Introduction and Basic Working example Transistors

Transistors are applied to control the flow of current. In digital electronics, this control takes the form of an on/off action, with the transistor working as an electronic switch. Transistors are also worked in analog electronics wherever they can utilize to amplify signals linearly.

But, these days, a better way to do this to do an integrated circuit (chip) that combines lots of transistors and different components into a single convenient package. This tutorial does not include all types of transistors or semiconductor devices just
instead focuses on the most common items, which are usually low in cost and easy to use. There are other exotic devices like the unijunction transistors and SCRs (silicon-controlled rectifiers) that used to in but are now seldom used.

≡ Choosing the Right Transistor

Attach to a basic set of go-to transistors for the largest applications until you have more important requirements and then find something that fits the bill. A good set of go-to transistors shown in Table

Transistor Type Package Max. Current Max. Volts
2N3904 Bipolar TO-92 200mA 40V
2N7000 MOSFET TO-92 200mA 60V
MPSA14 Darlington TO-92 500mA 30V
TIP120 Darlington TO-220 5A 60V
FQP30N06L MOSFET TO-220 30A 60V

If buying for an FQP30N06L, make sure the MOSFET is the “L” (for logic) version, with “L” on the end of the part name. unless the gate-threshold voltage demand may too high to connect the gate of the transistor to a microcontroller output very if it operating at 3.3V. The MPSA14 actually a pretty universally useful device for currents up to 1A, although at that current there a voltage drop of nearly 3V, and the device gets up to a temperature of 120°C! At 500mA, the voltage drop a more flexible 1.8V, and the temperature 60°C.

≡ Digital switches Using Transistors

Apply a low-cost bipolar junction transistor (BJT) to do this work. BJTs are similar to the 2N3904 cost just a few cents and are often used with a microcontroller output pin from an Arduino or Raspberry Pi to improve the current pin can control.

Bipolar Transistor Schematic Symbol and Actual Device
Bipolar Transistor Schematic Symbol and Actual

Shows the schematic symbol for a BJT alongside one of the most common
models of this kind of transistor, the 2N3904. The 2N3904  a black plastic package called a TO-92 and you will find that many different low-power transistor models come supplied in the TO-92 package. The transistor is normally shown with a circle around it, but sometimes just the symbol inside the circle is used.

The three connections to the transistor in Figure are from top to bottom:

  1. The collector—the main current to  controlled flows into the collector
  2. The base—the control connection
  3. The emitter—the main current flows out through the emitter

The main current flowing into the collector and out of the emitter controlled by a much smaller current flowing into the base and out of the emitter. The ratio of the base current to the collector current named the current gain of the transistor and
typically about between 100 and 400. So for a transistor with a gain of 100, a 1mA current flowing from base to emitter will provide a current of up to 100mA to flow from collector to emitter.

Schematic for Experimenting with a Transistor
Schematic for Experimenting with a Transistor

The figures showed To get a feel for how such a transistor could be used as a switch. The push switch will turn the LED on when it’s pressed. Although this could be done much more simply by just putting the switch in series with the LED and R2. the major point that the switch supplying current to the transistor through R1. A fast calculation shows that the maximum current that could possibly flow through R1 and the base is:

I = V /R = 9V /10k =  . 9mA

In reality, the current less than this, because we are ignoring the 0.5V within the base and emitter of the transistor. If you require to be more precise, then the current is actually:

I = V/ R =( 9V   . 6V)/ 10k =  . 84mA

This little current flowing through the base establishing a much bigger current of roughly (assuming Vf of LED 1.8V):

I = V /R =( 9V  1 . 8V )/270 = 26 . 67mA

Just like a diode, if the BJT is in control there will an almost constant voltage drop of around 0.5V to 1V between the base and emitter connections of the transistor.

The BJT transistor defined earlier the most basic and an NPN type of transistor (negative positive-negative). This not a case of the transistor being indecisive.

but links to the fact that the transistor made up of lunch with N-type (negative) semiconductor as the bread and P-type (positive) semiconductor as the filling.

There an extra less generally used type of BJT called a PNP (positive-negative positive) transistor. The filling in this semiconductor lunch negatively doped. This means that everything flipped around. In Figure Schematic for Experimenting with a Transistor, the load (LED and resistor)  joined to the positive end of the power supply and switched on the negative side
to the ground. If you were to use a PNP transistor the circuit would look like

Using a PNP Transistor
Using a PNP Transistor


≡ Switch a Current with Minimum Control Current:

Use a Darlington transistor for Switch a Current with Minimum Control Current. A usual BJT will typically only have again (ratio of base current to collector current) of perhaps 100. A lot of the time, this will enough, but sometimes more gain is wanted. A useful way of doing this is to use a Darlington transistor. which will typically have a gain of 10,000 or more.

Darlington Transistors
Figure: Darlington Transistors

A Darlington transistor is really made up of two normal BJTs in one package as shown in Figure: Darlington Transistors. The shorter one is the MPSA14 and the larger the TIP120. The overall current gain of the pair of transistors in this arrangement is the gain of the first transistor multiplied by the gain of the second. It’s simple to see why this is the case, as the base of the second transistor supplied with current from the collector of the first.

