Test Your EQ
— Issue #162
Each month, Test Your EQ presents
some basic engineering problems for you to test
your Engineering Quotient. What's your EQ?
|
A
designer was given the following requirements for an interface
circuit for a piece of marine electronics equipment.The
equipment has a digital input that is meant to accept
0- to 5-V signals. It consists of a 1000-W
resistor in series with the LED of an optoisolator. The
anode of the LED is connected to ground.
The
interface circuit must accept an input voltage that swings
between 0 and 12 V or 0 and 24 V nominally, depending
on whether the boat has a 12- or 24-V (negative ground)
electrical system. This input might come from a simple
mechanical switch tied to the positive terminal of the
boat battery. However, the boat’s electrical system can
go as high as 30 V when the battery charger is operating,
and brief surges to ±200 V can occur as well.
Also, the circuit might be hooked up backwards by inexperienced
personnel, so continuous inputs of up to –30 V might be
applied. A 5-V power supply is available from the equipment.
The
designer came up with the following circuit. VTH
is a 3-V rail supplied by a small three-terminal regulator
running off of the 5-V supply. Let’s
discuss some key points relating to this design.
Problem
1—Assume
that VBE for both transistors is 0.7 V and VCE(sat)
is 0.4 V. How much current gain is required in Q1 and
Q2?
Answer
Problem
2—At what input voltage does the output switch?
Answer
Problem 3—Why
is R2 required? What about R5?
Answer
Problem
4—What is the purpose of C1? Of D1?
Answer
Problem
5—The SWR required to maintain a healthy transmitter
should be less than what value?
Answer
Problem
6—Traveling on the perimeter of the SWR circle
of a Smith chart is equivalent to what?
Answer
Problem
7—The intersection of the SWR circle on the right-hand
side of the one and only straight line on the Smith chart
gives the value of what?
Answer
Problem
8—Suppose you have to convert the digit displayed
by a seven-segment display driver back to its binary equivalent.
Prove logically that five connections must be made to
the driver chip. The digit display patterns are:
Answer
Published January 2004
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