ujt

UJT

A Unijunction transistor is a three terminal semiconductor switching device.this device has a unique characteristics that when it is triggered , the emitter current increases regeneratively until is limited by emitter power supply the unijunction transistor can be employed in a variety of applications switching pulse generator saw tooth generator etc.

ConstructionIt consists of an N type silicon bar with an electrical connection on each

end the leads to these connection are called base leads. Base 1 B1 Base 2 B2 the bar between the two bases nearer to B2 than B1. A pn junction is formed between a p type emitter and Bar.the lead to the junction is called emitter lead E.

Operation

The device has normally B2 positive w.r.t B1 If voltage VBB is applied between B2 and B1 with emitter open. Voltage gradient is established along the n type bar since emitter is located nearer to B2 more than half of VBB appears between the emitter and B1. the voltage V1 between emitter and B1 establishes a reverse bias on the pn junction and the emitter current is cut off. A small leakage current flows from B2 to emitter due to minority carriers

If a positive voltage is applied at the emitter the pn junction will remain reverse biased so long as the input voltage is less than V1 if the input voltage to the

emitter exceeds V1 the pn junction becomes forward biased.under these conditions holes are injected from the p type material into the n type bar these holes are repelled by positive B2 terminal and they are attracted towards B1 terminal of the bar. This accumulation of holes in the emitter to B1 region results in the degrees of resistance in this section of the bar the internal voltage drop from emitter to b1 is decresed hence emitter curret Ie increases as more holes are injected a condition of saturation will eventually be reached at this point a emitter current limited by emitter power supply only . the devices is in on state.

If a negative pulse is applied to the emitter , the pn junction is reverse biased and the emitter current is cut off. The device is said to be off state.

Characteristics of UJT

The curve between Emitter voltage Ve and emitter current Ie of a UJT at a given voltage Vbb between the bases this is known as emitter characterstic of UJT

Initially in the cut off region as Ve increases from zero ,slight leakage current flows from terminal B2 to the emitter the current is due to the minority carriers in the reverse biased diode .

Above a certain value of Ve forward Ie begins to flow , increasing until the peak voltage Vp and current Ip are rreached at point P.

After the peak point P an attempt to increase Ve is followed by a sudden increases in emitter current Ie with decrease in Ve is a neagative resistance portion of the curve The negative portion of the curve lasts until the valley point V is reached with valley point voltage Vv.and valley point current Iv after the valley point the device is driven to saturation the difference Vp-Vv is a measure of a switching efficiency of UJT fall of Vbb decreases

Advantages of UJTIt is a Low cost device It has excellent characteristics It is a low-power absorbing device under normal operating conditions

The Unijunction Transistor (UJT)We need your help! This page requires proofreading - If you notice

any errors, please post on our forumsUnijunction transistor: Although a unijunction transistor is not a thyristor, this device can trigger larger thyristors with a pulse at base B1. A unijunction transistor is composed of a bar of N-type silicon having a P-type connection in the middle. See Figure below(a). The connections at the ends of the bar are known as bases B1 and B2; the P-type mid-point is the emitter. With the emitter disconnected, the total resistance RBBO, a datasheet item, is the sum of RB1 and RB2 as shown in Figure below(b). RBBO ranges from 4-12kΩ for different device types. The intrinsic standoff ratio η is the ratio of RB1 to RBBO. It varies from 0.4 to 0.8 for different devices. The schematic symbol is Figure below(c)

Unijunction transistor: (a) Construction, (b) Model, (c) SymbolThe Unijunction emitter current vs voltage characteristic curve (Figure below(a) ) shows that as VE increases, current IE increases up IP at the peak point. Beyond the peak point, current increases as voltage decreases in the negative resistance region. The voltage reaches a minimum at the valley point. The resistance of RB1, the saturation resistance is lowest at the valley point.IP and IV, are datasheet parameters; For a 2n2647, IP and IV are 2µA and 4mA, respectively. [AMS] VP is the voltage drop across RB1 plus a 0.7V diode drop; see Figure below(b). VV is estimated to be approximately 10% of VBB.

Unijunction transistor: (a) emitter characteristic curve, (b) model for VP .

