Service manual

PN 4237242C
2.2-6
INSTRUMENT DESCRIPTION
POWER SUPPLY
There are four plugs available to select the appropriate range. Though units are sold for a
specific power range, the other three plugs are available if the power does not match with the
plug being used.
+24 Volt Switching Power Supply
The +24 V power supply (Figure 2.2-2) is a switching power supply with a minimum
frequency of 100 kHz and a maximum rated output of 6 A at 24 Vdc. It senses and
automatically adjusts for 120 or 220 Vac input. Internal protection includes a 6.3-A fuse for
ac input, output voltage clamped at 30% overvoltage, and automatic shutdown for an
overtemperature condition.
The POWERFAIL (PF/PG) signal at J1 is monitored by the MD II. This is a TTL-compatible
signal referenced to the negative sense line at J1. A logical low indicates a fault condition and
is generated by a thermal shutdown or an ac failure longer than 38 ms.
The +24 V power supply is purchased as a unit. If it has problems, it should be replaced as a
unit, not repaired. Schematics and component parts are not available.
Inputs
r
TB1 - 90 - 264 Vac, 47 - 63 Hz
r
J1 - ON/OFF signal to turn supply ON
Outputs
r
TB2 - +24 Vdc
r
J1 - POWERFAIL (PF/PG) signal, internal regulator sense lines
Adjustments
V1 - This is set at the factory. Do not adjust.
Linear Power Supply Card
The Linear Power Supply card (see Figure 2.2-2 for location) provides voltages not supplied
by the AT power supply or +24 V power supply. Three ac voltages are supplied to the Linear
Power Supply card: 18.5, -18.5 and 165 Vrms (voltage root mean square). The white/orange
wires connected to J1-2 and J1-5 supply 18.5 Vrms. This is applied to bridge rectifier VR2,
producing an unregulated +25 Vdc. A regulator using an LM7815CK (U5) regulator circuit
produces the +15 V output from the card. This +15 V is also used onboard by regulator circuit
U4 to produce a 2.5-V supply that provides a constant current source for the Hgb LED.
Table 2.2-2 AC Power/Vacuum Relay Card Connectors and Line Input Ranges
Connector Line Input Range
100 VOLTS 90 - 110 Vac
120 VOLTS 110 - 132 Vac
220 VOLTS 198 - 242 Vac
240 VOLTS 220 - 264 Vac
C