Circuit Description; Power Source - Elenco Electronics XP-660 Instruction Manual

Circuit Description

The Elenco ® XP-660 and XP-760 use the same
basic power and regulator circuit.
difference is in the display function. The XP-660 has
two 0-1mA analog meters. These meters convert to
a voltmeter by placing a 20kΩ resistor in series or
an ampmeter by placing a .39Ω shunt resistor
across the meter.
The Model XP-760 has two 3 1/2 digit LED meters.
Voltage is read by connecting the meter to a voltage
divider on the power supply output. Current is read
by connecting the meter to a shunt resistor in series
with the power supply output.

Power Source

The power supply for Models XP-660 and XP-760
consists of a transformer with four isolated step
down windings. Connected to each winding is a
diode bridge and a filter capacitor. The components
on the Power One Supply are diodes D301, D302,
D303, D304 and capacitor C304.
components convert the 120VAC input to 25VDC.
0-20V Regulators
Figure 2 shows a simplified circuit of the 0-20V
regulator circuit. This circuit consists of a high gain
amplifier transistor Q305. The gain of this stage is
over 100,000 because of its extremely high load
impedance. This load consists of a current source
transistor Q301 and its biasing circuit. The effective
resistance of this circuit is over 500,000 ohms.
Transistor Q302 is a power device that controls the
output current. Transistors Q303 and 304 are
emitter followers used to prevent loading of the
current source. Q301, Q302, Q305 and VR303 form
a closed negative feedback loop. If you analyze this
loop you will find that when the output voltage goes
down due to increase output current, the voltage at
the base of transistor Q305 goes negative. This
reduces the current in transistor Q305 and thus the
The only
Figure 1 Block Diagram of 0-20V Power Supply
These
When referring to the schematic diagrams, the
circuit components associated with each supply can
be identified by its number. The 5V supply begins
with 100. The 0-20V Power One Supply begins with
300 numbers and the 0-20V Power Two Supply
begins with 200 numbers. The basic circuit of these
supplies consist of 1) The power source, 2) The
regulator and 3) The readout circuit. These circuits
are shown in Figure 1 and will be discussed in detail
in the following paragraphs.
collector voltage will increase, returning the output
voltage to very near its original value. The high gain
is essential to the output voltage. Diode D309 is
added to prevent drift with temperature changes
and to allow the output voltage to go to zero.
To protect the regulator from overloads and short
circuits, transistor Q306 is added. Whenever the
voltage drops across resistor R309 reaches .6V,
transistor Q306 will conduct and lower the collector
voltage of amplifier Q305. To obtain variable current
limiting, transistor Q306 is prebiased via resistor
R309 and VR301 (see XP-660 schematic). This will
allow the output current limit between .05 and 1
amp. Transistors Q307 and Q308 are added to light
the overload LED.
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