Global Specialties 1310 User Manual

Page 14

For best results, set the variable supplies to the desired voltage
before any connections are made. If the current equalizing resistors
are not well matched, current balance may be achieved by slightly
unbalancing the supplies while measuring the current output of
each supply. The voltage applied to the load must be moitored with
an external meter, since the current balancing resistors will introduce
a voltage drop, and will also adversely affect voltage regulation. For
these reasons, it is best to keep the value of the current balancing
resistors as low as possible. However, the lower value, the more
difficult it will be to balance the supplies. Each circuit powered this
way will required its own tradeoffs.

SECTION - 4

CIRCUIT DESCRIPTION

For the following circuit description, refer to the schematic.

5V SUPPLY :
Diodes D

through D9 make up a full-wave bridge which rectifies the

10 volt AC secondary winding of transformer T

used for the 5 volt DC

supply. This rectified output is filtered by capacitors C

and C

and

serves as the input to U

2’

an LM317 adjustable three-terminal regulator.

The output of U

is fixed at 5 volts by R

and R

7’

with C

11’

12’

and C

further filtering the output and improveing transient load response.
diode D11 protects U

from destructive current flow into its adjust pin,

such as might occur if the output is shorted. Diode D

provides

additional protection for U

from the inadvertent application of voltage

to the 5V terminals.

Resistors R

and R

are used to sense current flow into, and therefore

out of, the regulator. When currents exceed approximately 1 amp, the
voltage drop across these resistors will be sufficient to turn on transistor
Q

1’

which in turn lights D

12’

the OVERLOAD LED. Current flow is

ultimately limited by U

to a safe level.

VARIABLE SUPPLIES :
Since the operation of both variable supplies is identical, the following
discussion will deal only with the B variable supply.

The transformer secondary is rectified by the full-wave bridge
comprising D

through D

16’

and is filtered by capacitors C

and

15’

. This serves as the unregulated input to U

3’

an LM317 adjustable

three-terminal regulator. The output of U

is set by fixed resistor R

and variable resistor R

the B ADJUST control. Capacitors C

16’

17’

and C

accomplish additional filtering for the output, and improve the

transient load response. Diode D

protects the regulator from the

current flow that would occur into its adjust pin if the output becomes
shorted, and diode D

protects the regulator from the condition of its

output being raised above its input voltage.

Resistors R

and R

sense current flow through the supply, creating

a voltage drop that ultimetaly is used for the current reading on the
digital display. As there is a constant current flowing in the supply
circuitry even when there is no load attached, it is necessary to subtract
this so that the display shows true load current. This is done by the
constant current sink made up of transistor Q

resistors R

13'

14'

and

15'

and diodes D

and D

18'

Resistors R

11'

and R

scale this

resulting voltage to supply a 0.1mV / mA signal to display circuitry.
Similarly, resistors R

18'

19'

and R

scale the final supply output

voltage to procedure a 1mV / V signal for the display circuitry.