The unijunction transistor (UJT) and programmable unijunction transistor (PUT) are different internally, but are sufficiently similar in function to be combined in this entry.
Despite their names, the unijunction transistor (UJT) and programmable unijunction transistor (PUT) are not current-amplification devices like bipolar transistors. They are switching components that are more similar to diodes than to transistors.
The UJT can be used to build low- to mid- frequency oscillator circuits, while the PUT pro vides similar capability with the addition of more sophisticated control, and is capable of function ing at lower currents. The UJT declined in popularity during the 1980s after introduction of components such as the 555 timer, which offered more flexibility and a more stable output frequency, eventually at a competitive price. UJTs are now uncommon, but PUTs are still available in quantity as through-hole discrete components. Whereas an integrated circuit such as a 555 timer generates a square wave, unijunction transistors in oscillator circuits generate a series of voltage spikes.
The PUT is often used to trigger a thyristor (de scribed in Volume 2) and has applications in low- power circuits, where it can draw as little as a few Mmicroamps.
Schematic symbols for the two components are shown in Figure 27-1 and Figure 27-2. Although the symbol for the UJT is very similar to the symbol for a field-effect transistor (FET), its behavior is quite different. The bent arrow identifies the UJT, while a straight arrow identifies the FET. This difference is of significant importance.
The schematic symbol for a PUT indicates its function, as it resembles a diode with the addition of a gate connection.
Figure 27-1. Schematic symbol for a unijunction transistor (UJT). Note the bent arrow. The symbol for a field- effect transistor looks similar, but has a straight arrow. The functionality of the two components is very different.
In Figure 27-3, the transistors at left and center are old-original unijunction transistors, while the one at right is a programmable unijunction transistor. (Left: Maximum 300mW, 35V inter base voltage. Center: 450mW, 35V interbase volt age. Right: 300mW, 40V gate-cathode forward voltage, 40V anode-cathode voltage.)
Figure 27-2. Schematic symbol for a programmable unijunction transistor (PUT). The symbol accurately suggests the similarity in function to a diode, with the addition of a gate to adjust the threshold voltage.
Figure 27-3. The unijunction transistors at left and center are becoming obsolete; the one at the right is a program- mable unijunction transistor (PUT), still readily available and widely used as a thyristor trigger.
The UJT is a three-terminal semiconductor de vice, but contains only two sections sharing a single junction—hence its name. Leads attached to opposite ends of a single channel of N-type semiconductor are referred to as base 1 and base 2, with base 2 requiring a slightly higher potential than base 1. A smaller P-type insert, midway be tween base 1 and base 2, is known as the emitter.
The diagram in Figure 27-4 gives an approximate idea of internal function.
When no voltage is applied to the emitter, a relatively high resistance (usually more than 5K) prevents significant current flow from base 2 to base 1. When the positive potential at the emitter increases to a triggering voltage (similar to the junction threshold voltage of a forward-biased di ode), the internal resistance of the UJT drops very rapidly, allowing current to enter the component via both the emitter and base 2, exiting at base
1. (The term “current” refers, here, to convention al current; electron flow is opposite.) Current flowing from base 2 to base 1 is significantly greater than current flowing from the emitter to base 1.
Figure 27-4. Internal workings of a unijunction transistor.
The graph in Figure 27-5 outlines the behavior of a UJT. As the voltage applied to the emitter in creases, current flowing into the component from the emitter increases slightly, until the trig gering voltage is reached. The component’s internal resistance now drops rapidly. This pulls down the voltage at the emitter, while the cur rent continues to increase significantly. Because of the drop in resistance, this is referred to as a negative resistance region. The resistance actual ly cannot fall below zero, but its change is negative. After emitter voltage drops to a minimum known as the valley voltage, the current continues to increase with a small increase in voltage. On datasheets, the peak current is often referred to as Ip while valley current is Iv.
Figure 27-5. Response curve of a unijunction transistor (UJT). When positive potential at the emitter reaches the triggering voltage, internal resistance drops radically and the component goes through a phase known as “negative resistance” as current increases.
Figure 27-6 shows a test circuit to demonstrate the function of a UJT, with a volt meter indicating its status. A typical supply voltage would range from 9VDC to 20VDC.
Figure 27-6. A test circuit for a unijunction transistor (UJT) using a volt meter to show its response as a potentiometer increases the voltage applied to its emitter.
A PUT behaves similarly in many ways to a UJT but is internally quite different, consisting of four semiconducting layers and functioning similarly to a thyristor.
