Budget, Availability, Size, Alternate Vendors, Product Life and Quantity to be manufactured are some factors that determine the selection of a prudent and experienced design professional.
To choose the right type of part, an exhuastive knowledge of the behaviour of passives and discretes is essential. Some points i remember among the things that i learnt the hard way are listed below.
- Over rate components at least double, if you need a 1A-100V-100uS diode, use a 2A-200V-50uS diode.
- Electrolytic capacitors have a shelf life, if you need to store them you have to charge them every month.
- Gold plating is used in connectors because it has low contact resistance and does not corrode or react
- Use a 100uF and 0.1uF CD in parallel to filter because the inductance of 100uF is a lot (the electrodes are wound as in a coil) and high frequency passes over and is filtered by the 0.1uF which has negligible inductance.
- Ceramic capacitors leak a bit, electrolytics leak a lot but plastic or mica capacitors do not leak at all, near perfect. An oscillator with a RC time setting needs a plastic cap for stability.
- In industrial electronics equipment, connectors are a source of many problems, hence avoid connectors. vibrations, corrosion and frequent usage will result in strange problems.
- Small signal diode 1N4148 switches at 4nS, current of 70mA and 70V withstand capacity.
- 2N2222 and 2N2907 form a fast switching NPN-PNP pair and have been around for decades.
- LED displays are best indoors and are not so good for sunlit outdoors, LCD is good for that.
- All electromechanical parts like switches, relays, connectors and pots have limited number of operations.
- Failure of parts are when parts are stressed beyond limit, bad environment, misuse, infant mortality and normal aging.
- Most pots have a dielectric insulation between metal pot shaft and terminals of 1KV or more. Some commercial pots may have lesser breakdown voltage and that will be a product safety issue. So either way use Plastic Knobs or pots with plastic shafts.