AE252 Midterm exam
NAME:_______________________________________ DATE:___________
The questions in this exam relate specifically to the power supply
schematic for the Varifi JFET preamp kit, which can be downloaded
at:
http://www.tangibletechnology.com/ipr/AE230/html/wk_4/varifi/jfet_pre/VF_psu.html
PLEASE do the work in pencil, show all work (starting
with formulae) and be sure to specify quantity (volts, amps, watts, dB,
etc.).

The Varifi JFET mic preamplifier consists of
two gain stages each on its own printed circuit board (PCB). Each PCB requires
24volts dc @ 40mA from the power supply unit (PSU). The equivalent resistance
of each preamp board is _______________ohms and each dissipates __________________
watts.
_______________________________________________________________

The lowvoltage secondary of the power transformer (xfrmr)
connects to the PSU via connector ______________. The secondary has two
____volt windings that have a common connection at J1 pins ____ and ____.
This common connection becomes _______________ and travels, from left to
right, to pin____ of J2.

Power for the preamps starts at the transformer secondary
and is rectified by diodes _____ and _____, a __________wave rectifier,
after which the raw DC is filtered by __________.

This is followed by a voltage divider that consists
of _______ and ________ .

The voltage drop across R1 is____________________________________
_______________________________________________________________

The current through R1 is _______________________________________
_______________________________________________________________

The power dissipated by R1 is ___________________________________
_______________________________________________________________

The purpose of Z1 is __________________________________________
.

The baseemitter voltage drop for Q1 should be _____________________.

The measured baseemitter voltage is ______________________________.

The preamp PCBs require approximately 24volts and the
PSU delivers within ______ per cent.
_______________________________________________________________

As mentioned in question1, when testing the PSU, two
resistors simulated the load of each preamp module. These resistors were
connected between J2 pin3 and __________ (J____ , pin_____). In the
schematic, the power supply output voltage is slightly higher and the closest
standard resistor values (that were near at hand) were 620? @ 1 watt each.
What is the current through each resistor and the power dissipation?
________________________________________________________________
________________________________________________________________

For all device ratings  fuses (current), Resistors
(wattage), Capacitors (voltage)  here is the formula to correlate the
safe operating range so that it is within 75% of the devices rated value.

For example, a 20amp circuit breaker should see
no more than 15 amps of continuous current. By using the formula and crossmultiplying
(100x = 75*y = 100x = 75*20) you can see that (x = .75 * 20A = 15A) or
x = 1500/100 = 15A. Conversely, if you know the current, voltage or wattage
at the device, you can then calculate the safe rating above
which the device should be rated.

Based on this formula, calculate the safe power
rating for the resistors in question12 to the nearest roundedup wattage
(typical wattage values are 1/4, 1/2, 1, 2, 3, 5, 10).
_______________________________________________________________
_______________________________________________________________

The phantom power supply is fed from one leg of the
xfrmr secondary through ______ and ______ before reaching the rectifier,
which consists of ________ and _______. This circuit is essentially two
______wave rectifiers in a _____________ _____________ configuration,
named because it produces twice as much DC voltage than the RMS AC voltage
feeding it.

Once rectified, the raw DC is filtered by ________ and
clamped at approximately _______ volts by the two series _____________,
designated _____ and _____.

The phantom power output appears at pin_____ of J_____
________________________________________________________________
________________________________________________________________

The phantom power supply also fed a 3k4 resistive load
to simulate the worst case scenario (shorting the the two ____? phantom
resistors at pin2 and pin3 of the XLR to ground).

As stated in note3 on the schematic, the noload to
worst case fullload output of the power supply ranges from 51.3 volts
to 45.6 volts. This change in load is reflected back at R4. What is the
AC current through, and the power dissipated by R4 from noload to fullload?
_______________________________________________________________
_______________________________________________________________

Is the wattage dissipated by R4 within 75% of the resistor's
rating on the schematic? (show work)
_______________________________________________________________

The power indicating LED is designated _____. Is it
before or after the voltage regulator? ______________________________________________

The resistor, R _______, feeding the LED has to "throw
away" _____ volts (VR3)in
order to not blow up the LED. What is the current through R4 and DL1?
____________________________________________________________
_______________________________________________________________

Based on all the information available to you, what
is the voltage at DL1?
_______________________________________________________________