Homework 9
Reliability and Safety Analysis
For the MIDI Maestro
Written by
Ben Johanek
Introduction
The MIDI Maestro is an entertainment device that will be used in combination with a MIDI keyboard. Safety does not seem to be a problem since the device is used strictly as a game control mechanism, a MIDI data analyzer, and a visual and audio feedback tool. The device is not controlling anything that, if something fails, could hurt user. Damage of the attached MIDI keyboard is also very unlikely since it is optically isolated from the circuitry of the device. There is a possibility, however, that certain components could fail (which would make the device unusable) and these component failures are analyzed below.
Reliability Analysis
Four components in the MIDI Maestro stand out as components that could possibly fail. These four components are the L7805 voltage regulator, the L78L33AC voltage regulator, the MAX243 RS232 level converter, and the MAX4295 audio amplifier. These components were chosen because they are the components that could possibly rise in temperature during circuit operation and are therefore most likely to be devices that would have reliability issues. Below are calculations for and MTTF where and as stated in the Module 9 Lecture Notes for ECE477 Spring 2004 (Module 9):
L7805 5V Regulator:
and
0.020
16
0.0012
6.0
10
1
therefore, 3.272 failures per hours of operation
Explanation:All values were chosen based on (Module 9). They were verified in the Reliability Prediction of Electronic Equipment Military Handbook (MIL-HDBK-217F).
L78L33AC 3V Regulator:
0.020
16
0.0012
6.0
10
1
therefore, 3.272 failures per hours of operation
Explanation:The value for C1 was chosen based on the assumption that a 3V regulator has a similar number of transistors as a 5V regulator. This value was referenced in (Module 9). πT is the same as the 5V regulator at TJ less than 100° C since a 3V regulator falls into the same ‘Linear’ category as a 5V regulator. C2 is also the same for a 3V regulator as a 5V regulator since the number of pins and packaging is the same in both cases. This information was verified in (MIL-HDBK-217F). πE, πQ, and πL are assumed to be constant throughout these calculations.
MAX243 RS232 Level Converter:
0.040
16
0.0072
6.0
10
1
therefore, 6.832 failures per hours of operation
Explanation:C1was chosen assuming that an RS232 level converter is more complex than a voltage regulator (it is comprised of more transistors). If a voltage regulator has between 101 to 300 transistors, I have assumed that an RS232 level converter would fall into the next most complex group and would be comprised of 301 to 1000 transistors. πThere is similar to the voltage regulators and C2 has increased because the number of pins has increased from 3 to 16. This data was obtained from (MIL-HDBK-217F).
MAX4295 Mono Amplifier:
0.020
16
0.0072
6.0
10
1
therefore, 3.632 failures per hours of operation
Explanation:C1 was chosen assuming that an RS232 level converter is similar in complexity to a voltage regulator. πT here is similar to the voltage regulators and C2 has remained constant because both the amplifier and the RS232 level converter have 16 pins.
Reliability
Conclusion:It appears that the reliability of electronic components decreases as complexity increases. The highest reliability levels (λp = 3.272 failures per 106hours of operation) are those of the linear voltage regulators and the lowest reliability level (λp = 6.832 failures per 106hours of operation) is that of the RS232 level converter (which has more pins and has been assumed to be more complex in number of transistors). These calculations may be misleading, however. The junction temperature was assumed to be less than 100° C for each component above. This assumption would suggest that all of these devices are operating in a similar temperature range. In reality, however, the 5V voltage regulator will be running at the closest temperature to 100° C (due to the fact that it will be driving multiple components and will be providing a significant amount of current). The rest of the components will be running cooler and πT could feasibly be reduced when calculating λp. If this were done, the reliabilities of the cooler running devices would be increased.
FMECA Notes
The midi MAESTRO is broken down into the following blocks: Power, Sound, Logic, and Interface. The power block consists of the voltage regulators and the RS232 level converter. The Sound block consists of the ISD4003 sound record/playback chip, the MAX4295 amplifier, and the speaker. The Logic block consists of the microcontroller and its control lines. Finally, the Interface block consists of the RPG, the midi input and output circuits, and the LCD. The schematic is attached as a reference.
Criticality levels are defined as either HIGH or LOW. A criticality level of HIGH implies that the game is no longer operational while a criticality level of LOW implies that the game will still be playable but without enhanced features.
List of References
Module 9Module 9 Lecture Notes for ECE477 Spring 2004
MIL-HDBK-217FReliability Prediction of Electronic Equipment Military Handbook