ELEX Pre-Board Exam 11

A power derating factor of 3.33 mW/°C above 50°C means that

• you must decrease the DC power dissipation rating by 3.33 mW for every 1°C rise in the operating temperature above 50°C
• you will end up getting a power of 3.33 mW if temperature goes beyond 50°C
• you must increase the DC power dissipation rating by 3.33 mW for every 1°C rise in the operating temperature above 50°C
• your zener diode will dissipate a nominal DC power of 3.33 mW for temperature above 50°C

When you first turn a power supply on, the filter capacitor initially acts as a short circuit thus creating a high initial current that can possibly destroy your rectifier diodes. What is the best way to limit the said current but without sacrificing the output voltage?

• use a large-value series current-limiting resistor
• use large-value filter capacitors
• use inductive filters instead of capacitive filters
• don’t turn off your power supply

The electromagnetic force induced between a varying magnetic field and a stationary conductor.

• Mutually induced EMF
• Self-induced EMF
• Electro statically induced EMF
• Statically induced EMF

A wire has a resistance of 5 Ω at room temperature and a temperature coefficient α = 4 × 10-3/°C. Calculate the wire resistance if the temperature rises by 75°C.

• 6.5 Ω
• 6.1 Ω
• 6 Ω
• 6.16 Ω

A combination of two charges with equal charge magnitude but opposite signs.

• magnetic dipole
• static dipole
• dynamic dipole
• electric dipole

If a dc electromagnetic of 10-V produces a steady-state current of 100-mA in a 100 Ω coil, how much will the current be after 0.2 sec?

• 63 mA
• 100 mA
• 37 mA
• 200 Ma

What is the time constant of a 20-H coil having 100 Ω of series resistance?

• 5 s
• 0.5 s
• 0.2 s
• 20 ms

If the RC time constant is given to be 1 s, how long will it take for the voltage to drop from 100 V to 50 V?

• 0.63 s
• 0.37 s
• 0.5 s
• 0.7 s

The effect of little change in magnetic flux density when magnetic field intensity increases.

• Retentivity
• Coercivity
• Saturation
• Permeability

A series resonant circuit at 0.2 MHz develops 100 mV across a 250 µH coil with a 2 mV input. Calculate Q.

• 50
• 25
• 100
• 200