Lecture 4: RL Circuits. Inductive Kick. Diode Snubbers.

Transcription

1 Whites, EE 322 Lecture 4 Page 1 of 6 Lecture 4: RL Circuits. Inductive Kick. Diode Snubbers. Inductors are the third basic discrete coponent listed in Lecture 2. 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 soe of your own inductors for the NorCal 40A. (The others are axiallead inductors. They look like resistors, but are green with colored bands.) The inductors you wind will be wound on a toroidalshaped ferrite core. Toroid inductors are essentially self shielding at RF frequencies since ost agnetic flux ψ is contained in the core. ψ Consequently, these inductors can be placed close to each other on a PCB without too uch utual (and undesirable) interaction. However, be careful in your own designs. (For exaple, keep aircore inductors perpendicular to each other.) Inductors store energy in a agnetic field. They also oppose a change in the current through the Keith W. Whites

2 Whites, EE 322 Lecture 4 Page 2 of 6 I V L di = L This opposition can cause the inductor voltage to becoe enorous if there is a big change in the current. Called inductive kick. To see this explicitly, consider this siple circuit (an inductor connected directly to an AWG, for exaple) R s I(t) AWG V in L The open circuit source voltage is V A X B X t V C X T We will carefully analyze this circuit to predict the input voltage V in. In the following analysis we ll assue that τ = L/ R T / At A has reached steady state so that I(t) is nearly constant and approxiately equal to V/( Rs RL), where R L is the resistance of the inductor.

3 Whites, EE 322 Lecture 4 Page 3 of 6 The work done by the source against the agnetic force produces energy stored in the agnetic field 1 2 WL = LI [J] 2 2. Fro B to C the source is switching fro V to V volts. Since = L di, I cannot change instantly, but it can change rapidly. 3. Fro Faraday s Law of Induction dψ d ef = [V] or E dl = B ds cs ( ) sc ( ) where ψ = agnetic flux = Bt ( ) N A, where N = nuber of identical turns of wire and A is the crosssectional area. I(t) c B(t) Equivalent luped circuit Recall that as goes fro V to V, there will be a rapid decrease in I(t). It s not an instantaneous change fro V /( R R ) to V /( R R ) because of L, but a rapid change. s L s L However, Bt () It () decrease in Bt () and, hence, ( t ) which iplies there will be a rapid ψ. I(t) c ef

4 Whites, EE 322 Lecture 4 Page 4 of 6 4. Therefore, the ef = dψ will be large and positive. This ef (a net push on the charges) keeps current oving in the sae direction (fro top to botto in the figure) and thus opposing change. 5. Using the equivalent luped circuit above, we see that dψ VL = ef = Notice the negative sign! With ψ = LI, then 0 d di dl VL = ( LI) = L I di or VL = L [V] (1) which is what we originally stated on page 2. Now, we ll use (1) to predict the voltage V in circuit on page 2. = hown in the L I(t) Vin As goes fro V V. As goes fro V V. I(t) is changing rapidly V V fro R R R R I(t) is changing rapidly V V fro R R R R L s L s L s L s.. di Negative spike in voltage. (?) Positive spike in voltage.

5 Whites, EE 322 Lecture 4 Page 5 of 6 In graphical for: V V V in t V "Inductive kick" This rapidly changing current in an inductor can produce enorous V in (= ). Soeties this is useful, as in an autoobile spark ignition (see Fig. 2.19). Siilarly, this inductive kick can produce arcing in switches when they turn off electric otors. (I had a switch in a vacuu cleaner burn a hole through berylliucopper sliding contacts due to this source of arcing.) In sensitive electronic circuits, such inductive kick can be catastrophic and burn out transistors, for exaple. You will study this phenoenon in Probs. 5 and 6. Fro Fig. 2.32(b) in Prob. 5:

6 Whites, EE 322 Lecture 4 Page 6 of 6 V cc 1 H D 1N4148 R S 2 k 2 k b BJT used as a switch Q c e 2N2222A Snubber diode added later in Prob. 6 When Q turns off, there would be a very large and negative voltage 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 daaged. (Think of L as an equivalent inductance of an electric otor, for exaple.) With the snubber diode D, this reverse voltage on L is liited to the forward voltage drop of D! (Note that D ust 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 W171DIP7 reed relay.