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Final Answers
© 2000-2016   Gérard P. Michon, Ph.D.

Electronics 101

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Fundamentals of Practical Electronics


(2014-06-05)   Mechanical switches and snubber networks.
Current ratings are typically for AC only  (derate by a factor of 5 for DC).

We start our presentation of electronic circuit components with a quick study of the lowly mechanical on/off switch  (called a "hard switch" as opposed to a  soft switch  which merely powers down a circuit electronically, without ever completely disconnecting it from its power supply).

This engineering challenge can be solved by a  snubber network  which must be determined in ways that are rarely properly discussed, if ever...

For ordinary 50 Hz or 60 Hz AC mains, the voltage becomes zero 100 or 120 times per second.  This helps break the spark which is created when an inductive load is switched off.  With DC, we have no such luck, as self-sustaining arcing is possible for high enough currents, which could effectively prevent the circuit from being switched off.

To prevent this and/or reduce the long-term damage caused by sparks, one simple idea would be to connect a capacitor (C) in parallel with the switch.  This would solve the arcing problem which occurs when an inductive load is turned off but the new capacitor causes a different problem when the switch is used to turn the DC power supply  (or "battery")  back on...  In the off position the capacitor will typically see the entire voltage (U) of the battery and could store a substantial amount of energy which will be dissipated almost instantaneously into the switch when it's turned back on.

 snubber network

To solve this new problem, we may connect a resistor (R) in series with the capacitor C.  It will dissipate most of the aforementioned energy if its value is much greater than the unavoidable contact resistance of the switch.  However, that resistor limits the effectiveness of the capacitor in addressing the first issue raised.  Thus, there's a tradeoff which should be properly analyzed to work out optimal values for R and C.  Let's do so:

The above picture shows the situation just after the switch has been turned off.  The capacitor is discharged because it was previously short-circuited by the switch.  On the other hand, the current through the load still has its previous value (I) because of the assumed inductive properties of the load.  The current I is now flowing through the resistor and the voltage across the switch is therefore equal to  RI, which should not exceed the voltage rating for the switch  (U0).  So:

R   < U/I

With the switch off, the capacitor gets charged to the full voltage of the power supply through the connected load.  When the switch is turned back on, the capacitor discharges an initial current  U/R  through it.  That should not exceed the current rating of the switch  (I0).  So:

U/I   <   R

All told, we must have:   U/I   <   R   < U/I

The above analysis is for transient regimes only.  When two switches are used in parallel  (a DPST used as SPST)  one of them may have to withstand the whole transient before the other comes into play and the snubber network should be planned accordingly.  Let's use as an example the little DPDT slide switch sold by Radio-Schack as 275-0033  (pack of two).  As that switch is rated 500 mA at 30 VDC, we may consider using both sides in parallel to switch a power supply rated  1000 mA  at  9 V.  As a single side may have to absorb all the transients, we should use the above analysis with  I = 1 A  and  I0 = 0.5 A.  So:

U/I   =   18 W   <   R   < 30 W   =   U/I

Thus, we may want to choose for R a standard value of  22 W.

The above consideration are useful to obtain a rough figure.  However, they are based on a dubious type of "rating":  Switches are not rated in terms of the maximum voltage or current a switch can withstand.  Instead, a given switch can be given several such ratings, where the  power  obtained as the product of those two quantities is a constant  (e.g., the same switch can be rated for 3A at 120VAC or 1.5A at 240VAC to express a power rating of 360 VA).

The value we should use for C is derived from timing considerations which may or may not be predictable.  Some switches feature a spring-loaded mechanism which makes the switching time short and predictable.  Other switches are typically slower and less predictable  (the timing characteristics of slide switches depend very much on who operates them).

 Come back later, we're
 still working on this one...

DC switch rating  (Electro Tech forum).
Switch contact design  ("All about Circuits")
Design of snubbers for power circuits  by  Rudy Severns.
Wikipedia :   Snubber


(2014-06-05)   Flyback diode
The standard way to prevent negative spikes on DC-controlled relay coils.

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 still working on this one...

Wikipedia :   Flyback diode


(2014-06-06)   Semiconductor diodes
Junction diodes (PN and PIN).  Schottky diodes.  Zener avalanche.

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 still working on this one...

Wikipedia :   Diode


(2014-06-10)   Temperature coefficient (tempco) and compensation (TC)
How to minimize the influence of temperature on semiconductor circuits.

 Come back later, we're
 still working on this one...

Yet another bench supply  by Jebnor   |   Zener diode T.C.   |   Zener diodes Wikipedia :  


(2014-04-20)   Operational Amplifiers  ("Op-amps")
An easy component to use, for beginners and seasoned engineers alike.