The PUT is triggered by increasing the voltage on the anode. Figure 27-7 shows a test circuit for a PUT. This component is triggered when the volt age at its anode exceeds a threshold level, while the gate sets the threshold where this occurs. When the PUT is triggered, its internal resistance drops, and current flows from anode to cathode, with a smaller amount of current entering through the gate. This behavior is almost identical to that of a forward-biased diode, except that the threshold level can be controlled, or “programmed,” according to the value of the positive potential applied at the gate, with R1 and R2 establishing that potential by functioning as a volt age divider.
Figure 27-7. A test circuit for a programmable unijunction transistor (PUT) using a volt meter to show its response as a potentiometer increases the voltage applied to its anode.
The voltage output of a PUT follows a curve that is very similar to that shown in Figure 27-5, al though current and voltage would be measured at the cathode.
PUTs and UJTs are not made as surface-mount components.
UJTs are usually packaged in black plastic, al though older variants were manufactured in cans. PUTs are almost all packaged in black plas tic. With the leads pointing downward and the flat side facing toward the viewer, the lead functions of a PUT are usually anode, gate, and cathode, reading from left to right.
The triggering voltage of a UJT can be calculated from the values of R1 and R2 in Figure 27-7 and the voltage at base 1. The term Rbb is often used to represent the sum of R1 + R2, with Vbb repre senting the total voltage across the two resistors (this is the same as the supply voltage in Figure 27-6). Vt, the triggering voltage, is given by:
Vt = Vbb * (R1 / Rbb)
The term (R1/Rbb) is known as the standoff ratio, often represented by the Greek letter ƍ.
Typically the standoff ratio in a UJT is at least 0.7, as R1 is chosen to be larger than R2. Typical values for R1 and R2 could be 180Ω and 100Ω, respectively. If R4 is 50K and a 100K linear potentiometer is used for R3, the PUT should be triggered when the potentiometer is near the center of its range. The emitter saturation voltage is typically from 2V to 4V.
If using a PUT, typical values in the test circuit could be supply voltage ranging from 9VDC to 20VDC, with resistances 28K for R1 and 16K for R2, 20Ω for R5, 280K for R4, and a 500K linear po tentiometer for R3. The PUT should be triggered when the potentiometer is near the center of its range.
Sustained forward current from anode to cathode is usually a maximum of 150mA, while from gate to cathode the maximum is usually 50mA. Power dissipation should not exceed 300mW. These values should be lower at temperatures above 25 degrees Centigrade.
Depending on the PUT being used, power consumption can be radically decreased by upping the resistor values by a multiple of 100, while supply voltage can be decreased to 5V. The cath ode output from the PUT would then be connected with the base of an NPN transistor for amplification.
Figure 27-8 shows a simple oscillator circuit built around a UJT, Figure 27-9 shows a comparable circuit for a PUT. Initially the supply voltage charges the capacitor, until the potential at the emitter of the UJT or the gate of the PUT reaches the threshold voltage, at which point the capacitor discharges through the emitter and the cycle repeats. Resistor values would be similar to those used in the test circuits previously described, while a capacitor value of 2.2μF would provide a visible pulse of the LED. Smaller capacitor values would enable faster oscillation. In the PUT circuit, adjusting the values of R1 and R2 would allow fine control of triggering the semiconductor.
Figure 27-8. A basic oscillator circuit using a unijunction transistor (UJT). As the capacitor accumulates charge, the voltage on the emitter increases until it triggers the UJT, at which point the capacitor discharges through the emitter.
Probably the most common use for a PUT at this time is to trigger a thyristor.
Figure 27-9. A basic oscillator circuit using a programmable unijunction transistor (PUT). As the capacitor accumulates charge, the voltage on the anode increases until it triggers the PUT, at which point the capacitor discharges through the anode. The gate voltage is preset by R1 and R2 to adjust the triggering voltage.
A programmable unijunction transistor (PUT) is sometimes referred to simply as a “unijunction transistor” (UJT). Bearing in mind the totally different modes of operation of UJT and PUT, the PUT should always be identified by its acronym or by its full name. A circuit will not function if a UJT is substituted for a PUT, or a PUT is substituted for a UJT.
Neither the UJT nor the PUT is designed to operate with reverse bias. In the UJT, a small forward bias should be applied from base 2 to base 1 (that is, base 2 should be at a higher potential relative to base 1) regardless of the voltage on the emit ter. The emitter voltage may vary from 0 volts
What Can Go Wrong
upward. The PUT must be forward biased be tween its anode and cathode (the anode must have a higher potential relative to the cathode), with an intermediate positive voltage at the gate established by resistors R1 and R2 functioning as a voltage divider (see Figure 27-7). Failure to observe correct biasing will result in unpredictable behavior and possible damage to the component.
Like any semiconductor, the UJT and the PUT must be protected from excessive current, which can burn out the component. Never connect either of these components directly across a power source without appropriate resistances to limit current flow. Maximum continuous power dissipation for UJTs and PUTs is usually 300mW.
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