Theory and Operation of Programmable Unijunction Transistors (PUTs)

Theory and Operation of PUTs

• A PUT (Programmable Unijunction Transistor) has the same characteristics as a standard UJT.
• The cathode (B1) is the reference terminal relative to which other voltages are measured.
• The anode (E) voltage rises until it reaches a certain critical value called the peak voltage (VP) which causes the device to fire.
• The peak voltage (VP) is determined by the external circuitry, unlike a UJT where VP is determined by the intrinsic standoff ratio associated with the transistor itself.
• The gate (B2) receives a voltage from an external circuit.

PUT Relaxation Oscillator

• The oscillation frequency is adjusted by varying the dc voltage applied to the gate from the RG1- RG2 voltage divider.
• A PUT can be used in applications that need low supply voltage.

PUT for Triggering SCR

• PUTs are used for triggering SCRs due to their ability to be programmed via gate voltage.

PUT Construction Slop Ramp Generator

• The output ramps always maintain a constant slope, but the height of the ramps can be adjusted by programming the PUT via gate voltage VG.
• This circuit could be used to furnish the input signal to an industrial servomechanism.

Advantages of PUT over UJT

• The advantage of using PUT over UJT for pulse triggering is that the peak voltage (VP) can be controlled by external circuitry.

Application of Slop Ramp Generator

• The voltage ramp furnishes the set-point signal to the servo system, increasing the ramp height would coincide with increasing the mechanism's stroke distance.

SCR Triggering Pulse Parameters

• The SCR triggering pulse should have an amplitude of 15 V and a duration of 30 µsec.

PUT Parameters

• Vs = 25 V and Ig = 0.5 mA are the parameters of the Programmable Unijunction Transistor (PUT).

PUT Application

• PUT is used in applications that require a low supply voltage.

PUT Triggering Circuit for SCR

• A PUT can be used to trigger an SCR.

PUT Construction and Slope Ramp Generator

• The output ramps of a PUT always maintain a constant slope.
• The height of the ramps can be adjusted by programming the PUT via gate voltage VG.

Applications of Slope Ramp Generator

• Such a circuit can be used to furnish the input signal to an industrial servomechanism.
• The voltage ramp can be used to provide a set-point signal to the servo system.
• The ramp height is directly proportional to the mechanism's stroke distance.

Advantage of PUT over UJT for Pulse Triggering

• The advantage of using PUT over UJT for pulse triggering is not explicitly stated in the text, but it is implied that PUT is more suitable for certain applications.

Delay Angle Control of SCR

• To control the delay angle of the SCR in the range from 25° to 180°, Vz and all circuit parameters need to be found.
• Vz and circuit parameters are not provided in the text, but can be calculated based on the specific application.