The relaxation oscillator in Figure below is an application of the unijunction oscillator. RE charges CE until the peak point. The unijunction emitter terminal has no effect on the capacitor until this point is reached. Once the capacitor

voltage, VE, reaches the peak voltage point VP, the lowered emitter-base1 E-B1 resistance quickly discharges the capacitor. Once the capacitor discharges below the valley point VV, the E-RB1 resistance reverts back to high resistance, and the capacitor is free to charge again.

Unijunction transistor relaxation oscillator and waveforms. Oscillator drives SCR.During capacitor discharge through the E-B1 saturation resistance, a pulse may be seen on the external B1 and B2 load resistors, Figure above. The load resistor at B1 needs to be low to not affect the discharge time. The external resistor at B2 is optional. It may be replaced by a short circuit. The approximate frequency is given by 1/f = T = RC. A more accurate expression for frequency is given in Figure above.The charging resistor RE must fall within certain limits. It must be small enough to allow IP to flow based on the VBB supply less VP. It must be large enough to supply IV based on the VBBsupply less VV. [MHW] The equations and an example for a 2n2647:

Programmable Unijunction Transistor (PUT): Although the unijunction transistor is listed as obsolete (read expensive if obtainable), the programmable unijunction transistor is alive and well. It is inexpensive and in

production. Though it serves a function similar to the unijunction transistor, the PUT is a three terminal thyristor. The PUT shares the four-layer structure typical of thyristors shown in Figure below. Note that the gate, an N-type layer near the anode, is known as an “anode gate”. Moreover, the gate lead on the schematic symbol is attached to the anode end of the symbol.

Programmable unijunction transistor: Characteristic curve, internal construction, schematic symbol.The characteristic curve for the programmable unijunction transistor in Figure above is similar to that of the unijunction transistor. This is a plot of anode current IA versus anode voltage VA. The gate lead voltage sets, programs, the peak anode voltage VP. As anode current inceases, voltage increases up to the peak point. Thereafter, increasing current results in decreasing voltage, down to the valley point.The PUT equivalent of the unijunction transistor is shown in Figure below. External PUT resistors R1 and R2 replace unijunction transistor internal resistors RB1 and RB2, respectively. These resistors allow the calculation of the intrinsic standoff ratio η.

PUT equivalent of unijunction transistorFigure below shows the PUT version of the unijunction relaxation oscillator Figure previous. Resistor R charges the capacitor until the peak point, Figure previous, then heavy conduction moves the operating point down the negative resistance slope to the valley point. A current spike flows through the cathode during capacitor discharge, developing a voltage spike across the cathode resistors. After capacitor discharge, the operating point resets back to the slope up to the peak point.

PUT relaxation oscillatorProblem: What is the range of suitable values for R in Figure above, a relaxation oscillator? The charging resistor must be small enough to supply enough current to raise the anode to VP the peak point (Figure previous) while charging the capacitor. Once VP is reached, anode voltage decreases as current increases (negative resistance), which moves the operating point to the valley. It is the job of the capacitor to supply the valley current IV. Once it is discharged, the operating point resets back to the upward slope to the peak point. The resistor must be large enough so that it will never supply the high valley current IP. If the charging resistor ever could supply that much current, the resistor would supply the valley current after the capacitor was discharged and the operating point would never reset back to the high resistance condition to the left of the peak point.We select the same VBB=10V used for the unijunction transistor example. We select values of R1 and R2 so that η is about 2/3. We calculate η and VS. The parallel equivalent of R1, R2 is RG, which is only used to make selections from Table below. Along with VS=10, the closest value to our 6.3, we find VT=0.6V, in Table below and calculate VP.

We also find IP and IV, the peak and valley currents, respectively in Table below. We still need VV, the valley voltage. We used 10% of VBB= 1V, in the previous unijunction example. Consulting the datasheet, we find the forward voltage VF=0.8V at IF=50mA. The valley current IV=70µA is much less than IF=50mA. Therefore, VV must be less than VF=0.8V. How much less? To be safe we set VV=0V. This will raise the lower limit on the resistor range a little.

