Re: Variation on the 5v -> 12v newbie question...

"Jonathan Kirwan" <jkirwan@xxxxxxxxxxxxxx> wrote in
message news:l68rb1tsks25l38tpvl8s4ilsagnuavg1m@xxxxxxxxxx
> On 25 Jun 2005 10:34:26 -0700, cgwiita@xxxxxxxx wrote:
[snip]
> However, I
> wouldn't use Larry's KSB834 because I can't get them from Digikey or
> the hobbyist suppliers I checked.

Parts you can get are the best kind.

[snip]
> Notes: When you apply the 12V to the solenoid, it's probably going to
> take some 10s of milliseconds to rise up to the 1A or so you might
> expect with the 10 ohms (V/L effect.) Similarly, it's going to also
> be even slower to lose its energy, because using the diode will only
> allow a small voltage across the inductor during turn-off.

That is a common misconception. When the solenoid having L = Ls
and R = Rs is turned on by applying Vs, its current begins ramping at
Vs/Ls A/S and asymptotically approaches Vs/Rs with an exponential
decay time of Ls/Rs. When the switch is turned off, the current begins
ramping down at (Vs+Vd)/Ls A/S (where Vd is the forward voltage of
the clamp diode, treated as constant) and asymptotically approaches
-Vd/Rs with an exponential decay time of Ls/Rs. The current not only
ramps slightly faster, it is headed for a value on the other side of zero.
So the current decay is definitely faster than the onset.

If such a circuit is slower to release the solenoid than energize it, the
reason is either dissymmetry between pull-in and drop-out voltages,
or an increase in inductance when the moving element is in the
energized position. The clamp diode should only be blamed for
not dropping the voltage as fast as would be possible by applying
a larger reverse voltage to bring the current to zero.

If you doubt this, I urge you to simulate it and study the result.

> Finally,
> there may be a bit of voltage "ringing"/oscillation at the PNP's
> collector as the solenoid's inductance "fights" with the non-linearity
> of the diode near the point where the inductor's energy is nearly
> spent.

I have looked at lots of diole-clamped solenoids without seeing
such a thing. The circuit will certainly not support oscillation. The
usual appearance is a more rapid drop toward zero Volts across
the diode as its current approaches zero and its impedance goes up.

[snip]
--
--Larry Brasfield
email: donotspam_larry_brasfield@xxxxxxxxxxx
Above views may belong only to me.


.

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