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Daveís Pet Circuit Peeves
By: Dave Johnson  

After some 40+ years in the electronics industry I have seen a lot of things. But, there are a few things that bug me when it comes to some circuits.  Below are some of my peeves.  I would love to hear about what things make your list.

Impractical Battery Powered Circuits:   I often see a circuit running on a 3v or 9v battery which draws way to much current. Sure, if you are just interested in seeing how a circuit works then fine, but often a hobby circuit is turned into something that performs a practical function. But, also too often, the circuit will deplete a battery much faster than anyone would tolerate.  Even a single 555 timer circuit will deplete a 9v battery in about 3 months.  How would you like it if you had to replace the batteries in your home smoke detectors every 3 months?  Other circuits, such as some flashing LED systems, will suck the energy out of a battery in just days. If you are going to design a circuit powered by a battery, keep the average power low enough so the thing will run for at least 6 months. Many new ICs draw so little power that a 9v will power it from for 5 or more years.

 

741 Op Amp Circuits:   Please, give this poor thing a rest.  I have a drawer full of these things but they are just not worth much when compared with what is now available.  In its day, this popular IC was the latest technology but they are now very dated.  They are power hogs, they are slow, they require higher supply voltages, they canít operate near the supply rails and they introduce lots of noise due to crossover distortion.  Often a well designed 741 circuit can be made better by just replacing the 741 op Amp integrated circuit with a more modern device.

 

Op Amp Common Mode Range Violation:   I have seen a lot of op Amp designs operated from a single power supply with one of the inputs connected directly to circuit ground.  These circuits will just not work properly with such a connection.  The two inputs of all op Amps have a limited voltage range.  Unless the part is one of the newer ďrail to railĒ types, most designs will not operate properly when the inputs hover close to either the V+ or the V- supply voltages.  Even true rail to rail op Amps, canít produce an output which is truly zero volts.  Know the limitations of part before using it.

 

Light Dimmer Circuits Without a Hash Choke:   There are lots of AC light dimmer circuit designs posted on the Internet which save a bit of money by neglecting to add an inductor in series with the load.  You need an inductor to slow down the rate of rise of current drawn from the AC power line.  Without the inductor, RF trash will be radiated all over the place.  AM radios are especially sensitive to this kind of noise.  I was once hired by a major toy manufacturer, who was sitting on millions of products ready to go to the store shelves when they received complaints. You could not pick up any radio station on an AM radio when their product was in operation.   They all had to go back to the manufacturer because they didnít have the hash filter installed.  Donít leave out key components.

 

Transistor Relay Drivers without a Parallel Protection Diode:   This is a classic error but I still see designs which do not include this critical part. Sure, the circuit might work, for a while, but then, poof.  The energy stored in the inductance of a relay coil when energized can easily pop a transistor when the transistor is turned off.  Donít forget that diode.

 

Timers Using Large Electrolytic Capacitors:  555 timers are OK for many timing needs but they are not very practical for timing functions stretching out to tens of minutes and are very impractical when asked to operate for times measured in hours.  Such circuits, which use a 555 timer, require ridiculously large capacitors which will not be stable with temperature or age.  A circuit might work after Initial calibration time adjustment but it will soon drift.  A 30 minute time function might easily turn into 15 minutes or much less when the circuit is used in hot weather.  Use a microprocessor or a couple logic devices when long times are needed. 

 

Lack of Electrostatic Protection for Off-Board Connections:  I canít tell you how many circuit designs I have seen where an external switch, sometimes positioned a 100 feet away, is connected directly to a logic circuit.  Such a connection is asking for trouble.  It is best to add some static and noise protection my inserting a simple RC filter circuit between the switch and the logic circuit.

 

Microprocessor Circuits which Flash LEDs:  Microprocessors are wonderful devices and yes indeed, they can be programmed to flash a LED with few parts but not everyone will have the development hardware and software to support these devices. For such simple tasks, I strongly suggest using discrete parts.  Use a processor when it makes sense to do so.

 

Single Sided Circuit Boards for Products Used Outside:  This is a common practice, especially for products made in China, but it is asking for trouble. The wild temperature swings associated with any device used outside will put a strain on components soldered in the circuit board.  Single sided boards have a much smaller solder contact surface area than a double sided board.  Cracks will often form, leading to product failure.

 

No Conformal Coating on Boards Exposed to Outside Conditions:  I have seen many perfectly good designs fail in the field due to corrosion and condensation on the surface of a circuit board.  Condensation will often form on a circuit board and that water can easily short out sensitive parts.  A good practice is to cover the entire board with a non-conductive material. My favorite coating is a Dow Corning 1-2577.  This stuff is easy to apply to the board and if necessary, it can be removed to make board repairs.

 

Poor Battery Charger Designs:   Many hobby projects use rechargeable NiMH or lithium ion batteries.  Often little attention is paid toward the circuit used to recharge the batteries.  This lack of attention usually results in short battery life.  Take the time to design a proper charger.

 

Little or No Filter Capacitors in a Logic Circuit:  Modern logic ICs can handle fairly high speed data.  This high speed nature of the parts can trigger a host of problems due do noise induced on the power supply rails.  To counteract this, it is best to scatter some 0.1uF capacitors around on the board, connected across the power supply rails.  Still, I see many designs with no filter capacitors shown. 

These are some of mine. What are your pet peeves?  Email me.

July 2010     Issue 11

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