So my plan is to charge a powerbank with a bicycle dynamo. I have got it to work with a full bridge rectifier and a linear regulator, it just isn't very effective. I used the following circuit:
(The hatched box is the circuit that comes with the powerbank).
After some research I found that using a buck-boost converter is way more effective. So I ordered the following from AliExpress: https://www.aliexpress.com/item/Boost-Buck-DC-DC-adjustable-step-up-down-Converter-XL6009-Module-Solar-Voltage/32661520979.html?spm=a2g0s.90423188.8.131.52774c4dvUq5VL
I connected it to my DC voltage generator to set the voltage to approximately 4.5 V, since that's what the power bank would need. Until here it all worked fine.
Then I connected it to my circuit instead of the linear regulator. When I started spinning the dynamo, the output-voltage of the buck-boost converter would rise quickly to 20 or 25 V (probably even more, didn't want to try that though, in fear of breaking it) with only 3 to 4 V input. I also tried it without the capacitor (1500 μF), hoping that would solve the issue, which it unfortunately didn't.
I know there are different types of buck-boost converters and I might have picked the wrong kind... Could someone please help me to get this working?
Any help is appreciated.
Edit: The linear regulator I used is a L7805CV, which has an output voltage of 5V.
The Capacitor should be big enough. I measured it with an old oscilloscope and it looked fine as far as I can tell (by that I mean it was a more or less smooth line).