AVR-Chat

At 08:06 PM 7/20/06 +0000, Ryan wrote:
>How would one calculate the values needed if
>
>1) I don't know the inductance of the solenoid and

Measure a sample or two.  Although not built as inductors they will be 
reasonably consistent because they need a consistent magnetic field from 
one to another.  If you are using more than one type design for the largest 
inductance and build in a generous margin.

Also you can ask the manufacturers.  If they are decent they can provide 
with that information and give you an idea of the part to part 
variability.  If they can't that's reason enough to be wary of their product.

>2) The inductance changes with time (because it's a solenoid)

Yes, but you only care about the maximum since that's when the maximum 
energy is stored in the coil.  You care about the minimum when determining 
PWM frequencies.

>--- In AVR-Chat@yahoogroups.com, Ned Konz <ned@...> wrote:
> > >
> > What is done sometimes instead of a diode is to put an RC snubber
> > network across the solenoid.

Another possibility is to keep the diode and use a large cap on the 
protected side of the bus to take the freewheel energy.

IE 12V -- (Reverse polarity protection diode)>|--- cap -- 12Vout ---

      return --- other side of cap --- return out

Robert

http://www.aeolusdevelopment.com/

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