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

On Sat, 25 Jun 2005 20:59:03 -0700, "Larry Brasfield"
<donotspam_larry_brasfield@xxxxxxxxxxx> wrote:

>"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]

I first admit I'm just a hobbyist on this score. But I didn't need to
simulate it, as I've seen it before. Damped oscillations in the 50kHz
range. I chalked it up to that non-linearity issue. But, of course,
I could be wrong.

So, I just simulated it. And there it is! The simulator seems to see
it, too.

Jon
.

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