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Tutorials Graphic We provide tutorials to help our customers better understand semiconductors and other electronic devices. Whether you have a question regarding a particular component, or simply want to know more about semiconductors, browse through our tutorial section for help. If you are still stuck, give us a call at (973) 377-9566 and we'll be happy to recommend a device for your application.

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The UJIT, CUJT, PUT, FLD, SUS, SBS, SCS.... Tutorial

What’s in a Name? Very little if you go by the acronymistic world we live in i.e. DOD, DOT, NASA, NATO and so on. Every field of endeavor has its own alphabetic soup. In the medical field I never did understand the medical difference between and ECG AND EKG. Usually it doesn’t mean much unless your job or life depends upon it, and if it does (your job that is) this brings us to discrete semiconductor acronyms. We all know: BJT stands for bipolar junction transistor FET is Field Effect Transistor MOS is Metal Oxygen Silicon However, did you know that?

UJT is the acronym for Unijunction Transistor (which isn’t a transistor at all) treated in our tutorial section under Unijunction Transistors. The UJT comes in TO-5, TO-18, TO-92 packages some of the most common types are 2N2646, 2N2647, 2N4891, 2N4892, 2N1671A,B,C, 2N489, 2N490, 2N490, 2N491, 2N492, 2N493, AND 2N494.

CUJT, Complementary Unijunction Transistor, type numbers D5K1 and D5K2 are both in a TO-18 package; is similar to the normal unijunction transistor except the polarities are opposite to those of the normal unijunction transistor. The Complementary unijunction transistor is analogous to the standard unijuntion transistor as the PNP transistor is to the NPN transistor. The CUJT also incorporates a tighter intrinsic standoff ratio and is more stable than the normal UJT.

PUT, Programmable Unijunction Transistor, type numbers are 2N6027, 2N6028, D13T1, and D13T4 all in a TO-18 package. By selecting two resistors external to the PUT, the intrinsic standoff ratio(n) can be varied as well as the peak current (Ip), the valley current (Iv), the interbase resistance (Rbb).

FLD, a Four Layer Diode, a thyristor with only 2 leads which may not qualify it in a purists mind as a thyristor; however when its breakover voltage is exceeded it conducts and continues to do so as long as its current remains at or above its holding current (Ih). It is unidirectional some typical part numbers are--.

SUS, Silicon Unilateral Switch, type numbers 2N4987, 2N4988, 2N4989, and 2N4990 all in TO-98 epoxy packages; and the 2N4983, 2N4984, 2N4985, and 2N4986 ALL IN TO-18 metal packages are really integrated circuits but none the less are considered a discrete device. It operates similar to an ideal FLD and has a designed 8 volt breakover voltage. It has a gate lead that minimizes the rate effect as well as transients on the SUS and allows for lower breakover voltages. In addition it has a low temperature coefficient of switching.

SBS, Silicon Bilateral Switch, type numbers are 2N4991 and 2N4992 both in TO-98 epoxy packages is also in truth an integrated circuit but is considered a descrete device; a bilateral thyristor with a designed breakover voltage of 8 volts, same as the SUS. However the SBS is bilateral and the SUS unilateral. It also has a gate lead similar to the SUS to reduce the rate effect and allow for lower breakover voltages. It also has a low temperature coefficient of switching.

SCS, Silicon Controlled Switch, types are 3N84, 3N85 and 3n86 each in a TO-18 package are SCRs suited for low current applications and feature a fourth lead that eliminates the SCR rate effect (dv/dt). They are sensitive SCRs which feature a low holding current (Ih).

SCR, Silicon Controlled Rectifier, a true unilateral thyristor; gate controlled, with an “ON STATE” and holding current. See our tutorial on SCRs. Too many to list here.

CSCR, Complementary Silicon Controlled Rectifier, uses a negative gate pulse to turn on an SCR otherwise it behaves as a standard SCR.

GTO, Gate Turn- off Thyristor, an SCR that can be turned “OFF” as well as “ON” by a gate signal. There are many GTOs available, too many to mention. Call American Microsemiconductor for information.

TRIAC, A bilateral thyristor, gate controlled, with an “ON STATE” and the usual holding current. See our tutorial on triacs.

DIAC, A bilateral triggering device, two leads only, which switches from an “OFF STATE” to an “ON STATE” when its breakover voltage is reached. It is used primarily to trigger triacs and other bidirectional gate thyristors. See our tutorial on diacs.

SIDAC, another two leaded thyristor device that fires when its breakover voltage is met. However it is usually used to carry the lead current directly in contrast to a diac which often triggers another power device such as a triac which in turn carries the lead current. P.S. The dv/dt problem:

If the rate of voltage rise that any gate controlled thyristor sees is high enough, it can result in a gate current large enough to falsely trigger a thyristor (this can e caused by transient voltages from other parts of the circuit, power supply spikes, lightning etc.). In such case the determining gate current is: Ig= c dv/dt

Where dv/dt is the rate of the rise of the anode to gate voltage and C is the anode to gate capacitance of the thyristor junction. Since the capacitance C is intrinsic to the device, the device manufacturer specifies a maximum dv/dt that if exceeded could falsely trigger the thyristor. On way to control this rate of rise is with a: Snubber Circuit One very simple way to control the dv/dt of a thyristor circuit is to place an r-c circuit across the anode to cathode, slowing the rate of rise dv/dt of the thyristor circuit and the resulting generated gate current. This is the simplest way to solve the problem. There are many others. And who would you think is responsible for these ancronymistic devices? Well many are General Electric and Motorola. But guess what? Neither supplies these any longer. But who does? AMERICAN MICROSEMICONDUCTOR!

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