Interview with Jochen Kronjaeger

Featured Engineer

Jochen Kronjaeger

Jochen Kronjaeger - Physicist, Cold Atoms group, University of Birmingham (UK)

How did you get into electronics? Were there any first projects or mentors that got you started down this path?

I got started on electronics when as a kid my parents bought me a “Schuco Elektrotechnik” electricity kit. So I learned to how to make a relais, an electric motor, and electrically triggered fireworks (from matches). I went through a succession of electronics kits, started to roam electronics shops, got my own scope, started to etch pcbs etc – which all came in handy when I became interested in high voltage in particular.

How and when did you become interested in High Voltage/Power Electronics?

I was probably round 14 years old when I borrowed the book “Physik A-Z” by Hoefling and Wittmann from the local library. This physics dictionary for pupils has an appendix featuring a wealth of experiments, and among them quite a few on electrostatics. The best thing however was that it had instructions on how to make an electronic source of high voltage – a Cockroft-Walton style voltage multiplier. ElectroSTATICS is nice, but add some juice and it gets really fascinating! That’s how it started. Soon I was scavening TV sets and rummaging through the remote corners of electronics shops and car boot sales for exotic components and antique literature. Then the internet came along, with personal home pages and news groups about Tesla coils – and of course ebay, which opened up a whole new dimension. Still, some my best projects were assembled from “scratch”. In the pictured on my web site you’ll recognise hot water bottles, door knobs, yoghurt containers, and Mardi Gras plastic cups from New Orleans.

Electronics remained a hobby until I started my phd thesis at a Cold Atoms experiment in Hamburg, where I enjoyed the company of some very knowledgable colleagues and got my hands on professional (expensive) equipment. But even today, I mostly teach myself – which is probably not always the most efficient way of learning, but certainly helps with understanding.

What is on your bookshelf?

Two classics of electronics: “Electronic Circuits” by Tietze, Schenk and Gamm and “The Art of Electronics” by Horowitz and Hill. These two books complement each other, “Electronic Circuits” is a comprehensive and mathematically sound treatment while “The Art of Electronics” offers a more hands-on approach. Another book that offers unique insight in a way very much suited to physicists is “Noise Reduction Techniques in Electronic Systems” by Ott. The relatively large and complex experiments typical of modern ultracold atoms research are particularly vulnerable to technical noise, and it’s important to know your enemy.

Besides my job, I have a strong interest in badly designed high voltage electronics – the more it sparks the better! There’s quite a lot of good information available on the internet, and some of it has been condensed into printed books (among them my own book “Experimente mit Hochspannung”, published as part of a series by Franzis Verlag aimed at the amateur – available in German only).

What are your favorite hardware tools that you use?

My field of research, ultracold atom physics, involves a multitude of technolgies: ultrahigh vacuum, lasers, high power electronics, rf and microwave and others. Our requirements are very specific and often at the “cutting edge” of technology, so we design a lot of our own electronics – servo loops, signal conditioning, etc. For electronic development I use the classic tools of the trade, signal generator and scope – a 50MHz DDS synthesizer and any decent digital scope generally do the job. For noise analysis not just of electronics but also of e.g. lasers, a high-end spectrum analyser like Rohde&Schwarz FSP7 is indispensible.

I tend to avoid having sensitive equipment around when tinkering with high voltages – it’s hard enough to keep any electronic components in the generator from self-destructing. A simple spark gap tells me everything I need to know.

What are your favorite software tools that you use?

I use Eagle (CadSoft, now owned by Farnell) for schematics and pcb design. It’s inexpensive and widely spread among hobbyists, which is an advantage at the university – chances are, a new phd student interested in electronics will already be familiar with it. It also does a very good job and is certainly adequate for our relatively manageable projects. The user interface takes a while to get accustomed to but is quite efficient once taken to heart.

What is the hardest/trickiest bug you have ever fixed?

My whole job as a researcher boils down to find “bugs” and subsequently “fix” them, in a way. In fact, there’s generally no shortage of bugs. The tricky bit is to choose the one that’s worth fixing!

Do you have any tricks up your sleeve? (special way to analyze circuits, special process you use to make something, etc.)

Everyone has their tricks and most are far too specific to be worth sharing. However, the one advice I always give to students is: try to understand the WHY. Even if someone tells you how to do it – ask, why that way? Understanding is key to progress, and besides it makes life a lot easier as it minimises the amount of facts you have to memorise!

Do you have any note-worthy engineering experiences? (blowing up things, getting shocked, etc.)

You’d think playing around with hundreds of kV was pretty dangerous, but in fact the worst shock I ever had was from mains voltage, when I was foolishly changing a fuse without pulling the plug. The least expected shock I ever had was also from mains voltage, and it took a very close look to find that the brand-new moulded plug had a single copper whisker sticking out by some fraction of a millimeter. The most dangerous situation involving kilovolts was when a jacob’s ladder setup I wanted to take pictures of collapsed in the dark while energised, and came to rest somewhere between me and the main switch – I’ve definitely learned the lesson from this one.

Even supposedly low-power electronics has its risks – I’ve recently had some tantalum capacitors blow up more or less into my face, which for the comparatively minute amount of energy involved I found rather impressive!

What are you currently working on?

My job at the moment is setting up an experiment to cool Rubidium and Potassium atoms into a quantum gas at temperatures around 100nK, which can then hopefully be used to shed light on some deep problems of quantum mechanics. It may actually have applications as a “quantum simulator”, a kind of analog computer that mimicks the behaviour of e.g. electrons in superconductors but is much better under control than the “real thing”. This is going to be a pretty complex machine, and has taken resp. will take years and a team of several students and researchers to get to work. It also involves a lot of fun with lasers and other high-tech toys!

The most pressing project on the high voltage side is probably updating my website. Limited resources in time and space have forced me to all but give up on any real experiments for the time being, but I’m steadily aquiring ideas and materials. One of the fascinating aspects I intend to look into at some point is the very short time scales involved in high voltage discharges. Rise times in the nanosecond range can easily be achieved with a simple low inductance spark gap, a fact that is used e.g. in atmospheric air nitrogen lasers. The downside is that sparks can lead to massive ground bounce spikes, sufficient to destroy equipment sharing the same ground e.g. via the mains lead.

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