Programmable Unijunction Transistor (PUT) Basic Construction Working and Symbol

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 transistor

Figure 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 oscillator

Problem: 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]

VT    V
 VS=10V, RG=1Meg0.20.71.6 
 VS=10V, RG=10k0.20.350.6 
IP    µA
 VS=10V, RG=1Meg1.252.0 
 VS=10V, RG=10k4.05.0 
IV    µA
 VS=10V, RG=1Meg1850 
 VS=10V, RG=10k70150 
 VS=10V, RG=200Ω1500 

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 usable 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.

  • 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.

Article Extracted from Tony R. Kuphaldt Lessons In Electric Circuits — Volume III Chapter 7 under the terms and conditions of the CC BY License.

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