Current Vs Voltage Piezzo diesel injector 2

[CarlosReis]

Hello

I connected my scope to the two wires of a Siemens piezzo diesel injector. One chanel reading voltage and the other reading current with a current clamp.

I get voltage growing around to 50v, stepping up and then falling to zero (This is normal).
In exactly the same time, current starts going up from zero to around 10 Amps and then falls to zero for a while but (here is were i don't understand...) it goes negative the same amount it has been positive before and goes back to zero.
I have a picture that is more explicit but I don't know how to post it here.

In a book that I have, Bosch has the same pattern for the same kind of injector (so, its a normall pattern) but does not explain how it works.

Question: How can I have, at the same time, in the same circuit, voltage in one sense and current that reverses?


Best regards
Carlos Reis
Lisbon, Portugal

 

[Fabrico]

You might take a look here:

http://www.drivven.com/visitor_download/DI_Driver_UM_RevD.pdf

 

[TStaples]

CarlosReis,

The Piezo elements that are being driven have a very large capacitance. In order to change their shape, you apply voltage. This voltage has to "charge" their capacitance, which you see as "positive pulse" current. In order to return them to their original shape, that capacitance has to be "drained" away. Hence, the "reverse pulse" current.



-Tony Staples

http://www.tscombustion.com

 

[CarlosReis]

Thanks for the reply(s).

As far as I have understand, the piezo works like a capacitor and the feed circuit "discharge" it at the end of each cycle in order to close the injector.

Did I got it?

Thanks
Carlos Reis

 

[Ron07663]

Fabrico, thanks for the great reference. What it doe not explain fully is that the piezo stack expands when you apply the voltage to turn on the injector. When you turn off the injector the piezo stack contracts and generates voltage. To prevent extremly high voltages and to control the turn off time, the injectors are discharged into a controlled impedance which develops the reverse current.

 

[TStaples]

Carlos,

You've got it! Good luck!


-Tony Staples

http://www.tscombustion.com