I can't seem to find an answer to this, or the answers available are too advanced for me to understand.

I have an AC source 0VAC-40VAC. I also have a buck converter rated at 6VDC-24VDC input, and 5V output.

To connect the two, I've soldered 4 diodes together to convert VAC to VDC. I have put a 50V capacitor at the output. But my question is, how do I convert the DC source (after the rectifier) so that it caps at 24 VDC? If my memory serves, the Vrms is what comes out of the AC source and so the DC out of the rectifier is the same as the Vrms minus 0.7 (diode) multiplied by 2? Do I add more diodes in series to drop the voltage from 29VDC (0.7 x 40V) until it drops to 24VDC?

The problem is I can't measure because the AC source is not at a constant voltage, it goes anywhere between 0 and 40 at any time.

Please point me in the right direction to figure this out.

Thanks for reading.

  • \$\begingroup\$ What is the source impedance? Maybe you need a buck-boost with a higher rating \$\endgroup\$ May 16, 2019 at 23:25
  • \$\begingroup\$ Maybe it's not possible to do what you just asked if you cannot measure AC \$\endgroup\$ May 16, 2019 at 23:36
  • \$\begingroup\$ Thanks. I don't know the source impedance unfortunately. The source is actually a dynamo. It depends on the water flow that drives the dynamo. Sometimes lots, other times the water level doesn't reach the cups to turn the dynamo. \$\endgroup\$
    – Azrudi
    May 17, 2019 at 0:17
  • \$\begingroup\$ Dynamo doesn't output AC voltage. \$\endgroup\$
    – user208862
    May 17, 2019 at 0:29
  • \$\begingroup\$ Do you mean it outputs DC voltage? \$\endgroup\$
    – Azrudi
    May 17, 2019 at 1:27

1 Answer 1


Best way is to use a transformer after your 40V AC to get voltage down to about 15V. Then place a rectifier.

Or use a buck with higher input voltage.

Btw. the rectified voltage is equal (little less) to max value of AC voltage, not rms.


To make it clear I added a waveform of rectifier with 4OV input amplitude (blue) which is a 28Vrms and rectified output (green).

enter image description here

  • 1
    \$\begingroup\$ oh, I guess that's my biggest problem then, that I thought the rectified voltage is equal to rms. \$\endgroup\$
    – Azrudi
    May 17, 2019 at 0:15

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