to design a completely new circuit from scratch. I had a new goal: Make a good, simple, reliable battery charger, but it had to be backwards compatible to the 5061A. HP didn’t want to support the old design any more (huge mica capacitors and crazy relays), and the new circuit needed to be a replacement assembly too. This presented another priceless lesson: real work design is often constrained, and not clean sheet like those university projects. I’m still proud of the final battery charger design. So proud that attached the 5061B circuit diagram if you want to be impressed. 😊 I ditched the timing circuits altogether, and just went a simple trickle charger. If a 5061B application has a power outage of 30+ minutes, how likely are they going to have another long power outage within the next few hours? Not very. The new circuit used standard, inexpensive, robust components in the whole design. There are other clever (to me) aspects, and you can discover them on your own if you are interested. One last story on this. My first prototype of the circuit was hand soldered together on proto-board material. That generic PCB with grids of 0.10” pitch holes all over it. Roberto told me to go to the 5061A production line tech’s, and have them try out the new circuit. Picture a brand new engineer who looks about 17 years old, with this rats nest of a proto circuit for them to test. I recall one of the Tech’s, Ray, putting on safety glasses before he put it in a test 5061A and powered it up. They never wore safety glasses. I don’t think it worked on the first try. But I recall that the problem was something simple to resolve. Over the next few years, HPs iconic reputation would continue to loose it’s luster in my eyes, as I learned that mortal people worked there, and that real business had real problems. But there were also a lot of really good engineers and technicians there, and they would help you if you asked. It was a fantastic start to a 35 year (and counting) career at HP. Happy Chinese New Year from Singapore, Hugh Rice << File: 5061B Ch8 A2 Battery Charger.pdf >> _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
