this is a follow up to my last question. I am working on a hand crank battery charger project. My current set up is that I am using a geared nema 17 stepper motor as a generator and have a bridge rectifier on each coil. I have the both of the two rectified coils wired in parallel to get max current output. I also have a 25V 4700uf capacitor across each rectifier to stabilise the output. I then feed the rectified voltage into a buck/boost converter to step it down to 5V which is what my 18650 li-ion BMS requires.

However I notice that when I first turn the crank there is a 10V spike from the output of the buck/boost module before for about 1 second before it stabilises to 5V (slightly less under load).

My question is what is causing this and is it a problem for my BMS circuit? If so how can I avoid it? I have a feeling it’s to do with that capacitors but I don’t know for sure. Help wound be much appreciated, happy to provide more details if need be. Thanks Billy.

  • \$\begingroup\$ What output voltage are you getting out of the stepper motor when you crank it? Can you put it on a scope and view the initial voltage whilst cranking and at steady state. What's the spec for the buck converter e.g. max input voltage? Perhaps you may need to clip a startup transient with a TVS before it gets to the buck converter. \$\endgroup\$
    – mhaselup
    Sep 19, 2020 at 12:34
  • \$\begingroup\$ @mhaselup I am getting about 15 volts DC from stepper after rectification. Unfortunately I don’t have access to a scope at the moment. I’m using the XL6009 35Vin max. Not sure what you mean by clip a startup transient with TVS. Do you mind explaining further please? Thanks. \$\endgroup\$
    – Billy
    Sep 19, 2020 at 12:51
  • \$\begingroup\$ You may get a high voltage spike when you start cranking, obviously without a scope it's hard to say. A transient voltage suppressor (tvs) or a zener setup could be used to clip voltage above 35v. \$\endgroup\$
    – mhaselup
    Sep 19, 2020 at 13:08

1 Answer 1


I think the three items will help provide some context. I think you may still be able to use the buck converter approach with a zener voltage limiting front end on your circuit.

What is the role of zener diode in this clipper circuit?

AC overvoltage protection for bike dynamo (alternator)

Calculating max power of bicycle dynamo


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