PUT (industrial electronic)

Chapter 2
Programmable Unijunction Transistor (PUT)

Basic Operation

(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 : 2N2067
The only similarity to a UJT is that the PUT can be used in the
same oscillator to replace the UJT. EAD 3043 (Industrial Electronic)
Nor Aida Idayu Binti Abdullah. 2012

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.

EAD 3043 (Industrial Electronic)

Characteristic of PUT

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


Characteristic-(Data Sheet)


Characteristic (con’t)


UJT’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


PUT relaxation oscillator

 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 EAD 3043 (Industrial Electronic)

Summary
 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.


Example


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.


Solution
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 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.


Quiz


Assignment


PUT (industrial electronic)

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