Choosing R > 143k guarantees that the operating point can reset from the valley point after capacitor discharge. R < 755k allows charging up to VP at the peak point.Selected 2n6027 PUT parameters, adapted from 2n6027 datasheet. [ON1]

Parameter Conditions min typical max units

VT         V

  VS=10V, RG=1Meg 0.2 0.7 1.6  

  VS=10V, RG=10k 0.2 0.35 0.6  

IP         µA

  VS=10V, RG=1Meg - 1.25 2.0  

  VS=10V, RG=10k - 4.0 5.0  

IV         µA

  VS=10V, RG=1Meg - 18 50  

  VS=10V, RG=10k 70 150 -  

  VS=10V, RG=200Ω 1500 - -  

VF IF=50mA - 0.8 1.5 V

Figure below show the PUT relaxation oscillator with the final resistor values. A practical application of a PUT triggering an SCR is also shown. This circuit needs a VBB unfiltered supply (not shown) divided down from the bridge rectifier to reset the relaxation oscillator after each power zero crossing. The variable resistor should have a minimum resistor in series with it to prevent a low pot setting from hanging at the valley point.

PUT relaxation oscillator with component values. PUT drives SCR lamp dimmer.PUT timing circuits are said to be useable to 10kHz. If a linear ramp is required instead of an exponential ramp, replace the charging resistor with a constant current source such as a FET based constant current diode. A substitute PUT may be built from a PNP and NPN silicon transistor as shown for the SCS equivalent circuit in Figure below by omitting the cathode gate and using the anode gate. REVIEW: A unijunction transistor consists of two bases (B1, B2) attached to a

resistive bar of silicon, and an emitter in the center. The E-B1 junction has negative resistance properties; it can switch between high and low resistance.

A PUT (programmable unijunction transistor) is a 3-terminal 4-layer thyristor acting like a unijunction transistor. An external resistor network “programs” η.

The intrinsic standoff ratio is η=R1/(R1+R2) for a PUT; substitute RB1 and RB2, respectively, for a unijunction transistor. The trigger voltage is determined by η.

Unijunction transistors and programmable unijunction transistors are applied to oscillators, timing circuits, and thyristor triggering.

The UJT TransistorThe Uni Junction Transistor

This transistor is rarely used in a hobby-circuit because there are a number of alternate ways to produce the same effect, however it is interesting to know how the Uni Junction Transistor works, so you can see why it lost favour. The UJT is especially designed to connect to SCR's and this is where its main application lies. It only needs 3 or 4 external components and the UJT will produce an oscillator. The output of the oscillator can be a saw-tooth, if you carefully pick off the waveform across the capacitor, but more often the output is taken between B1 and the negative rail. This is where the LOAD is designed to be placed. The waveform here is a spike, and has an amplitude of about 2v for a 6v supply (from 0v to +2v).The diagram below shows the UJT in a typical arrangement with a low-impedance device, such as a speaker, between B1 and 0v rail. 

The way the circuit works is this: The capacitor charges via the 10k resistor During this time the resistance between B1 and B2 is infinite. The charging is a sawtooth waveform because the charging is a rising gradient and the discharge is very rapid - into the speaker. The emitter detects when the voltage rises to 4v (for a 6v circuit) and at this point the transistor "turns on" and the resistance between the Emitter and B1 becomes very low and is effectively equivalent to a diode in forward-bias. The voltage (the energy) in the capacitor is then passed to the speaker and this produces a "click." This is the only way the speaker gets its energy. The voltage across the capacitor falls very rapidly and when it reaches about 1v (for a 6v circuit), the transistor turns off and the

cycle repeats. It is important to note that the speaker does not receive any current via the resistor connected to B2 and this  resistor has very little effect (almost no effect) on the volume produced by the speaker. The only way to increase the loudness of the tone is by adding a buffer transistor.  Doubling the supply voltage will alter the frequency of the output very slightly and the voltage across the speaker will increase to 10v, but the width of the spike is so narrow that very little energy is passed to it. The circuit is reliable in operation. It will work on almost flat batteries (a high impedance supply) but you have to be careful with the choice of resistors and capacitors. For instance, if the 10k is reduced to 1k, the circuit will simply refuse to work with the 2u2 electrolytic so the circuit below may "freeze-up" if the wrong ratio of R and C is selected. The "freeze-up" may also depend on supply voltage so, all in all, the circuit has some limitations that must be overcome by building a prototype before deciding on the final design.  

