Weir 413 - Page 7

The action of the feedback loop is as follows:- Should the output voltage increase, the extra current through
RV6 causes VTI to turn off slightly, reducing its collector current. Some of the current from R8, therefore,
diverts through MR9 and into the emitter of the common base stage VT3. This current flows from the
collector of VT3 into the base of VT4, causing its collector current to increase. As a result, the emitter
follower VT5 also draws more current from V T7, reducing the drive to VT9 and, hence, the output stages.
This, therefore, causes a change in output to oppose the initial change and stabilises the output voltage.
C3 gives increased gain at a.c. to minimise ripple. R42 helps to define the gain of VT1. 817 and C4
provide the main loop time constants. C14 and C7 suppress possible parasitic oscillations. When S6 is -
depressed, the variable control system is disconnected from V Tl base. An adjustable reference current through
R3 and RV3 and a fixed feedback current through R4 and MR6 are connected to VT1 base to give the fixed
oV output.
The current limit circuitry operates in a similar manner. The reference current is derived from the adjustable
divider R1 and RV1, with R9, R10 and R11 as the series resistors. The switching in this case is arranged to
allow only the lowest current limit selected to operate. The feedback current is through R12 and MR8. The
voltage drop across MR8 cancels the base emitter voltage of VT2 so that the voltage across R12 is equal to
that across R29. Similarly, the base emitter voltages of VT2 and VT6 cancel to stabilise the reference
current. The output of the current sensing stage is OR-gated into the emitter of VT3 by MR10. C6 provides
extra stabilisation of the loop in current limit mode.
The meter switching is entirely straightforward. RV4 on the voltage ranges or RV5 on the current ranges is
adjusted to shunt the meter and obtain calibration. S1A and S1B switch the meter between voltage and
current ranges. S2A on voltage and S2B on current short out part of the series chain to give the more
sensitive meter ranges.
The circuit of the centre tap section, used only on 'D' option units, is shown in Fig.3. On the 'D' option
units, the auxiliary unregulated supply is regulated by R31 and MR14. This supply provides, via R34 and
VT12, a constant current to the differential pair VT13 and VT14. These compare the centre tap voltage
with a centre point voltage derived by divider R32 and R33. This differential pair drives the Darlington
connected pair VT16 and VT17 which, in turn, drive the class 'B' push/pull output stage VT18, VT19,
VT20 and VT21. 837 and VT15 form another current source, providing a dynamic load for VT16 and VT17.
Output capacitors C8 and C9 are added to give good centre tap ripple and transient response. C10 and R39
determine the loop freguency response.
4.0 SETTING-UP PROCEDURE
Remove the cover from the unit by removing the two screws holding the carrying handle and
the four small screws (two on each side) from the sides of the cover. Access to all preset potentiometers is
gained through holes in the printed circuit board; their functions are marked on both sides of the board.
Prior to switching on, zero the meter if necessary (see Safety Regulations 64.5 (a) 8 (a)).
Switch on the supply, set the variable voltage control fully clockwise and depress the 7-5V (15V on 423)
push-button. Adjust RV2 (SET V) to give an output of 7-6V (15V on 423). Release the 7.5V (15V on 423)
push-button and depress the 15V (30V on 423) button. With the variable voltage control still fully clockwise,
check that the output voltage is 15.4 +0-4V (30-8 +0-8V on 423). Release the 15V (30V on 423) button
and depress the 30V (60V on 423) push-button. With the variable voltage control still fully clockwise,
check that this output voltage is 30-8 £0.8V (61.5 +1.5V on 423).
By means of the variable voltage control, set the output voltage to 30-0V (60-0V on 423). Adjust RV4
(SET METER V) so that the meter reads 30-0V (60-0V on 423) with both meter push-buttons released. By
means of the variable voltage control, reduce the output voltage to 10.0V (20.0V on 423). Depress the
lower meter push-button and check that the meter reads 10-0 +0.2V (20-0 £0.4V on 423). Release the
lower meter button.
Set the variable voltage control fully clockwise and short circuit the unit through a d.c. ammeter. Adjust
RV1 (SET A) to give a short circuit current of 0.55A (1-1A on 413). Release the 30V (60V on 423) push-
button and depress the 15V (30V on 423) push-button. Check that the short circuit current is 1.1 +0-1A
(2.2 +0-2A on 413). Release the 15V (30V on 423) push-button and depress the 7.5V (15V on 423) push-
button. Check that the short circuit current is 2.2 +0-2A (4.4 +0-4A on 413).
Remove the short circuit, release the 7-5V (15V on 423) push-button and depress the 5-00V push-button.
Adjust RV3 (SET 5V) to give an output of 5-00 £0-01V.