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Electronics II Tutorials
   Overview Analog Wireless Communcation
   Resistors, Capacitors, RC Networks
   Diodes, Amplitute Modulation, Diode Detection
   RL Circuits, Inductive Kicks, Diode Snubbers
   RC filters. Series resonance and quality factor, Matching, Soldering
   Ladder filters Butterworth and Chebyshev filters Filter tables ADS
   Bandpass ladder filters Quartz crystals
   Impedance inverter
   Ideal Transformers
   Transformer shunt inductance
   BJT-Large signal models
   Transistor switches. Voltage regulators
   Transistor switches. Voltage regulators
   Common emitter amplifier. Max. efficiency of class A amps. Transformer coupled loads
   Available power. Distortion. Emitter degeneration. Miller effect
   Emitter follower and differential amplifiers
   JFET Source follower amplifier
   Oscillators. Clapp oscillator. VFO startup
   Variable frequency oscillator. Gain limiting
   Receiver incremental tuning. Crystal oscillators
   Mixers. Gilbert cell
   Superheterodyne receivers. Spurious responses of mixers
   Decreasing channel bandwidth by using CW
   Audio amplifiers
   JFETs as variable resistors
   Automatic gain control
   Noise, SNR, MDS, noise power density and NEP
   Nyquist noise formula. Cascading noisy components. Noise figure
   Receiver intermodulation and dynamic range
Other Electronics 1 Tutorials
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   BJT Tutorials
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RL Circuits Inductive Kick Diode Snubbers

RL Circuits. Inductive Kick. Diode Snubbers.
Inductors are the third basic discrete component listed in . Uses for inductors in the NorCal 40A include filters and RF “chokes.” The latter provides essentially a short circuit at DC and nearly an open circuit at RF frequencies. (Essentially the opposite function of a DC blocking capacitor!) You will wind some of your own inductors for the NorCal 40A. (The others are axial-lead inductors. They look like resistors, but are green with colored bands.)
The inductors you wind will be wound on a toroidal-shaped ferrite core. Toroid inductors are essentially “self shielding” at RF frequencies since most magnetic flux ?m is contained in the core.

Consequently, these inductors can be placed close to each other on a PCB without too much mutual (and undesirable) interaction. However, be careful in your own designs. (For example, keep air-core inductors perpendicular to each other.) Inductors store energy in a magnetic field. They also oppose a change in the current through them.

This opposition can cause the inductor voltage to become enormous if there is a big change in the current. Called “inductive kick.” To see this explicitly, consider this simple circuit (an inductor connected directly to an AWG, for example)

The open circuit source voltage is

We will carefully analyze this circuit to predict the input voltage Vin. In the following analysis we’ll assume that ? = L / R ??T / 2.
1. At “A” Vs has reached steady state so that I(t) is nearly constant and approximately equal to V R + R , where RL is the resistance of the inductor. The work done by the source against the magnetic force produces energy stored in the magnetic field

2. From “B” to “C” the source Vs is switching from Vm to –Vm volts. Since L V = LdI dt , I cannot change instantly, but it can change rapidly.
3. From Faraday’s Law of Induction

where m ? = magnetic flux = B( t) ? N ? A, where N = number of identical turns of wire and A is the cross-sectional area.

Recall that as Vs goes from Vm to –Vm, there will be a rapid decrease in I(t). It’s not an instantaneous change from Vm|( Rs + RL )to ?Vm (R s+ RL) because of L, but a rapid change.
However, B (t )? I (t ) which implies there will be a rapid decrease in B(t ) and, hence, m ?( t) .
4. Therefore, the emf = ?d?m dt will be large and positive. This emf (a net “push” on the charges) keeps current moving in the same direction (from top to bottom in the figure) and thus opposing change.
5. Using the equivalent lumped circuit above, we see that

Notice the negative sign! With m ? = LI , then

which is what we originally stated on page 2. Now, we’ll use (1) to predict the voltage Vin =VL shown in the circuit on page 2.

In graphical form:

This rapidly changing current in an inductor can produce enormous Vin (= VL). Sometimes this is useful, as in an automobile spark ignition (see Fig. 2.19). Similarly, this “inductive kick” can produce arcing in switches when they turn off electric motors. (I had a switch in a vacuum cleaner burn a hole through beryllium-copper sliding contacts due to this source of arcing.) In sensitive electronic circuits, such inductive kick can be catastrophic and burn out transistors, for example. You will study this phenomenon in Probs. 5 and 6. From Fig. 2.32(b) in Prob. 5:

When Q turns off, there would be a very large and negative voltage VL if D were not present. This large voltage appears across c and e of Q. If this voltage is too large, then Q could be damaged. (Think of L as an equivalent inductance of an electric motor, for example.)
With the snubber diode D, this reverse voltage on L is limited to the forward voltage drop of D! (Note that D must be able to withstand all of the current that initially exists in L just before D begins to conduct.) We’ll see the snubber diode again in Prob. 20 inside the Magnecraft W171DIP-7 reed relay.

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