EE 461G Introduction to Electronics

 

Homework # 5 Due October 5, 1999

 

In Sedra and Smith do problems 3.31 and 3.47 at the end of chapter 3.

 

Problem 1

 

An approximate model for a florescent light is consists of two back to back Zener diodes.

 

Assume the diodes have a reverse breakdown voltage equal to 90V. The light is connected to a standard 115Vrms 60Hz power source. The resistance of the connecting wire is 1W.

 

  1. Draw the characteristic curve of this composite device (current versus voltage).
  2. Plot Vin, V across the light, and the current into the light versus time.

 

As you see, the current into the light is excessive, it will blow a 15A fuse. To limit this current the resistance in series with the light could be increased. Unfortunately there will be power dissipated in this resistor lowering the effective light efficiency. Can you think of a way to limit the current that consumes no power? I will tell you if you can not think of it on your own. Put an inductor in series with the light. This inductor is called ballast in the terminology of florescent lights.

 

  1. Estimate a value of inductance to limit the peak value current into the light to 1A. An exact calculation is not possible.
  2. Build a spice model of your circuit, Use a Zener diode model with IS = 10nA, n =1, bv = 90V, and ibv = 500mA. What is your final choice of tend and tstep?
  3. Adjust your estimated ballast inductance to get a peak current of 1A in the light. What is L?
  4. Use spice to plot the input voltage, light voltage, and the light current.
  5. Estimate if the current leads or lags the input voltage.

 

Problem 2

a) For a solid, the allowed energy states for electrons form into bands separated by energy gaps. No electron can posses any energy value in the energy gap. For semiconductors the two most important energy bands are the valance band and the conduction band. Which has more energy, an electron in the conduction band or or one in the valance band?

b) In terms of the valance and conduction bands what is an electron and what is a hole?

c) When a sample of intrinsic Si is at absolute zero how full are its conduction and valance bands.

d) When a sample of intrinsic Si is at room temperature

i) how many holes per cm3 are in the valance band

ii) how many electrons per cm3 are in the conduction band.

e) Why can bands other than the conduction and valance bands, with energy values below the valance energies, be neglected in the treatment of the electrical behavior of semiconductors?