Microcontrollers are primarily concerned with interfacing with our analog world.  No discussion of microcontrollers would be complete without a discussion of the analog circuitry that can achieve that.  One of the easiest analog building block to work with is the  operational amplifier  ("op-amp" to the initiated)...

An  ideal  operational amplifier is a circuit powered by a DC source and endowed with one poutput pin and two input pins  (an inverting input and a noninverting input).  The voltage at the output is a very large multiple of the difference in voltage between the inputs.  That multiple  (the so-called  gain  of the amplifier is so large, in fact, that the feedback of such an amplifier, within a properly designed circuit, will reduce to virtually zero the voltage difference between its two inputs.

Ideally, the voltage between the inputs is zero as is the current flowing from one to the other.  On the other hand, the current and the voltage delivered by the output can both be anything the circuit requires. 

Elsewhere on this site, I present an elegant way to introduce the functionality of an operational amplifier as a combination of two strange dipoles:  One  (called  nullator)  imposes the value of both the current and the voltage between its pins  (making them both zero)  while the other  (called  norator)  imposes neither.  The combination of both is called a  nullor :

 Operational amplifier presented as a nullor

A modern operational amplifier will provide a good approximation of this ideal behavior with a few flaws that can be partially corrected only at a substantial cost.  For general applications, though, even the cheap ones are extremely good.  Let's present a few of the more common ones:

Dual-Opamp ICs :

The TL072 was introduced as a low-noise version of the extremely popular TL082.  Nowadays, it seems that only one version is produced (the low-noise one) with different identification stamps for commercial reasons only.  However, if you get an IC stamped "TL082", you have no guarantee that it will be a low-noise version, since it may have been manufactured before that practice came about or before the TL072 even existed.  For most applications, that makes little or no difference, though.

Characteristics of the most popular operational amplifiers
Name (dual op-amp IC) TL082 TL072 LM833 LTC6244
Technology JFETBJT
Known for: Huge input impedanceLow noise
Input noise on
100W at 1 kHz
nV / ÖHz 1845
pA / ÖHz 0.7

Quadruple Opamps :

The standard pinout is that of the LM324, which packs four operational amplifiers in a single 14-pin package; 3 pins per amplifier and 2 pins for the power supply.

Trimmable Single Opamps :

The standard pinout for single op-amps is that of the most common LM741  which includes two pins (1 and 5) for connection of an optional 10k external trimmer  (the midpoint of the trimmer goes to the negative rail.  The trimming is performed by wiring the inputs together and attempting to make an adjustement which does not make the output go to saturation  (or, rather, find a point close to where saturation swiches polarity).

Single-amplifier versions of the above dual op-amp packages are drop-in replacements for the LM741 but the balance pins aren't connected at all  (the internally-trimmed value of the input offset cannot be ajusted externally).  One JFET op-amp whose offset can be trimmed externally like the LM741 is the LF411.

The LM709  can be frequency-compensated with external cirduitry using pin 1 (balance input) and pin 5 (balance output).

A 14-pin version of the LM709 exists which is pin compatible with (obsolete) first-generation operational amplifiers, which had to be stabilized with external components.

Wikipedia :   Precision rectifier (super diode)


(2016-09-14)   Bipolar junction transistors  are current amplifiers.
The devices which started the silicon age and the information era.

 NPN transistor, idealized

 Come back later, we're
 still working on this one...

Wikipedia :   Transistors   |   Bipolar junction transistor
 
Video :   Genesis of the transistor,  Bell Labs  (1965)  with modern introduction by George Kupczak  (2012).


(2016-09-14)   Thyristor   (GE's  Silicon-Controlled Rectifier,  July 1957)
An SCR is a high-current bistable switch made from  4  layers of silicon.

A thyristor is  rigorously  equivalent to two transistors of opposite polarities in series.  In a pinch, or for low-current experimentation purposes, you can  make  thyristors this way, which behave exactly like commercial ones.

 Thyristor

 P-Gate Thyristor  The above equivalence is also a good way to analyze the behavior of the thyristor.  When the gate  (G)  is brought to the potential of the cathode  (K)  or below,  the thyristor is nonconducting.  Conversely, when the gate is positive enough to allow some current from G to K, the device becomes conductive and remains so without the need to maintain that current.

 N-Gate Thyristor  The above describes the standard  P-gate,  or  cathode-gated  thyristor,  which is now synonymous with  SCR.  The less common  N-gate  polarity is dubbed  anode-gated thyristor  (AGT).  The  AGT  polarity is so rarely used that some practitioners aren't even familiar with the corresponding symbol, shown at right  (which is mandatory to properly describe triacs).  The previous schematics apply, with the only change that the external gate terminal is connected to the other intermediate electrode.

