Make Electronics

Switching Basics and More 83

Experiment 11: A Modular Project

essentIAls

All about programmable unijunction transistors

The schematic symbol for a programmable unijunction transistor, or PUT, looks
very different from the symbol for a bipolar transistor, and its parts are named
differently, too. Nevertheless, it does have a similar function as a solid-state
switch. The symbol and the names of the three connections are shown in Figure
2-99.

Note that this is a rare case (maybe the only one in the whole of electronics!) in
which you won’t run into confusing variations of the basic schematic symbol. A
PUT always seems to look the way I’ve drawn it here. Personally I think it would be
clearer if we added a circle around it, but no one seems to do that, so I won’t, either.

The 2N6027 is probably the most common PUT, and seems to be standardized
in its packaging and pin-outs. I’ve only seen it in a plastic module rather than
a little tin can. Figure 2-100 shows the functions of the leads if your 2N6027 is
manufactured by Motorola or On Semiconductor. If you have one from another
source, you should check the data sheet.

Note that the flat side of the plastic module faces the opposite way around
compared with the 2N2222 bipolar transistor, when the two devices are func-
tioning similarly.

The PUT blocks current until its internal resistance drops to allow flow from the
“anode” to the “cathode.” In this way, it seems very similar to an NPN transistor, but
there’s a big difference in the circumstances that cause the PUT to lower its resis-
tance. The voltage at the anode determines when the PUT allows current to flow.

Suppose you start with, say, 1 volt at the anode. Slowly, you increase this volt-
age. The transistor blocks it until the anode gets close to 6 volts. Suddenly this
pressure breaks down the resistance and current surges from the anode to the
cathode. If the voltage goes back down again, the transistor reverts to its origi-
nal state and blocks the flow.

I’ve included another version of the “finger on the button” drawing to convey
this concept. The voltage on the anode is itself responsible for pushing the but-
ton that opens the pathway to the cathode. See Figure 2-101.

Anode

Cathode

Gate

Figure 2-99. The schematic symbol for
a PUT.

Anode

Gate

Cathode

Figure 2-100. In PUTs manufactured by

On Semiconductor and Motorola, the

leads have these functions.