Farnell TOPS 3D - Circuit Description

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CIRCUIT DESCRIPTION
Each of the raw d.c. lines is provided by a secondary winding on
the mains tranformer, feeding a bridge rectifier and resevoir
capacitor.
Control section
The-17 volt output uses Z1 as a reference zener fed from the out
put with R1, D1, D2, D3 providing the start up condition. IC2 is
used asa high gain amplifier to compare any difference between
the reference voltage provided by Zi and a fraction of the output
voltage given by the divider chain R10, P2, P4, P3 and R11.
The output of lC2 is fed via base/emitter of VT1 to drive the
Current limiting is provided by sensing across R4 and R7 with
VT3. At a pre-determined level set by P1,VT3 is turned 'on'
absorbing the drive current to the compound pair, and holding
the current constant whilst pulling the output down. In the
situation of VT3 being turned 'on' the drive current flow is such
that VT1 is turned 'on' and led LD2 illuminated to indicate
'overload'.
The negative supply is used as the reference for the + ve supply.
R12 and R13 sense the + ve voltage and R21, P5 and R22 the
Overall voltage between +ve and -ve. The circuit components
function on the same principle as for the -17 volt output.
In the case of the 6 volt output a 'ring of two' circuit provides a
suitable reference at Z3. VT11 and VT13 compare the difference
between the reference voltage and the fraction fed back from the
output via P8, P9, P10 and R41. Any error is amplified and fed via
base/emitter VT14 to the ermitter foldback pair VT15 and VT17.
This drives VT18 and VT19 in such a way as to oppose the output
change. Current limiting is provided by VT16 and at a level preset
by P7. In the event of current limit, i.e. when VT16 is turned on
VT14 is also turned on illuminating the overload led. When the
output voltage falls below a given level, foldback limiting is
provided by D6/ R33 driving VT16 harder on. Overvoltage prot
ection is provided by Z5 turning SCR1 on. The SCR is placed
directly across the output terminals so that the output'switch can
be used to reset the crowbar.
Compound series regulator formed by VT2 and VT4 in such a way
as to oppose any output changes.