Building discrete semiconductor and passive designs, using Opamps and Mixed Design Circuits are the first step in electronics. Most consumer Electronics have a high analog circuit content. Even in Embedded Systems the interface design that matters for real time systems is an analog circuit. Here are some more analog points in my old notes. (the links have been added to provide additional reference to the reader and are not related to these notes of mine form the late 80s)
SMPS designs should have clear line or patch of isolation for product safety and long term reliability. If you use opto-couplers they need to have 5kV or better isolation and 10mm between pins. The Live circuitry and the output circuitry should not overlap on PCB, tracks should not stray to the opposite side, The Transformers can be split bobbin if possible. A 10 to 20 mm desert (means no tracks) running from end to end of PCB under Transformer and under optos etc., Even ground tracks and ground planes should not stray.
The high voltage side must have layer to layer (mylar) insulation, means wind a layer and then put the yellow mylar tape, make tape concave so no strands should slip over to next layer, use split bobbin if possible. have terminations far away and enclosed for safety. vacuum impregnate with natural resin or epoxy depending on voltage and environment. product safety is very important.
If in your power supply you have a varistor, then you should have a fast acting fuse in series, as varistor fail as a short.
Wire and crimping of wires for supporting the high currents and high voltages must be carefully chosen. The copper cross section area gives its ability to carry current, the quality, thickness and flexibility of the insulation gives its voltage capability. The Tightness of the crimped contact will ensure long term reliability. Mechanical Stress prevention for all wiring is very important and guarded termination for user-operator safety.
BTA16600 and triacs of this series from ST have the metal TO220 tab electrically insulated from the device.
MOC3041 and others, switch triacs at zero crossover which reduces EMI-RFI and spikes. It means when the sine wave is close to zero volts the triac is turned on.
You have to ensure proper air circulation and fix proper heat-sinks with thermally conductive heat-sink compound or silicone grease with alumina. Anodized Aluminum heat-sinks with large surface areas and small fans are used in power electronic products for this purpose. If a component works very hot the specs will get derated, precision is lost and undesirable and unpredictable results will happen.
A thermistor must be used in series with huge power electrolytic capacitors to limit the enormous current inrush on start, or spikes may stress components, cap or EMI-RFI.
A freewheeling diode should be used across a relay, solenoid coil or motor because inductive kickback will damage transistor or mosfet. High current loads like relays can reset logic circuits if ground is not connected well, It also produces more ripple in supply, so it is better to have a separate supply for such parts.
EMI-RFI causes-Switching Loads Simultaneously (inductive loads). Power factor correction capacitors and devices. Lightning strikes (enhanced by earth faults). Line Inductance (inductive kickback, resonance). DC and AC Drives for Motors. Rectifiers with large filters and stray inductance.
Small battery operated gadgets when turned on after a very long time may not work properly due to a thin oxide layer at the battery connector, replace cells if required, scratch and clean battery and connector surface, the reason is some gadgets consume so low power that the current cannot break a micronic oxide layer. Keep using regularly.
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.
All Electronic Products have interfaces to external electrical circuits. The power supply is also derived using converters or power supplies. Motors, Heaters and Transformers are examples of components in the electric power circuits.
Points to bear in mind
12V lead acid batteries should be charged with a constant current CC and constant voltage charger the CV being 13.6V. Deep discharge and over charging will reduce the cycles.
Lead acid batteries need a monthly distilled water update to ensure long life, sealed lead acid dont need attention. Keep lead acid batteries in well ventilated area due to acid fumes.
Electric Shock can kill hence take great care, Electrolytic capacitors burst and Transformers catch fire. turn off equipment after use. turn off all electricity when going out of town.
Inductive kickback starts a tubelight or automobile as high voltages are produced when current in inductor is turned off. This also is the cause of failure in power semiconductors.
Earthing is important, ensure it's perfect, the neutral to earth could be 5V AC max., also a circuit breaker and fuse a must. Use a electrical earth leakage circuit breaker to protect both man and machine.
Extra care should be taken to prevent injury to eyes while working. Also 230V/110V shock can be fatal. working on high energy electrical circuits and power electronics should be with all precautions.
You need a Fuse, Circuit breaker and earth leakage circuit breaker for safety against shock and fire hazard.
High voltage or current stress, mechanical vibration, user misuse, High Energy stress (freq and voltage), aging (cycles) cause failure.
Good earthing, servo motor regulated mains supply, earth leakage circuit breaker and over current circuit breaker, HRC fuse are some safety steps to take.
When working with HV or HE circuits wear shoes, keep yourself dry, the supply board should have above protections in red.
Use silicone shrink sleeves to insulate or close exposed high voltage wire ends or joints.
Sparking at the high voltage mains supply power contacts like a plug can cause electronic gadgets to fail.
An Embedded Microcontroller or DSP system is made of Chips, Circuits and Firmware. The digital voltage levels, speed, bus width, fan out, power consumption are some factors that a designer has to keep in mind. As portable and wireless gadgets are becoming more popular, RF, Ethernet and Energy Efficient Design aspects should be studied. Power electronics and Analog Circuits knowledge is used around the system, all this is integrated to make an instrument, equipment or gadget.
80C51 ports can sink more current but source very less, hence use a 10k pull up at all the ports or outputs.
