Hi

I came across the attached circuit in a paper. I assume D1 rectifies half the input wave (since the current source and capacitor in parallel are representative of a piezoelectric source) but I'm not sure of the purpose of diode D2.

Could anyone please explain the effect of D2?

Thanks!

 

It looks like a voltage doubler circuit, like the circuit partway down this link:

http://www.play-hookey.com/ac_theory/ps_v_multipliers.html

 

It looks like a voltage doubler circuit, like the circuit partway down this link:

http://www.play-hookey.com/ac_theory/ps_v_multipliers.html

Looks like it to me too.

 

Don't you need two capacitors for a voltage doubler?

 

You have two. The PZT device acts as a capacitor. A perfect current source has infinite resistance, so does not short the PZT capacitor.

awright

 

You have two. The PZT device acts as a capacitor. A perfect current source has infinite resistance, so does not short the PZT capacitor.

OK so I'm being thick here, please explain in more detail how this works?

The capacitor in the piezo element is in parallel with the supply and the D1/D2 junction, not in series. A series cap provides the charge pump function inherent in the Cockcroft multiplier described by others

Label the top of the current source A and the bottom B.

If A is positive, relative to B D1 conducts, D2 blocks and C is charged accordingly.
If A then becomes negative D1 now blocks, cutting off C from discharge, and D2 conducts, providing a path for the current.

Thus C is maintaned at the positive voltage on A.

If the piezo element is now triggered, generating a positive voltage at A this will be added to the voltage already on C.

If the piezo element is now triggered, generating a negative voltage at A, D1 again blocks, thus the voltage on C is unaffected.

So this circuit appear to me to be a circuit to add a positive peizo generated voltage to any impressed on the output capacitor by the rest of the circuit.

Not quite a multiplier.

 

I think its a clipper circuit.

 

There is a mechanical delay with the piezo, compared to the time to charge the capacitor. Exact delay depends on size of the element for the resonant frequency.

I'm in the same boat as studiot, though the constant current source may be tripping me up in quick mental visualization.

 

Note also that piezo devices are essentially voltage devices. They are not capable of supplying much current. So all analyses will depend upon the size of the capacitors.

 

But what is the practical use of this circuit then ?

 

But what is the practical use of this circuit then ?

Good question, to which we would all like the answer.