4.2 Transistors, Relays, and Switches

4.2 Transistors, Relays, and Switches

Active switching and amplification circuits govern electrical control, signal processing, and high-power distribution across modern aircraft systems. This section details Bipolar Junction Transistors (BJTs), mechanical and solid-state switch configurations, electromagnetic relays, solenoids, and inductive flyback protection.

Bipolar Junction Transistor (BJT) Fundamentals

A Bipolar Junction Transistor (BJT) is a three-terminal, current-controlled semiconductor device consisting of two back-to-back P-N junctions within a single crystal structure. Conduction involves both majority and minority charge carriers (hence bipolar).

Construction and Current Relationships

BJTs are constructed in two structural configurations:

• NPN Transistor: Features a thin P-type base layer sandwiched between an N-type emitter and an N-type collector. In schematic symbols, the emitter arrow points outward away from the base ("Not Pointing iN").
• PNP Transistor: Features a thin N-type base layer sandwiched between a P-type emitter and a P-type collector. In schematic symbols, the emitter arrow points inward toward the base ("Pointing iN Permanently").

The three BJT terminals perform distinct physical functions:

1. Emitter (E): Heavily doped to inject charge carriers into the base region.
2. Base (B): Extremely thin and lightly doped to allow most injected carriers to pass through to the collector.
3. Collector (C): Moderately doped and physically larger to collect charge carriers and dissipate operating heat.

According to Kirchhoff's current law, the total current entering or leaving the transistor obeys the fundamental equation: $I_E = I_B + I_C$

Two critical DC current gain parameters define transistor amplification:

• DC Beta ($\beta$ or $h_{FE}$): The ratio of DC collector current to DC base current: $\beta = \frac{I_C}{I_B}$ In typical general-purpose and switching transistors, $\beta$ ranges from 20 to 300.
• DC Alpha ($\alpha$): The ratio of DC collector current to DC emitter current: $\alpha = \frac{I_C}{I_E} = \frac{\beta}{\beta + 1}$ Because collector current is slightly less than emitter current due to base recombination, $\alpha$ is always slightly less than unity (typically 0.95 to 0.999).

BJT Operating Regions and Characteristic Curves

A transistor operates in one of three distinct regions based on the bias voltages applied across its base-emitter and base-collector junctions.

Cutoff, Active, and Saturation Modes

1. Cutoff Region: Both the base-emitter and base-collector junctions are reverse-biased ($V_{BE} < 0.7\text{ V}$ for silicon). Base current is zero ($I_B = 0$), resulting in zero collector current ($I_C = 0$). The collector-to-emitter voltage equals the supply voltage ($V_{CE} \approx V_{CC}$). In this state, the transistor acts as an open switch.
2. Active Region: The base-emitter junction is forward-biased ($V_{BE} \approx 0.7\text{ V}$), and the base-collector junction is reverse-biased. Collector current is directly proportional to base current ($I_C = \beta I_B$). The transistor operates as a linear current amplifier, commonly employed in audio amplifiers, RF receivers, and sensor signal conditioning.
3. Saturation Region: Both the base-emitter and base-collector junctions are forward-biased. Collector current reaches its maximum circuit-limited value ($I_{C,sat} \approx \frac{V_{CC}}{R_L}$). Further increases in base current produce no increase in collector current. The collector-to-emitter saturation voltage drops to a minimal value ($V_{CE,sat} \approx 0.1\text{ V} - 0.3\text{ V}$). In this state, the transistor acts as a closed switch.

Worked Formula: BJT Relay Driver Calculation

An avionics engineer is designing a switching circuit where a 5.0 V digital logic output from a flight computer drives an NPN transistor to energize a 28 V DC landing light control relay coil ($R_{coil} = 140\ \Omega$). The transistor has a minimum specified beta $\beta_{min} = 40$ and a forward base-emitter drop $V_{BE} = 0.7\text{ V}$. Calculate the minimum collector current ($I_{C,sat}$), the minimum base current ($I_{B,min}$), the required drive base current ($I_{B,drive}$) using a forced-beta overdrive factor of 2, and the value of the base resistor ($R_B$).

