Key to the field effect transistor

Perhaps even a distant person from electronics heard that there is such an element as a relay. The simplest electromagnetic relay contains an electromagnet, when a voltage is applied to which the other two contacts close. With the help of a relay, we can switch a fairly powerful load, applying or vice versa, removing the voltage from control contacts. The most widespread relays, controlled by 12 volts. Also, there are relays for voltage of 3, 5, 24 volts.
The key on the field-effect transistor
However, it is possible to commute a powerful load not only with the help of a relay. Recently, powerful field-effect transistors have become widespread. One of their main goals is to work in a key mode, i.e. the transistor is either closed or completely open when the resistance of the Stok-Istok transition is practically zero. You can open a field effect transistor by applying voltage to the gate relative to its source.It is possible to compare the operation of the key on the field-effect transistor with the operation of the relay - the voltage is applied to the gate, the transistor has opened, the circuit has closed. They removed the voltage from the gate - the circuit was opened, the load was de-energized. In this case, the key on the field-effect transistor has some advantages over the relay, such as:
  • High durability. Quite often, relays fail due to the presence of mechanically moving parts, while the transistor, under proper operating conditions, has a much longer service life.
  • Efficiency. The relay coil consumes a current, and sometimes very significant. The gate of the transistor consumes current only at the moment when voltage is applied to it, then it practically does not consume current.
  • No clicks when switching.


The key diagram for the field-effect transistor is shown below:
The key on the field-effect transistor
The resistor R1 in it is current-limiting, it is needed in order to reduce the current consumed by the gate at the time of opening, without it the transistor can fail. The value of this resistor can be easily changed over a wide range, from 10 to 100 Ohm, this will not affect the operation of the circuit. Resistor R2 pulls the gate to the source,thereby equalizing their potentials when no voltage is applied to the gate. Without it, the gate will remain “hanging in the air” and the transistor will not be able to shut down guaranteed. The value of this resistor can also be changed over a wide range - from 1 to 10 kΩ. Transistor T1 - field N-channel transistor. It must be selected on the basis of the power consumed by the load and the magnitude of the control voltage. If it is less than 7 volts, you should take the so-called "logical" field-effect transistor, which reliably opens from 3.3 - 5 volts. They can be found on computer motherboards. If the control voltage is in the range of 7-15 volts, you can take the "ordinary" field-effect transistor, for example, IRF630, IRF730, IRF540 or any other similar. In this case, it is necessary to pay attention to such characteristics as the resistance of the open channel. Transistors are not perfect, and even in the open state, the resistance of the Stoke-Istok transition is not zero. Most often, it makes up hundredths of Ohm, which is absolutely not critical when switching a load of low power, but very significantly at high currents. Therefore, in order to reduce the voltage drop across the transistor and, accordingly, reduce its heating,You need to choose a transistor with the lowest resistance of the open channel. "N" in the diagram - any load. The disadvantage of a key on a transistor is that it can only work in DC circuits, because current flows only from Stock to Source.

Making a key on a field-effect transistor

Assemble such a simple the circuit can be mounted, but I decided to make a miniature PCB using laser-iron technology (LUT). The procedure is as follows: 1) Cut out a piece of PCB that fits the size of the circuit board pattern, clean it with fine emery paper and degrease with alcohol or solvent.
FET transistor key
2) On special thermotransfer paper we print a printed circuit board drawing. You can use glossy paper from magazines or tracing paper. The density of the toner on the printer should be set to maximum.
Key on the FET
3) Transfer the pattern from the paper to the textolite using the iron. It should be monitored so that the paper with the pattern does not move relative to the PCB. The heating time depends on the temperature of the iron and lies within 30 - 90 seconds.
Key on the field-effect transistor
4) As a result, a pattern of the tracks appears in the mirror image. If the toner sticks poorly to the future board, you can fix the flaws with the help of women's nail polish.
The key on the field-effect transistor
5) Next, we put the textolite to be etched. There are many ways to make etching solution, I use a mixture of citric acid, salt and hydrogen peroxide.
Key on the field-effect transistor
After etching, the board takes the following form:
Key on the field-effect transistor
6) Then you need to remove the toner from the PCB, the easiest way to do this is using nail polish remover. You can use acetone and other similar solvents, I used petroleum solvent.
FET transistor key
7) The case for small - now it remains to drill holes in the right places and zaludit fee. After that, it takes the following form:
Key on the field-effect transistor
The board is ready for soldering the parts into it. Only two resistors and a transistor are required.

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