# High-Reliability Circuits

## Digital Circuits

• #### Question 1

 Don’t just sit there! Build something!!

Learning to analyze relay circuits requires much study and practice. Typically, students practice by working through lots of sample problems and checking their answers against those provided by the textbook or the instructor. While this is good, there is a much better way.

You will learn much more by actually building and analyzing real circuits, letting your test equipment provide the “answers” instead of a book or another person. For successful circuit-building exercises, follow these steps:

1. Draw the schematic diagram for the relay circuit to be analyzed.
2. Carefully build this circuit on a breadboard or other convenient medium.
3. Check the accuracy of the circuit’s construction, following each wire to each connection point, and verifying these elements one-by-one on the diagram.
4. Analyze the circuit, determining all logic states for given input conditions.
5. Carefully measure those logic states, to verify the accuracy of your analysis.
6. If there are any errors, carefully check your circuit’s construction against the diagram, then carefully re-analyze the circuit and re-measure.

Always be sure that the power supply voltage levels are within specification for the relay coils you plan to use. I recommend using PC-board relays with coil voltages suitable for single-battery power (6 volt is good). Relay coils draw quite a bit more current than, say, semiconductor logic gates, so use a “lantern” size 6 volt battery for adequate operating life.

One way you can save time and reduce the possibility of error is to begin with a very simple circuit and incrementally add components to increase its complexity after each analysis, rather than building a whole new circuit for each practice problem. Another time-saving technique is to re-use the same components in a variety of different circuit configurations. This way, you won’t have to measure any component’s value more than once.

• #### Question 2

The equation relating probability of continued performance for a component or a system versus time may be expressed as follows:

 x = e−t / m

Where,

x = Probability (a number between 0 and 1, inclusive)

e = Euler’s constant ( ≈ 2.7182818)

t = Time of continuous operation

m = Mean Time Between Failure of the component or system

The unit of time for both t and m must be the same. That is, if t is measured in years, then m must also be expressed in years or else the equation will give very misleading answers.

Suppose, though, we were given m in years, and the operating time t in days. Substitute the relationship td = 365 ty into the reliability equation so that we will have a new equation that can take t in days (td) and m in years, and still provide the correct answer.

• #### Question 3

Explain what the following statement means, with regard to the design of electronic circuits:

Faults are inevitable, but failure is not.

Specifically, what does this philosophy mean for your career as an electronics professional, entrusted with the installation, maintenance, and possibly design of complex systems?

• #### Question 4

An important parameter of high-reliability systems is abbreviated MTBF. What does this acronym stand for?

• #### Question 5

The component failure rate of complex systems usually follows a trend known in the industry as the “bathtub curve”:

While the “Useful life” and “Wear-out” phases of the system life-cycle are easy to understand, the initial “Infant mortality” phase is not so intuitive. Explain what factors might lead to premature component failure during this initial phase of a system’s lifespan.

• #### Question 6

For the following electronic components, determine whether they are more likely to fail open or fail shorted (this includes partial, or high-resistance, shorts):

Resistors:
Capacitors:
Inductors:
Switches:
Transformers:
Diodes:
Bipolar transistors:
Field-effect transistors:
Crystals:

I encourage you to research information on these devices’ failure modes, as well as glean from your own experiences building and troubleshooting electronic circuits.

• #### Question 7

The Orbiting Astronomical Observatory was a NASA project during the late 1960’s and 1970’s to place precision observational instruments in earth orbit for scientific purposes. Satellites designed for this program had to have “hardened” circuitry to withstand the radiation, extreme temperatures, and other harsh conditions of space.

An example of some of this “fail-safe” circuitry is shown here: a passive, quad-redundant, two-input AND gate:

First, draw a schematic for a non-redundant, passive AND gate. Which components shown in the above schematic are “redundant,” and which are essential?

Then, explain why the circuit is referred to as quad-redundant. How many individual component failures, minimum, must occur before the gate’s functionality is compromised? Prove your answer through an analysis of the circuit’s operation.

• #### Question 8

One use for “rectifying” diodes is to parallel multiple power supplies for extra reliability in powering a critical system:

However, as an experienced electronics technician, you should know that diodes are not immune to failure. Modify this schematic diagram to include three extra (redundant) diodes that will allow normal operation if any one of the three original diodes were to fail, assuming the most common failure mode of rectifier-type diodes.

• #### Question 9

The Orbiting Astronomical Observatory was a NASA project during the late 1960’s and 1970’s to place precision observational instruments in earth orbit for scientific purposes. Satellites designed for this program had to have “hardened” circuitry to withstand the radiation, extreme temperatures, and other harsh conditions of space.

An example of some of this “fail-safe” circuitry is shown here: a quad-redundant inverter (NOT) gate:

Explain why the circuit is referred to as quad-redundant. How many individual component failures, minimum, must occur before the gate’s functionality is compromised? Prove your answer through an analysis of the circuit’s operation.