QUASAR PROJECT KIT # 3125 - NEGATIVE ADJUSTABLE POWER SUPPLY MODULE
PAGE 1
This kit is a compact, easy to build and use negative
variable power supply module. It is ideal for powering
any application requiring a DC supply at current levels
up to 1.5A
FEATURES:
•
Output reverse polarity and back-voltage protection
•
LED power on indication
•
On board heatsink for IC regulator
•
Variable output voltage
•
AC or DC input voltage
•
Low noise (uses linear regulator)
•
Screw terminals for input, output and LED indicator
•
Compact dimensions (84 x 37 mm)
This kit can be combined with Kit 3124 to create a dual-
rail supply.
The kit is constructured on a single-sided printed circuit
board (PCB). Protel Autotrax & Schematic were used to
design the board.
ASSEMBLY INSTRUCTIONS
Follow the overlay on the PCB when inserting
components. Start with the lowest height components
first. Leave the 2200uF capacitor, heatsink and regulator
IC until last. Make sure that the polarised components,
diodes and electrolytic capacitors, are inserted the correct
way round.
Note:
Take care when bending the diode leads. Use a
pair of long nose pliers to hold the lead close to the body
to avoid stressing the diode itself. Make sure that the
diode leads are bent accurately to fit into the PCB.
Lightly secure the regulator IC to the heatsink using the
screw and nut provided. Use some heatsink compound if
available. Fit the whole assembly to the PCB and solder
into position. Tighten the screw and nut. Now fit the
2200uF capacitor.
CIRCUIT DESCRIPTION
Diodes D1-4 form a bridge rectifier which converts the
AC input voltage into a DC level. They also allow a DC
input voltage to be connected either way around.
Capacitor C1 smooths the DC output of the bridge whilst
C2 provides high frequency decoupling. The LM337T is
an adjustable regulator IC providing the desired output
voltage. Diode D5 is reversed biased during normal
operation and is used to protect the regulator if the output
is connected to a voltage of the same polarity (eg battery).
Diode D6 protects the regulator if a reverse polarity
voltage is connected to the output.
The regulator develops a nominal 1.25V reference
voltage between the output and adjust terminals. This
constant voltage is applied across R1, causing a constant
current to flow. This constant current flows through
trimpot VR1. By varying VR1, the voltage across it will
vary and hence the output voltage can be set.
The output voltage is calculated by:
V
VR
R
OUT
=
+
1 25 1
1
1
.
(
)
Capacitor C3 improves the ripple rejection of the
regulator while capacitorS C4 and C5 provide high and
low frequency decoupling respectively.
The LED indicates that power is present at the output.
The current through the LED should be between 5 and
20mA and is set by resistor R2. The value of R2 varies
depending on the required output voltage and is
calculated by
R
V
V
x
OUT
LED
2
3
10 10
=
−
−
where
V
V
LED
≈
2
INPUT CONFIGURATION
There are a number of different input configurations that
can be used with this kit. The configuration chosen will
depend on the available voltage source. Wherever
possible, keep the input voltage as low as possible to
achieve the required output. This minimises the heat
dissipation on the regulator.
AC Transformer - single output
AC Transformer - centre-tapped output
DC input source
Note:
DC source can be connected either way around