PUT (industrial electronic)

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Chapter 2Chapter 2Programmable Unijunction Transistor (PUT)

Basic OperationBasic Operation

EAD 3043 (Industrial Electronic)Nor Aida Idayu Binti Abdullah. 2012

(a) symbol (b) construction

Like the thyristor, its consists of 4 P-N layers . Has anode and cathode connected to the first and last layer and gate connected to the one of inner layer. Not directly interchangeable with conventional UJTs but perform a similar function. In a proper circuit configuration with two ‘programming’ resistor for setting the parameter η, they behave like a conventional UJT. Example : 2N2067The only similarity to a UJT is that the PUT can be used in the same oscillator to replace the UJT.

Basic Operation (con’t)Basic Operation (con’t) When we bias the PUT properly, the current can not be flow

because the gate terminal is positive w.r.t cathode, when the anode voltage is increase form the cut off, the PN junction is forward bias, the PUT turn ON. The PUT remains in ON state until the anode voltage decreases below the cut off level and at that time the PUT is turn off.

The gate terminal of PUT can be biased through voltage divider network to active the desired voltage as shown in the given diagram.

Characteristic of PUTCharacteristic of PUT

The characteristic curve for the PUT is similar to the UJT.This is a plot of anode current IA versus anode voltage VAAs anode current increase, voltage increases up to the peak pointThereafter, increasing current results in decreasing voltage, down to valley point

Characteristic-(Data Characteristic-(Data Sheet)Sheet)

Characteristic (con’t)Characteristic (con’t)

UJT’s and PUT CircuitUJT’s and PUT Circuit

The PUT equivalent of the UJT is shown as the Figure above. External PUT resistor R1 and R2 replace UJT RB1 and RB2, respectively.These resistors allow the calculation of the intrinsic standoff ratio, η

Figure above shows the PUT version of the unijunction relaxation oscillator from the topic UJT before. Resistor R charges the capacitor until the peak point 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 oscillatorPUT relaxation oscillator

SummarySummary

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.

ExampleExample

Problem: What is the range of suitable values for R, a relaxation oscillator? The charging resistor must be small enough to supply enough current to raise the anode to VP the peak point 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.

SolutionWe 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 Along with VS=10, the closest value to our 6.3, we find VT=0.6V, in Table and calculate VP.

We also find IP and IV, the peak and valley currents, respectively in Table 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.

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.

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PUT (industrial electronic)

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