(2014-06-05) Current-ratings are typically for AC only (derate by a factor of 5 for DC).
We start our presentation of electronic circuit components witha 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 inways 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 withthe switch. This would solve the arcing problem which occurswhen an inductive load is turned off but the new capacitor causesa different problem when the switch is usedto 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 whichwill be dissipated almost instantaneously into the switch when it's turned back on.
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 greaterthan the unavoidable contact resistance of the switch. However, that resistor limits the effectiveness of the capacitor in addressingthe first issue raised. Thus, there's a tradeoff which should be properly analyzed towork out optimal values for R and C. Let's do so:
The above picture shows the situation just after the switch has beenturned off. The capacitor is discharged because it was previouslyshort-circuited by the switch. On the other hand, the current throughthe load still has its previous value (I) because of the assumed inductiveproperties of the load. The current I is now flowing through theresistor and the voltage across the switch is therefore equal to RI,which should not exceed the voltage rating for the switch (U0). So:
R < U0 /I
With the switch off, the capacitor gets charged to the full voltage of thepower 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/I0 < R
All told, we must have: U/I0 < R < U0 /I
The above analysis is for transient regimes only. When two switches are used in parallel (a DPST used as SPST) oneof them may have to withstand the whole transient before the other comes intoplay and the snubber network should be planned accordingly. Let's use as an example the little DPDT slide switch sold by Radio-Schackas 275-0033 (pack of two). As that switch is rated 500 mA at30 VDC, we may consider using both sides in parallel to switch apower supply rated 1000 mA at 9 V. As a single side may have to absorb all the transients, we should usethe above analysis with I = 1 A and I0 = 0.5 A. So:
U/I0 = 18 < R < 30 = U0 /I
Thus, we may want to choose for R a standard value of 22 .
The value we should use for C is derived from timing considerationswhich may or may not be predictable. Some switches featurea 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).
(2014-06-05) The standard way to prevent negative spikes on DC-controlled relay coils.
(2014-06-06) Junction diodes (PN and PIN). Schottky diodes. Zener avalanche.
(2014-06-10) How to minimize the influence of temperature on semiconductor circuits.
(2014-04-20) 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 circuitrythat 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 andendowed 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 fromone to the other. On the other hand, the current and the voltage delivered by theoutput can both be anything the circuit requires.
Elsewhere on this site, I present an elegant way to introducethe 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 :
A modern operational amplifier will provide a good approximation of this ideal behaviorwith 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
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 commonLM741 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 tomake 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 butthe balance pins aren't connected at all (the internally-trimmedvalue of the input offset cannot be ajusted externally). One JFET op-amp whose offset can be trimmed externally like the LM741 is theLF411.
TheLM709 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-generationoperational amplifiers, which had to be stabilized with external components.
(2016-09-14) The devices which started the silicon age and the information era.
(2016-09-14) An SCR is a high-current bistable switch made from 4 layers of silicon.
A thyristor is rigorously equivalent to two transistors ofopposite polarities in series. In a pinch, or for low-current experimentation purposes,you can make thyristors this way, which behave exactly like commercial ones. (Diagram below courtesy ofElectronics Tutorials.)
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.
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 familiarwith the corresponding symbol, shown at right (which is mandatory to properly describetriacs). The previous schematics apply, with the only change that the external gateterminal is connected to the other intermediate electrode.
Some thyristors are commercially available which have external connectionsto both internal gates. Such a four-pin device is known as a thyristor tetrode or a silicon controlled switch (SCS). Video (ECG 239).
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 deviceto 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 beentoo expensive to defend in view of thepioneeringwork on "PNPN switches" at Bell Laboratories, byJohn 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.
(2016-09-14) 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.
A triac is meant to conduct in either direction using whichever of its two consituentthyristors is relevant. The controlling voltage of the gate isalways understood to be with respect to the first main terminal labeled MT1 (which is the cathode of the cathode-gated thyristor and theanode of the anode-gated thyristor). The triac and its symbol areasymmetrical.
When used with AC a triggered triac will stop conductingwhen the main current returns to zero. To reduce the power (RMS) delivered to a resistive load by an AC current, a triac in series istypically 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 generateslittle heat even when traversed by a fairly large current.
(2016-09-20) The standard way to correct assymmetric triggering on the gate of atriac.
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.
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 theDB3bidirectional trigger diode (32 V nominal breakover voltage) which comes in a DO35 axial package(usually dark blue) similar to a ¼ W carbon resistor.
Above is the schematics for the popular dimmer switch to power an incandescent lamp fromthe mains (120 V or 240 V AC). When the voltage becomes zero, the triac is offand will start cinducting again when the voltage of the capacitor exceeds the diac's breakover voltage (about 32 V).
(2016-09-18) Voltage
(2014-06-05) Arguably the most flexible digital circuit (and one of the fastest).
(2019-03-05) Sideways voltage in a current crossing a magnetic field B.