The circuit below shows a common use for a UJT - to   "fire" an SCR after a pre-determined period of time. The LOAD will remain active (the SCR will remain "triggered") and the supply voltage must be removed to de-energise the LOAD - a bit messy - in the days of microcontroller designs, this can be done much more conveniently with a chip. 

All Talking Electronics projects are now microcontroller based and it is only a matter of adding a few extra lines of code to produce almost any sound you need. UJT transistors are expensive and difficult to obtain, however the circuit consumes only about 1mA @6v or 2mA at 12v and this may give it an application where very little is required to be consumed. Note: many hobby circuits show the UJT drawn incorrectly. B1 must be connected to the device being activated as the energy from the capacitor flows via the Emitter-B1 circuit to the device. The transistor will not work around the other way, however it will not be damaged if momentarily connected incorrectly. 

 The tab is actually between B2 and E.

The UJT can be gated "on and off" by  controlling the 47k "charging resistor."The circuit below shows an "up-side-down" multivibrator turning on the tone circuit (the UJT circuit):

The Talking Electronics Digital Course studies the "standard" multivibrator as shown below. This standard circuit can be turned up-side-down if needed. The animation below shows how one side of the multivibrator turns on then the other side to activate each LED.  In the circuit above, the multivibrator takes the 47k to the positive rail, to activate the tone circuit.

Don't forget: UJT's are difficult to get and expensive, when compared to transistors and other standard semiconductors. 

     - Colin Mitchell  21-7-2001 and 15-3-2004

Unijunction Transistorby Matthew H. Williams

Back to the British Amateur Electronics Club.The basic structure of a unijunction transistor (UJT) is shown in Fig.1. It is essentially a bar of N type semiconductor material into which P type material has been diffused somewhere along its length. Contacts are then made to the device as shown; these are referred to as the emitter, base 1 and base 2 respectively. Fig.2 shows the schematic symbol used to denote a UJT in circuit diagrams. For ease of manufacture alternative methods of making contact with the bar have been developed, giving rise to the two types of structure - bar and cube - shown in Fig.3

The equivalent circuit shown in Fig.4 has been developed to explain how the device works, and it is necessary to define the terms used in this explanation.

RBB is known as the interbase resistance, and is the sum of RB1 and RB2:

RBB = RB1 + RB2 (1)

N.B. This is only true when the emitter is open circuit.

VRB1 is the voltage developed across RB1; this is given by the voltage divider rule:

RB1

VRB1 = (2) RB1 + RB2

Since the denominator of equation 2 is equal to equation 1, the former can be rewritten as: RB1 VRB1 = x VBB (3) RBB

The ratio RB1 / RBB is referred to as the intrinsic standoff ratio and is denoted by   (the Greek letter eta).

If an external voltage Ve is connected to the emitter, the equivalent circuit can be redrawn as shown in Fig.5.

If Ve is less than VRB1, the diode is reverse biased and the circuit behaves as though the emitter was open circuit. If however Ve is increased so that it exceeds VRB1 by at least 0.7V, the diode becomes forward biased and emitter current Ie flows into the base 1 region. Because of this, the value of RB1 decreases. It has been suggested that this is due to the presence of additional charge carriers (holes) in the bar. Further increase in Ve causes the emitter current to increase which in turn reduces RB1 and this causes a further increase in current. This runaway effect is termed regeneration. The value of emitter voltage at which this occurs is known as the peak voltage VP and is given by: VP =   AVVBB + VD (4)

The characteristics of the UJT are illustrated by the graph of emitter voltage against emitter current (Fig.6).

As the emitter voltage is increased, the current is very small - just a few microamps. When the peak point is reached, the current rises rapidly, until at the valley point the device runs into saturation. At this point RB1 is at its lowest value, which is known as thesaturation resistance.

