Design and Development Notes 2004-4

• In a circuit swap all PNP and NPN, reverse all diodes and el-caps, reverse the supplies and input polarities, most of the circuits may still work like they did before.
• In a 3-1/2 digit meter circuit you may need 0.1% Resistors 10ppm or less, so in a 4-1/2 meter we need 0.01% Resistors 1ppm or less or temperature compensation circuits and trimpots.
• A sharp bend in a PCB track is a huge inductance (say 1nH ) when it comes to RF. it may radiate or reflect, it may even act like an antenna and pick up RF from the SMPS, Motor or Cellphones nearby.
• If one layer of PCB is a ground plane and second layer has wide tracks they may form a capacitance (say 1pF) with the PCB glass epoxy as dielectric, more so in multilayer PCB as dielectric is thinner.
• If you need a 10.00K resistor 0.1% and you only have 1% in the market, you can trim lower values to higher values by removing the paint of the resistor and scratching the black metal film till you get the desired value.
• 7107 and 7135 of Intersil can be tested in-circuit, check Vref for 100mV or 1V reference, then short Vin and Vref the reading should be 1000 counts for 7107 and 10000 counts for 7135. ground plane should be good, no ground loops, PCB only glass epoxy and low leakage caps to be used, then reading will be stable, use some coating on soldered PCB.
• An electrically Noisy environment can be simulated with a hair drier, or hot air blower. looping the NC (normally closed) contacts of a 230V Electrical Contacter-Relay can make it oscillate which makes a lot of electrical noise rich in EMI-RFI. This may be required for basic testing of timers, counters, uP, uC and embedded systems as they may malfunction in noisy environment.
• Glass epoxy PCB have high insulation resistance, above 10 tera ohms, and are not hygroscopic which means they do not drink water vapor, this makes them very suitable for precision instrumentation and sensitive circuits.
• Loose Contacts may have a resistance like 100mE (milliohm). A bit of corrosion and 10A of current will make that dissipate 100 * 10² = 10,000mW = 10W, if the contact area is 1 sqmm the heat will cause the resistance to increase, then the dissipation becomes more, sparking and welding may happen, it can even cause fire.
• When using a sharp blade the cutting stroke must move away from you and make sure nobody is close, Wear protective or even plain glasses, If you use a blade with the stroke moving towards your body it will cut badly.
• Power tools and machinery must be used only after exhaustive training and with safety precautions. The power tools used carelessly and wrongly can cause an injury which will last a lifetime or may even handicap you.