1. Calculate Saturation Collector Current ($I_{C,sat}$): Assuming $V_{CE,sat} = 0.2\text{ V}$: $I_{C,sat} = \frac{V_{CC} - V_{CE,sat}}{R_{coil}} = \frac{28.0\text{ V} - 0.2\text{ V}}{140\ \Omega} = \frac{27.8\text{ V}}{140\ \Omega} = 0.19857\text{ A} \approx 200\text{ mA}$

2. Calculate Minimum Base Current for Saturation ($I_{B,min}$): $I_{B,min} = \frac{I_{C,sat}}{\beta_{min}} = \frac{200\text{ mA}}{40} = 5.0\text{ mA}$

3. Apply Forced-Beta Overdrive Factor ($k_{overdrive} = 2$): To guarantee hard saturation across temperature variations, double the base current: $I_{B,drive} = 2 \times I_{B,min} = 2 \times 5.0\text{ mA} = 10.0\text{ mA}$

4. Calculate Base Resistor ($R_B$): $R_B = \frac{V_{logic} - V_{BE}}{I_{B,drive}} = \frac{5.0\text{ V} - 0.7\text{ V}}{0.010\text{ A}} = \frac{4.3\text{ V}}{0.010\text{ A}} = 430\ \Omega$ The designer selects a standard $430\ \Omega$ resistor to ensure reliable transistor switching.

Switches and Switching Topologies in Avionics

Switches are mechanical or solid-state devices used to make, break, or change the connections in an electrical circuit.

SPST, SPDT, and DPDT Contact Configurations

• Single-Pole Single-Throw (SPST): Contains one movable contact (pole) and one stationary contact (throw). Provides basic ON-OFF control for a single circuit.
• Single-Pole Double-Throw (SPDT): Contains one movable pole that toggles between two stationary throw contacts (Common, Normally Open - NO, and Normally Closed - NC). Used to switch power or signals between two alternative pathways.
• Double-Pole Double-Throw (DPDT): Contains two separate poles mechanically linked to toggle simultaneously between two sets of throw contacts. Functions as two synchronized SPDT switches, allowing dual-circuit reversal or dual-bus switching.

Switch contact action is categorized as Break-Before-Make (BBM)—where the pole disconnects from the existing contact before connecting to the new contact to prevent shorting two power sources—or Make-Before-Break (MBB), used where continuous signal continuity is required.

Electromagnetic Relays, Solenoids, and Flyback Protection

Relays and solenoids utilize electromagnetism to control mechanical contacts or actuators remotely.

Coils, Armatures, and Inductive Spike Suppression

• Electromagnetic Relay: Consists of an insulated wire coil wound around a soft iron core and a spring-loaded movable armature attached to contact points. Applying a small control current through the coil creates a magnetic field that pulls the armature, opening or closing high-current electrical contacts. Relays provide vital electrical isolation between low-voltage digital avionics logic and high-power 28 V DC or 115 V AC distribution buses.
• Solenoid: Uses an electromagnetic coil to drive a movable iron plunger in a linear mechanical stroke. Solenoids actuate mechanical valves, door locks, and starter-generator engagement mechanisms.
• Flyback (Freewheeling) Diode Protection: An electromagnetic coil is a strong inductor ($L$). When the driving transistor abruptly switches from saturation (ON) to cutoff (OFF), the collapsing magnetic field induces a severe reverse voltage spike across the coil, known as inductive counter-EMF ($v_L = L \frac{di}{dt}$). This voltage spike can reach several hundred volts and destroy the driving transistor's collector junction. To suppress this spike, a rectifier diode is connected directly in parallel across the relay coil in a reverse-biased orientation relative to supply voltage. During normal operation, the diode is non-conducting. When the transistor turns off, the inductive counter-EMF forward-biases the diode, safely recirculating and dissipating stored magnetic energy through the coil resistance.

Avionics Traps & Maintenance Considerations

• Relay Contact Pitting & Welding: Switching heavy inductive loads creates electrical arcing across mechanical contacts upon opening. Over time, arcing causes contact erosion, material transfer (pitting), high contact resistance, or physical contact welding (sticking closed).
• Reversed Flyback Diode Short: If a replacement flyback diode is accidentally installed backward (anode to positive supply), turning on the control transistor creates a direct dead short from the 28 V DC bus through the transistor to ground, instantly destroying the transistor or blowing the circuit breaker.
• Mechanical Contact Bounce: Physical switch contacts bounce upon closure for 1 to 10 milliseconds, producing a rapid burst of digital logic transitions. High-speed digital systems must incorporate hardware RC low-pass debouncing or software debouncing algorithms to prevent false triggering.