Firmware must be developed in increments, tested in increments, backed up in increments, must be modular (include) reuse.
Tristate output, High Impedance and Floating all mean the same when it comes to IC Inputs-Outputs. It means the pin is insulated from rest of circuit in the IC. That means it will not influence the node or bus it is connected to. A DMM terminals are floating means that the hand held plastic DMM has no electrical conductive link to earth or ground.
You can use hyper terminal to upload code to single board computers 80C51 like in BINARY or ASCII.
ASICs are for large volume production, or for products which have a long product life cycle.
Low volume production use FPGA or CPLD, or even flash based microcontrollers, so that all your inventory can be reused and recycled.
Cell phones or a LAN card ASIC is ideal as volumes are good in cell phones and for LAN cards the technology is matured. For either FPGA or ASIC's you get IP Modules or Code Libraries for many functions and applications.
Whatever the method keep design flexible and modular for reuse and to save cost. remember the hardware is difficult to alter, software can be altered even at customer site, flash has made this possible
In the future chips may be both analog and digital programmable with flash.
Unused CMOS inputs should have a pull up or pull down resistor, it should not float, or it oscillates.
Have a decoupling capacitor 104 that is 0.1uF or 100nF across the supply of every IC very near the IC supply pins.
A watchdog timer should be used in every microcomputer circuit like 8051 so that the system resets on hanging.
The reset on a microcomputer should be applied till the supply to it is stabilized, this will enable a clean start.
Analog ground (opamps), digital ground (CMOS) and power ground (relays and LED) should be separate, (linked at root)
Pull up or pull down resistors in TTL can be 10K and in CMOS 100K and in battery operated systems 1M.
CMOS gates and Opamps have a output drive capability of ~ 10-20mA, so when you drive a load say an LED use a series resistor to limit the current to 5mA to 10mA.
When the number of digital chips you use in a project goes above 20 or 30 then it is better to use PLD or CPLD types.
Try to use same family ICs in a circuit, like only LS or only HCT, if you mix up then you have to do a design review.
In a industrial environment many motors, DC drives and AC drives will be running, this will produce EMI, RFI, kickback spikes which cause microcontroller based equipment to hang. Use a watchdog timer for uC.
More EMI immunity by using opto couplers for all input and outputs, 4-20mA current signals for input and output and an isolated wide range SMPS.
Making a Logical Choice in Speed-n-Power
The glue/interface Logic
families are many. TTL and CMOS is what we all know. TTL is made of the
trusted NPN and PNP transistors. CMOS logic is made of Mosfets/FET
types. I like HCT but these days it appears
ACT is better and affordable too.
CD40XX and Intersil chips were the first CMOS types of IC
that became popular. The former were versatile Building Blocks and the
Latter created what i would call Flexible Advanced Public ASICs.
are low power consumption and TTL are nice Speed. In intel chips you
could see the transition at 8031 to 80C31, a CMOS with good speed. Now
its CMOS everywhere. FET's rule.
Power Supplies and SMPS, Transformers, Drive circuits for Motors, Heaters and Solenoids are all Power Electronics. Thyristors, Mosfets and High Power Transistors are important components used in these circuits.
Tek Power Supply Facia - Observe the user friendly interface design.
Power Design Points to note - delabs
Foldback circuits in the form of constant current or voltage can protect a Power Circuit from overload. eg. Charger.
Air gap in inductor core prevents saturation of the Magnetics, it must be made like that when required. eg. Choke.
High power equipment should have a soft start, so that fuses dont blow on start, inrush can cause damage to parts.
Caps can be put in series to double voltage withstand capability, when we put el-caps in series they have to be identical in value. put caps in parallel to increase the cap uF value and it also lowers ESR which is equivalent series resistance, this is good when filtering in SMPS. Two el-caps with the negatives of both connected and the positive terminals serving as the two terminals will give a non-polar cap, used in crossover networks.
When smps has to work at 100Khz or more, the primary of transformer must have multiple strands, and secondary can be a copper flat ribbon. this is due to skin effect, as high frequency current flows on surface of conductor.
MOSFET needs a turn on pulse and a turn off pulse as gate capacitance is huge.
Do not club Heatsinks unless the heatsink is very big or the clubbed components form a part of a current sharing set. If you club say two TO220 devices one dissipating 10W with a 2 Watt heating device, you may reduce reliability of the 2W device. Then clubbing parallel current sharing devices on one heatsink may be a good idea as it may reduce thermal runaway, also parallel power transistors should each have 0.1E in emitter path, this also can force load sharing on lazy devices.
Test a mosfet, charge the gate with the DMM in diode mode, then there is a short between source and drain.
Snubbers consisting of R-C should be used across switches, relay contacts, MOSFETS when switching inductive loads, this will absorb the spike and save the device.
Transformers used in SMPS could be split bobbin for product safety, and transformers should be resin impregnated, hazard may arise when using a badly made product.
In a SMPS which is not earthed the floating (unconnected) earth terminal will give a slight shock due to the two Y caps connected to earth from phase and neutral.
Power mosfets, transistors or IGBT's should be electrically isolated very well from heatsink with alumina, mica or silpad. It could withstand 2KV -5KV DC, related to product safety. Also these heatsinks should not be earthed but floating, do not fix to the metal chassis for maximum safety.