How transistors work as switches shawn
Transistor as a switch
Transistors are suitable for contactless switching of small and medium powers.
The actual switch is the collector-emitter path (CE path) of the transistor. The base connection is the control electrode. The applied voltage UBE at the control electrode, the decisive factor is whether a current flows through the transistor or not. If a current flows through the transistor, then it is low-resistance, if no current flows through the transistor, then it is high-resistance.
|Input voltage Ue||0 V||12 V|
|Base-emitter voltage UBE||~ 0 V||~ 0.7V|
|Collector current I.C.||~ 0 mA||~ 50 mA|
|Resistance between collector and emitter RCE||~ 100 MΩ||~ 4 Ω|
|Voltage between collector and emitter UCE||~ 12 V||~ 0 V|
|Voltage at the collector resistor URC||~ 0 V||~ 12 V|
|State of the transistor||locks||directs|
Principle: transistor as a switch
In order to understand the principle of "transistor as switch" or "switching transistor", one has to take a closer look at the behavior of the transistor. The principle is that a blocking transistor corresponds to an open switch and a conductive transistor to a closed switch. The switching principle can be illustrated with the help of an equivalent circuit consisting of two resistors.
Locking transistor - open switch
If the transistor does not receive a base voltage UBE, no base current can flow. This means that no collector current flows. The RCELine is high-resistance and the entire operating voltage UB. drops at the transistor (CE path) as voltage UCE from.
The transistor blocks from the point of view of the current. For the mode of operation this means that the switch is open.
In the blocking state, the transistor acts like a high-value resistor. Since the resistance RC. is smaller than the resistance of the blocking transistor, most of the operating voltage drops across the transistor (CE path).
Conducting transistor - closed switch
If the transistor receives a positive base voltage UBE, a base current and a collector current flow. The RCE-Way is low resistance. A very low voltage U fallsCE on the transistor.
The transistor conducts from the point of view of the current. For the mode of operation, this means that the switch is closed.
In the conductive state, the transistor acts like a very low resistance. Since the resistance RC. in this case has a higher resistance than the conductive transistor, most of the operating voltage drops across the resistor R.C. from.
Operating point shift
When the transistor is switched, the operating point in the characteristic field changes its location from "On" to "Off" or vice versa. The working point crosses the forbidden area Pdead. In this area, the power consumed is very high, which heats the transistor. If the operating point takes too long for this path, the transistor will be destroyed.
Switching ohmic load (e.g. resistance)
Switching under an ohmic load (resistance) is not a problem because the operating point is in the range P.dead only briefly roamed.
Switching capacitive load (e.g. capacitor)
If there is a capacitor in the collector circuit, then it is switched under capacitive load. This results in a high current when switching on, which heats the transistor considerably. If this current is not limited, the operating point moves through area P.dead, whereby the transistor is destroyed.
Switching inductive loads (e.g. coil, relay or motor)
If there is an inductance in the collector circuit, a high induction voltage is generated when the device is switched off.
Background: In the conductive state, a magnetic field builds up due to the flow of current in the inductance, which suddenly collapses when switched off. The coil tries to keep the voltage switched off and generates an induction voltage. Therefore, a diode must be connected in parallel to limit the voltage. This diode is called a freewheeling diode. It is a completely normal silicon diode.
Free-wheeling diode for inductive loads
The freewheeling diode is connected in parallel to the relay or the inductance, which generates a high induction voltage when the transistor is blocking. The diode acts here as a protective diode. The diode short-circuits the induced voltage and limits it to the value of the diode forward voltage. However, the disadvantage of this method is an increased drop-out delay of the relay.
Fast transistor switch with diode
Through the diode D1, between the base and collector, is when the transistor T1 the base current is limited. This is carried off to the collector by the low forward voltage and therefore the transistor is less saturated. When switching off, the transistor takes much less time because fewer charge carriers have to be removed. The BAT85 diode is a small signal Schottky diode. These diodes switch very quickly. That is why they are especially suitable for this application. In addition, the forward voltage of around 0.25 V is significantly lower than that of a conventional silicon diode with around 0.65 V.
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