Some thyristors are commercially available which have external connections to  both  internal gates.  Such a four-pin device is known as a  thyristor tetrode  or a  silicon controlled switch  (SCS).  Video  (ECG 239).

 BT151 pinout  BT151 pinout In the early days, the undisputed champion for the SCR at  General Electric  was  Frank W. "Bill" Gutzwiller  (c.1926-2011). Gutzwiller himself gave credit for the device to the development team at GE led by  Gordon Hall.  The thyristor was never patented, because GE's Patent Attorney (Bob Mooney) felt the claim would have been too expensive to defend in view of the pioneering work on "PNPN switches" at  Bell Laboratories,  by John L. Moll (1921-2011)  in Sept. 1956, and by  J.J. Ebers (1921-1959)  in 1952.  The original idea was first formulated by the Nobelist  William Shockley (1910-1989)  in 1950...  Thus, there's no inventor on record for the  thyristor.

The Early History of the Silicon Controlled Rectifier  (interview of Bill Gutzwiller).
The Sixth Decade of the Thyristor  Goce L. Arsov and Slobodan Mircevski (June 2010)
BT151-500R (or 650R, or 800R)   |   Driving a Tesla Coil
SCS for detection of ionizing radiation  MS Thesis by Lt. Karl J. Kjono, USN  (Dec. 2015).
Types of thyristors   |   Wikipedia :   Thyristor (SCR)


(2016-09-14)   TRIAC   (triode for alternating current,  1964)
A triggered triac lets current flow in one direction, until it vanishes...

A triac consists of two  thyristors  of  opposite polarities  in reverse-parallel  (i.e.,  the cathose of one is the anode of the other)  sharing the same gate.

 TRIAC  TRIAC  TRIAC

A triac is meant to conduct in either direction using whichever of its two consituent thyristors is relevant.  The controlling voltage of the gate is always understood to be with respect to the  first main terminal  labeled  MT1  (which is the cathode of the cathode-gated thyristor and the anode of the anode-gated thyristor).  The triac and its symbol are asymmetrical.

When used with  AC  a triggered triac will stop conducting when the main current returns to zero.  To reduce the power  (RMS)  delivered to a resistive load by an AC current, a triac in series is typically retrigerred after some adjustable delay every time it stops conducting.

Because it acts like a switch  (either on or off)  between its main terminals, the triac itself consumes little power and generates little heat even when traversed by a fairly large current.

"Semiconductor Bidirectional Switch" US Patent 3504241 A  (1967, 1970)   |   BTA16   |   Wikipedia :   TRIAC


(2016-09-20)   DIAC   (diode for alternating current,  1964)
The standard way to correct assymmetric triggering on the gate of a triac.

A diac is a symmetrical device which starts conducting upon reaching its  breakover voltage  (usually, 30 V or 40 V).  and remains conducting as long as the current doesn't fall below the  holding current.

The simplest type of diac  (e.g., DB3, discussed below)  is just a low-current NPN transistor without an external base connection and with emitter/collector symmetry.  As the voltage reaches the brakover voltage, enough current leaks into the isolated base to force the transistor into the saturated state, which it retains until current drops to zero again.  The device is engineered to withstand that regime repeatedly, unlike most ordinary transistors.

Most commonly, a diac is used in series with the gate of a  triac  to make it trigger at a predictable fairly symmetric voltage  (dissymetry less than 10%).  That configuration of a triac with a diac feeding its gate  greatly reduces the production of odd harmonics on the switched load.  It's commercially availaible as a single device, known as a  quadrac.

The most common type of diac, by far, is the DB3 bidirectional trigger diode  (32 V  nominal breakover voltage)  which comes in a DO35 axial package (usually dark blue) similar to a  ¼ W  carbon resistor.

 Lamp Dimmer circuit, with Triac and Diac

Above is the schematics for the popular  dimmer switch  to power an incandescent lamp from the mains  (120 V or 240 V AC).  When the voltage becomes zero, the triac is off and will start cinducting again when the voltage of the capacitor exceeds the diac's breakover voltage  (about 32 V).

DB3, DB31, DB32, DB4   |   Wikipedia :   DIAC


(2016-09-18)   Field-Effect Transistor
Voltage


(2014-06-05)   Microphones
Capturing sound.

 Come back later, we're
 still working on this one...

Tutorial by Doug Ford & Dave Jones (2014)
Introduction | Cardioid | Condenser & electret | Condenser design | Electret design | Phantom power


(2014-06-05)   Field-Programmable Gate Arrays   (FPGA)
Arguably the most flexible digital circuit  (and one of the fastest).

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 still working on this one...

Dave Jones :   What's an FPGA?  (2013-07-19)   |   FPGA for Beginners & FPGA Demo Boards  (2014-06-29)
Wikipedia :   Field-programmable gate array

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