hello everyone
i am having a problem, which is converting AC to DC
i have a two wire speed sensor, it is generating about 10V in AC
now i would like to reduce it to under 5V in DC
i tried to do the conversion, the oscilloscope showed me like this, is it normal ? am i doing something wrong ?

 

Your circuit is half wave rectifier, scope picture is correct. Your generator is not producing a nice sine wave. Depending on your application, half wave rectification may be ok? Alternatively full wave, in which it is easiest to use a full wave bridge rectifier. To set a stable 5V output, use a pos. voltage regulator --(eg LM78L05)
Make choice of regulator to suit circuit current.

 

You need to consider the output current capacity of the sensor. Placing a capacitor that is essentially directly across the output is probably not a dood idea. Inserting a a series resisitor of some value may help.

The speed sensor is probably not a good source of rectified power, if that is your intent. Without knowing your design and interface spec's, it's not easy to provide circuit advice.

 

Another approach might be to buy an IC that would convert the AC signal to a DC signal proportional to the RMS voltage of the AC signal. This might make it the only component needed other than perhaps a voltage divider on the input.

 

Look at an LM2917EP-8 IC.
http://www.national.com/mpf/LM/LM2917EP.html#Overview
You need to get the -8 model for a magnetic transducer pickup.
The LM2907EP is similar, but it does not have an internal Zener regulator, so it requires more external components.