Lars Krüger is a Potsdam, Germany-based teacher who has been reading Elektor magazine for eight years and designing electric bicycles since 1992. As you can imagine, he has learned a lot over the years about high current control, circuit testing, and e-bikes.

High Current Control

“In a former time and when I was younger, I tried to get things ready in a very short time. A quick idea in the morning had to be ready in the evening. I didn't care a lot about some malfunctions, when the rest was quite OK and running. But that led to a lot of broken and burnt semiconductors when it came to higher voltage and/or currents! For instance, in 1992, I built my first electric bicycle. Having read only a quarter of a book, I prepared a PWM, running at some 100Hz, a set of 10 'BUZ' MOSFETs, a huge 100Ah car battery and the 2kW starter of a Citroen CX 2.5D. After the first ‘throttling,’ the motor was just ‘on’ (MOSFETs were burnt) and the bike, getting a speed of around 60kph+, was quite unstoppable. Of course, switching off the supply voltage of the PWM was useless because the burnt FETs didn’t care about gate voltage. Lucky me that I built in a fuse and heavy braking lead to blow it. Next steps were that I built a large switch, made of thick copper and a spring. This was connected to the Bowden cable instead of the former potentiometer. The bike was just fun with this pure setup — 10m pedaling and then ‘puuush.’ Unable to control in the narrow streets of my hometown, but making me smile from one ear to the other when outwards. What I learnt from that story was that controlling (limiting) current is a necessary thing, when scaling up the size of a motor, you can't skip or forget it. And that this control has to be fast enough to cope with the gradient of the current rise. Otherwise, you'll have oscillations in your control loop. Later on, I studied and heard ‘control techniques,’ all this ‘s+1 stuff’ with minor effect. It had little to do with real life. A better experience were the upcoming simulation tools like PSpice or its follower SIMetrix. From that time on, I had much less problems with controlling high currents. But, of course, burning MOSFETs is not completely banned from my home lab. Sometimes, a probe slips off and produces a short while measuring. Good, if you're wearing goggles and the light circuit of the room is separated from the circuit you're testing at. By the way, the photo shows my second e-bike from 1994. The first was too fast and seems that I had no time to take a picture of it.” – Lars Krüger
Krüger e-bike projects taught him a lot about high current control
Krüger e-bike projects taught him about high current control and more. Check out his e-bike from 1994.

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