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APPENDIX
A1.1 Model 4234 Power Supply Unit
Two different power supply units are available to suit different
supply voltages. See Spare Parts List.
A1.1.1 Functional Description
A circuit diagram for each type is shown in Fig. A1 (240V), Fig.
A2 (110V).
A stable supply voltage is produced across zener diodes Z3
and Z4 by utilizing the forward slope resistance of zener
diodes Z1 and Z2 in the full-wave rectifier bridge connected to
the secondary winding of transformer T1. A reference voltage
is produced across C103 by zener diode Z101 in conjunction
with
R101,
with
diode
compensation. This reference voltage is applied to the base of
TR101, which is used to drive the power transistor TR102 to
produce a constant current output of 350mA. The small preset
potentiometer RV101 is used to provide a fine adjustment for
the current output.
The output current is restricted by inviolate resistors which
ensure that the requirements of the intrinsic safety certification
are met, even under a '2-fault' condition.
Warning. This unit is part of the certified
intrinsically safe system, Appropriate safety precautions
must be taken to prevent any incendive electrical
discharges in the hazardous area when carrying out
maintenance tasks.
A1.1.2 Fault Finding
Caution. Do not operate this unit without an
electrical load on the output.
If testing indicates that this unit is defective, further fault finding
may be carried out based on the typical test point values given
in Table A1. There are several test points available on the unit.
Reference should be made to Figs. A1, A2 and the markings
on the circuit boards for the location of test points and
components.
Table A1 Test Point Values - Model 4234 Power Supply Unit
Voltage
Test Point
V
TP1
9.1
TP2
3.5
TP3
3.0
TP4
2.4
TP5
5.0
TP6
1.7
T1
9.1
38
D103
providing
temperature
Form
Conditions
d.c.
With respect to 0V
d.c.
With respect to 0V
d.c.
With respect to 0V
d.c.
With respect to 0V
With respect to 0V using
d.c.
10Ω dummy load
d.c.
With respect to 0V
a.c.
At secondary
Note. The primary winding of the transformer T1
incorporates a thermal cutout device to prevent
overloading under fault conditions. Sufficient time must
be allowed for this to cool and reset after a fault has
occurred, and before continuing further testing.
A1.1.3 Parts List
Repair Maintenance Parts
Description
Resistor
R101, 910R, ±2%, 0.5W, metal oxide
R102, 3k3, ±2%. 0.5W, metal oxide
R103, 4R7, ±1%, 9W, wirewound
R104, 100R, ±2%, 0.5W, metal oxide
Variable resistor
RV 101, 1k0, Spectrol Reliance, CW51
Capacitor
C101, 1000µF, 16V, elect., Mullard 0175 15102
C102, 1000µF, 16V, elect., Mullard 0175 15102
C103, 100µF, 16V, elect, Mullard 0165 14101
Transistor
TR101, BC 108, Mullard
TR 102, 2N 3766, Motorola
Diode
D101, BYX 36 - 600, Mullard
D102, BYX 36 - 600, Mullard
D103, AAZ - 15, Mullard
Zener diode
Z1, BZY93C9V1, Mullard
Z2, BZY93C9V1. Mullard
Z3, BZY93C9V1, Mullard
Z4, BZY93C9V1, Mullard
Z101, BZY88C3V3, Mullard
Fuse
FS1, 500mA, hbc cartridge, Belling Lee L693
Transformer
T1, 110 - 120V primary
T1, 200 - 220 - 240V primary
Part No.
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
4234 130
4234 140
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