The simplest application of a UJT is as a relaxation oscillator, which is defined as one in which a capacitor is charged gradually and then discharged rapidly. The basic circuit is shown in Fig.7; in the practical circuit of Fig.8 R3 limits the emitter current and provides a voltage pulse, while R2 provides a measure of temperature compensation. Fig. 9 shows the waveforms occurring at the emitter and base 1; the first is an approximation to a sawtooth and the second is a pulse of short duration.

The operation of the circuit is as follows: C1 charges through R1 until the voltage across it reaches the peak point. The emitter current then rises rapidly, discharging C1 through the base 1 region and R3. The sudden rise of current through R3 produces the voltage pulse. When the current falls to IV the UJT switches off and the cycle is repeated.

It can be shown that the time t between successive pulses is given by:

VBB - VV

t + R1C ln secs (5) N.B. R measured in Megaohms. C in µF. VBB - VP

Design for a lKHz relaxation oscillator

The oscillator uses a 2N2646 UJT, which is the most readily available device, and is to operate from a 10V D.C. power supply.

From the relevant data sheet the specifications for the 2N2646 are:

VEB2O IE(peak) PTOT(max) IP(max) IV(max) Case style TO18 30V 2A 300mw 5µA 4ma 0.56 - 0.75

It is important that the value of R1 is small enough to allow the emitter current to reach IP when the capacitor voltage reaches VP and large enough so that the emitter current is less than IV when the capacitor discharges to VV. The limiting values for R1 are given by: VBB - VP VBB - VV

R1(max) = and R2(min) = IP IV

From the specifications for the 2N2646 the average value of   is 0.56 + 0.75/2 = 0.655. Substituting this value in equation (4) and assuming VD = 0/7V: VP = 0.655 x 10 + 0.7 = 7.25V.

So R1(max) = 10 - 7.25/5µA = 550K, and if VV = approx VBB/10, R1(min) = 10 - 1/4mA = 2.25K.

If we choose a value for R1 somewhere between these limits, e.g. lOK, the value of C can be calculated from equation (5)

If f = 1MHz, t = 1/f = 1msec. VBB - VP = 10 - 7.25 = 2.75 and VBB - VV = 10 - 1 = 9

t Rearranging equation(5) to make C the subject: C = VBB - VV

R1 ln VBB - VP

0.001so C = = approx 84nF. 104 ln (9/2.75)

Because of component and UJT tolerances it is sufficient in most circumstances to use an approximate formula: f = 1/CR, which assumes that   is 0.63 - well within 5% of the average value for the 2N2646. In practice one would use a variable resistance (or a variable resistance in series with a fixed resistance) for R1 so that the frequency of oscillation could be adjusted to give the required value.

R2 is not essential; if it is included, a value of 470 ohms is appropriate for the 2N2646. The value of R3 should be small in comparison with RBB, with which it is in series, so as to prevent it from affecting the value of the peak voltage. A value of 47 ohms or thereabouts is satisfactory.

Editor's notes: The above design points are illustrated in the circuit of the enlarger timer which was described earlier this year in the April Newsletter. In that circuit the UJT provides clock pulses at 20Hz. R1 is a combination of a 47K variable and a 150K fixed resistance; R2 is omitted and R3 is 33 ohms. The timing capacitor has a value of 220nF. In addition to the 2N2646, the component list for this timer also includes the TIS43 and the 2N4891. Most suppliers list only the 2N2646, but Maplin also include the TIS43. This device was used with a transistor constant current generator as the sawtooth oscillator in the timebase of the "Student's Oscilloscope" published in "Practical Wireless" in August 1973.

In his book "110 Semiconductor Projects for the Home Constructor" (2nd edition 1978), R.M.Marston gives twenty circuits for UJT projects using the 2N2646. These include pulse and sawtooth generators, analogue/digital converters, relay time delay circuits and frequency

dividers. If any member would like to experiment with UJTs there is a good number of 2N2646 and TIS43 in Cyril's stock, and Ray Marston's book can be borrowed from me for 38p postage.

There is also a device called a programmable UJT - the BRY39 is an example so called because its parameters can be set by external components. It is a PNPN device, similar in some ways to a thyristor, and can be used in applications similar to those for the UJT. Perhaps we could have an article about this in a future Newsletter.