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Sunday, June 10th, 2012

     Back in the 60s my dad spent about $25 to buy a small transistor radio.  That was a lot of money in those days, but well worth it.  The new transistor technology allowed for a much less cumbersome radio to be produced.  No more lugging around big radios armed with heavy vacuum tubes.  In the years that followed the word transistor became a household word.  They were employed in a variety of ways within televisions and other electronic devices, increasing both their reliability and functionality.

     So what is a transistor and what does it do?  It’s an electronic component, developed in the late 1940s.  The first transistor was about as big as a softball and crudely made.  As such, it was too impractical for commercial use.  Then in the l950s technological advancements made commercial production of smaller, high-quality transistors possible.  Transistors enjoyed widespread introduction to the consuming mainstream in the l960s, and since then they’ve been made in many different types, shapes, and sizes.  Some are shown in Figure 1 below.


Figure 1


     A commonly used type of transistor is called a field effect transistor, or FET, one of which is shown in Figure 2.  The FET has three metal leads which allow it to be connected into electrical circuits.  These leads are referred to as the drain (D), the source (S), and the gate (G).

Figure 2


     FET’s control the flow of current within an electronic circuit.  A good way to understand what they do is to consider the analogy of water flowing through a faucet.

Transistor Faucet Analogy

Figure 3


    Figure 3 shows a faucet, complete with valve and handle.  With the valve closed the flow of water is completely shut off.  If the valve is opened partway by rotation of the handle, a trickle of water emerges.  The more the handle is turned and valve is opened, the greater the flow of water. 

     The FET shown in Figure 4 operates a lot like a faucet, but with regard to electrical current.


Figure 4


     The FET controls the flow of current flowing through its D and S leads, but it does not employ a valve or handle to do it.  Rather, flow rate is controlled by application of a small amount of voltage to the G lead.  The voltage’s influence on the G lead influences the FET to permit current to flow in through the D lead, then out through the S lead.  The amount of voltage applied to the G lead is directly related to how much current will be allowed to flow.  

     In this example the D lead on the FET is connected to a 10 volt direct current (VDC) power supply.  The S lead is connected to a flashlight bulb which is connected to electrical ground.  If you will remember from previous blogs, electric current naturally wants to flow from the supply source to ground, much like water wants to naturally flow downhill.

     If the bulb was connected directly to the 10 VDC power supply, current would flow through unimpeded and the bulb would light.  However, in Figure 4 the FET acts as a regulating device.  It’s connected between the 10 VDC power supply and the bulb.  When no voltage is applied to the G lead the FET acts like a closed valve and current is unable to flow.  Without current we, of course, have no light.

     When a low amount of voltage, say one volt, is applied to the G lead, the FET acts like a partially opened valve.  It permits a trickle of current to flow from the 10 VDC supply to the bulb, and the bulb glows dimly.  As voltage to G increases the FET valve opens further, permitting more current to flow.  The bulb glows with increasing brightness.

     When the voltage applied to G increases to the point the FET valve is opened fully, in our example that is 2 volts, full current is allowed to flow from the 10 VDC supply to the bulb.  The bulb glows brightly.  Generally speaking, the voltage required to be applied to G for control of current flow through an FET depends on overall design and the particular application within an electrical circuit. 

     FETs are often used within electronic devices to turn things on and off, with no other function in between.  Next time we’ll look at some example circuits to see how it’s done.