≡ Switch High Current Loads

Use a MOSFET for Switch High Current Loads. MOSFETs (metal-oxide-semiconductor field-effect transistors) do not have an emitter, base, and collector, but rather a source, gate, and drain. Like BJTs, MOSFETs come in two flavors: N-channel and P-channel. It the N-channel that most utilized and well described in this recipe.

Figure MOSFETs

Figure MOSFETs shows the schematic symbol for an N-channel MOSFET with a couple of common MOSFETs next to it. The larger transistor (in a package called TO-220) is an FQP30N06 transistor capable of switching 30A
at 60V. The hole in the TO-220 package utilized to bolt it to a heatsink, something that only necessary when switching high currents. The small transistor on the right is the 2N7000, which good for 500mA at 60V.

Rather than multiply a current in the way that a BJT does, there is no electrical connection between the gate and other connections of the MOSFET. The gate isolated from the other connections by an insulating layer. If the gate-drain voltage exceeds the threshold voltage of the MOSFET, then the MOSFET conducts and a large current can flow between the drain and source connections of the MOSFET.

The threshold voltage varies within 2V and 10V. MOSFETs designed to work with digital outputs from a microcontroller such as an Arduino or Raspberry Pi are called logic-level MOSFETs and have a gate-threshold voltage guaranteed to be below 3V. If you look at the datasheet for a MOSFET you will see that it specifies on and off resistances for the transistor. An on resistance might be as low as a few mΩ and the off resistance many MΩ. This means that MOSFETs can switch much higher currents than BJTs before they start to get hot.

Schematic for Experimenting with a MOSFET
Figure Schematic for Experimenting with a MOSFET

The variable resistor now applied to set the gate voltage from 0V to the battery voltage. With the trimpot’s knob at the 0V end of its track, the LED will not lit. As the gate voltage rises to about 2V, the LED will begin to light and when the gate voltage gets to about 2.5V the LED should fully on.
Try disconnecting the end of the leaders working to the slider of the pot and touch it to the positive supply from the battery.

This should turn the LED on, and it should stay on even after you take the lead from the gate away from the positive supply. This because there enough charge sitting on the gate of the MOSFET to keep its gate voltage above the threshold. As soon as you touch the gate to ground, the charge will be conducted away to the ground and the LED will extinguish.

Since MOSFETs are voltage- rather than current-controlled devices you might be shocked to find that under some cases you do have to consider the current
flowing into the gate. That is because the gate acts like one terminal of a capacitor. This capacitor has to charge and discharge and so when pulsed at high frequency the gate current can become significant. Doing a current-limiting resistor to the gate will prevent too much current from flowing. Another difference within using a MOSFET and a BJT that if the gate connection left floating, then the MOSFET can turn on if you aren’t expecting it. This can be checked by connecting a resistor between the gate and source of the MOSFET.

≡ Switch Very High Voltages

An IGBT (insulated-gate bipolar transistor) an exotic species of transistor found in high-power, high-voltage switching applications. They are fast switching and usually are particularly well-specified if it comes to the operating voltage. Switching voltages of 1000V are not unique. A BJT has a base, emitter, and collector; a MOSFET has a gate, source, and drain; and
an IGBT combines the two, having a gate, emitter, and collector.

The Schematic Symbol for an IGBT and Two IGBTs
Figure The Schematic Symbol for an IGBT and Two IGBTs

schematic symbol for an IGBT beside two IGBTs. The less of the two
(STGF3NC120HD) capable of switching 7A at 1.2kV and the even larger one
(IRG4PC30UPBF) 23A at 600V.

IGBTs are voltage-controlled devices just like a MOSFET, but the switching side of the transistor works just like a BJT.

The gate of an IGBT will have a threshold voltage simply like a MOSFET. IGBTs are sometimes done in the same role as high-power MOSFETs but have the advantage over MOSFETs of being able to switch higher voltages at both large currents.

≡ Switching Alternating Current

TRIAC (TRIode for alternating current) a semiconductor-switching device created especially to switch AC. BJT and MOSFETs are not useful for switching AC. They can only do that if you split the positive and negative halves of the cycle and switch each with a separate transistor. It far better to use a TRIAC that can think of as a switchable pair of back-to-back diodes.

Switching AC with a TRIAC
Figure Switching AC with a TRIAC

The figure shows how a TRIAC might apply to switch a high AC load using a small current switch. A circuit like this often done so that a small low-power
A mechanical switch can apply to switch a large AC current.

If the switch pressed a small current (tens of milliamps) flows into the gate of
the TRIAC. The TRIAC conducts and will continue conducting until the AC crosses zero volts again. This has the benefit that the power switched off at low power at the voltage close to zero, reducing the power surges and electrical noise that would otherwise occur if switching inductive loads like motors.

But, the load could still be switched on at any point in the circuit, generating considerable noise. Zero-crossing circuits are applied to wait until the next zero crossings of the AC before turning the load on, reducing electrical noise further

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