Body
Hi Lutz,
You're right. I totally agree. This was a mistake on that page.
Regards
Ole
---In [email protected], <lutzliebers@...> wrote :
http://www.sharetechnote.com/html/Electronics_Diode_KickBack.html http://www.sharetechnote.com/html/Electronics_Diode_KickBack.html
Something is wrong with the explanations in figure (upper C): it's not the current that reverses its
direction in the moment of opening the switch but the created EMK is reversed in respect to the polarity
of the battery.
In other words:
- when you power the inductor from the battery, the current will lag due to the inductors' self inductance
creating a counter EMK until the current reaches its stationary rate defined by the Ohmic share of the
inductors' impedance.
- when you open the switch, the current is hindered from flowing through the inductor, so its rate over
time dI/dt is decreasing rapidly. According to the law of self inductance this negative dI/dt will create
a reversed EMK called kick back spike. As a consequence, an arc is created between the contacts and
the current will continue to flow through the arc in the same direction but at a decreasing rate
until the energy of the magnetic flux is fully consumed.
If there is a free wheeling diode hooked parallel to the inductor (figure middle C), the current will flow from
the plus side of the battery through the closed switch downwards through the inductor to the minus side of
the battery while the diode is sleeping.
In the moment of opening the switch, the decreasing current flow, still going in the same direction makes
the polarity at the inductor reverse and instead of forming an arc the diode wakes up and overtakes the
decreasing current flow.
Thus the negative kick back spike gets limited to the forward voltage of the diode which means to protect
the contacts of the switch. So the situation has changed to far extend, almost as if a resistor or filament
was switched.
...