- Edited
@martin and anyone who is interested: With the help of the above link I managed to make my BPW34 very responsive for oscilloscope use. Older posts here helped me too, like TechSensitive mentioning a 100,000 Ohm resistor.
I also bought another oscilloscope, the PicoScope 2204A. It is their cheapest oscilloscope for ~140 € and does the job. They have excellent online support and replied to my questions in detail, recommending this model. In fact I do not understand why I had to search that long to find PicoTech when it seems to be the best USB oscilloscope manufacturer.
This is how I made the BPW34 responsive:
Note how the BPW34 has a dot on its detector surface, pointing to the end that gets connected to the resistor. It is a 100K resistor. The hook and grapple hold the resistor. The grapple also holds the black battery cable, negative pole (-).
The battery's positive pole (+) goes directly to the BPW34.
I do not know the maximum frequency that this setup can measure. The BPW34 PDF says the maximum responsivity is 100 ns with 10 V and 1K resistor. A 100K resistor amplifies the current voltage so it is easily detectable by the oscilloscope, however it may make the diode less responsive.
However, with this I measured 200 kHz very low amplitude ripple on my "flicker-free" monitor, so there is that. Anyone has an idea what the limits of this setup are?