Make Electronics

Experiment 11: A Modular Project

84 Chapter 2

essentIAls

All about   programmable    unijunction transistors 

(continued)

This may cause you to wonder what the function of the gate is. You can think of

it as “assisting” the finger on the button. In fact, the gate is the “programmable”

part of a PUT. By choosing a voltage for the gate, you establish the threshold

point when current starts to flow.

Here’s a simple take-home summary:

• The anode has to be more positive than the cathode, and the gate should
  be between those two extremes.


• If anode voltage increases above a threshold point, current bursts through
  and flows from the anode to the cathode.


• If anode voltage drops back down below the threshold, the transistor stops
  the flow.


• The voltage you apply to the gate determines how high the threshold is.


• The gate voltage is adjusted with two resistors, shown as R1 and R2 in the
  simple schematic in Figure 2-102. Typically, each resistor is around 20K. The
  PUT is protected from full positive voltage by R3, which can have a high
  value, 100K or greater, because very little current is needed to bias the
  transistor.


• You add your input signal in the form of positive voltage at the anode.
  When it exceeds the threshold, it flows out of the cathode and can work
  some kind of output device.
  The only remaining question is how we make a PUT oscillate, to create a stream
  of on/off pulses. The answer is the capacitor that you included in the circuit that
  you breadboarded at the beginning of Experiment 11.

GATE

ANODE

CATHODE

Figure 2-101. When voltage at the anode

of a PUT crosses a threshold (deter-

mined by a preset voltage at the gate),

current breaks through and surges from

the anode to the cathode. In this sense,

the anode voltage acts as if it presses a

button itself to open a connection inside

the PUT, with some assistance from

control voltage at the gate.

R2

R1 R3

Input

signal

Output

device

Figure 2-102. This simple schematic

shows how a PUT is used. R1 and R2

determine the voltage at the gate, which

sets the threshold point for the input at

the anode. Above the threshold, current

flows from anode to cathode.