practical troubleshooting of electronic circuits for engineers and technicians

Technology Training that works

Practical Troubleshooting of Electronic Circuits for Engineers and Technicians

Technology Training that Workswww.idc-online.com/slideshare

Technology Training that worksTechnology Training that Workswww.idc-online.com/slideshare

Objectives

• Understand the technical meaning of failure

• Know the symptoms of failure

• Study the causes of failure

• Study Early Failure, Random Failure and Wear out Failure

• Know what is Failure Rate, Mean Time to Failure, Mean Time between Failure and Mean Time to Repair

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Failure Symptoms • Failure

A deviation in the properties of a component or item from prescribed condition is considered as a fault

• Failure Curve

• Traditional bathtub curve indicates component life in three stages


Failure Symptoms

• Reliability Curve (generalized)

First stage Failure rate begins high and decreases rapidly with time. This stage is known as Infant Mortality Period and has a Decreasing Failure Rate (DFR)

Second stage

State failure rate period, which is usually long also known as random failure period or useful life of the equipment

Third stage Beyond useful life period, there is a gradual increase in failure rate. A period of ageing and wear out with increasing failure rate


Equipment Failure

Partial failures Resulting from deviation in characteristics or parameters beyond the specified limits

Complete failure If characteristics deviate beyond specified limits causing complete breakdown

Catastrophic failure A sudden and complete change in an equipment's performance

Degradation failure Failures occurring gradually and in a partial manner

• Failed item:

Completely inoperable

Operable but no longer able to perform its intended function satisfactorily

Serious deterioration made it unreliable


Failure Causes

• Classification

• Design Deficiency

a) Little engineering efforts / changes in designb) Improper choice of componentsc) Upgrading of a partd) Inadequate information on failure analysis of componentse) Insufficient design criteriaf) Insufficient prototype testing for meeting both reliability and performance specificationg) Excessive heat development inside equipment and lack of cooling h) Poor mechanical layout of components, assemblies and panels


Failure Causes

•Production Deficiency

Improper / substandard purchase of material

Unsuitable storage method and long storage period of components

Absence of training programs to employees

Negligence in performing environmental tests like temperature cycling, operation of equipment at elevated temperature for specified period

Lack of proper working environment, ill-ventilated, poor illumination and dusty assembly shops

Lack of inspection of goods, sampling tests and inspection

Insufficient testing and inspection of finished product


Assembly Errors

• Careless Storage and Transport

Excessive vibrations and mechanical shocks during transportation

Improper packaging

Undue long storage of equipment

• Undetected errors in assembly by manufacturer / purchaser

• Improper matching of various signals used or generated by the components on the terminal board at the time of assembly

• Improper prescribed order of the components collected on the terminal board


Improper Working Conditions

• Presence of abnormal severe conditions of speed, temperature etc.

• Using equipment without following manufacturer's instructions, warnings and cautions

• Inadequate maintenance policy and after sale service

• Usage of equipment beyond its lifetime

• Aging of the equipment

• Operator error

• Lack of preventive measures


Failure Types

• Early Failure

• Random Failure• Wear out Failure


Failure Types

a) Early Failure

b) Random Failure

c) Wear out Failure

Poor quality control techniques during production

Causes:

Also known as Infant Failures, can be eliminated by de-bugging or burn-in process

Weak and substandard components


Failure Types

c) Wear out Failure

Predominant during actual operation of system

Occur at random, irregularly and unexpectedly

Caused by sudden stress accumulations beyond specification of component

Phase during which only chance failure occur is called useful life of the system

Reliability techniques help to reduce occurrence of random failures

b) Random Failure

a) Early Failure


Failure Types

b) Random Failurec) Wear out Failure

a) Early Failure

Caused due to aging or wearing out of components

Improper maintenance

Need of preventive maintenance

Period between two maintenances should be shorter than the mean wear out life of the system


Some Useful Terms in Failure

• Failure Rate

Failure Rate = f / T

Where:

f = number of failures during the test interval

T = total test time

• Smaller the value of the failure rate, the higher is the reliability of the system

• Mean Time to Failure (MTTF)

For one transistor used in the system,MTTF = 1 / 5 * 10 -5 = 20,000 hours



• Mean Time to Failure (MTTF)

• Calculated on the basis of the results of life testing of components

MTTF = Total test time / number of components

• Mean Time between Failures (MTBF)

MTBF = 1 / Failure rate= T / f

Where:f = number of failures during the test intervalT= total test time



• Mean Time to Repair (MTTR)

Average time required to bring a system from a failed state to an operational state.

Defined as the total corrective maintenance time divided by the total number of corrective maintenance actions during a given period of time.


Summary

• A deviation in the properties of the component or item from prescribed condition is considered as a fault. A state of fault is denoted by the term failure

• Component life is divided into three stages. Infant Mortality Period, Random Failure Period and Wear out stage

Production deficiency, Processing deficiency, Assembly errors, inadequate storage and transport conditions

Failure Rate, Mean Time to Failure, Mean Time between Failure and Mean Time to Repair

• Important mathematical terms in failure

• Some of the major causes of failure:


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