Power Supply Service Sheet - Agilent Technologies 4284A Service Manual

4-8. A 1 POWER SUPPlY SERVICE SHEET
4-8-1 . CIRCUIT DESCRIPTION
The A1 Power Sup ply board provides +5 V, +8 V, ± 1 2 V, ± 1 5 V, -2 1 V, and ±48 V to the A 1 1
M other Board. The A 1 Power Supply board is divided into two sections at transformer A 1 T3 :
the primary circuit and the secondary circuit.
The primary circuit consists the followi ng.
1 . Primary rectifier
2. Turn-on surge current limiter
3. Slow start circuit
4. Supply voltage controller
5 . Switching circuit
6 . Sh utdown circuit
The secondary circuit consists the following.
1 . Secondary rectifier for each output voltage
2. Overvoltage detector
An explanation for each circuit follows.
[ Primary Rectifier ]
The primary rectifier, composed of A1 C R1 , A1 C2, A1 C3 , A1 C4, and A1 C5, rectifies the AC
source voltage to supply the unregulated DC voltages. A 1 C R 1 acts as a full wave rectifier
when the line voltage selector is set to 220/240 V, and as a voltage doubler when the line
voltage selector is set to 1 00/1 20 V.
[ Surge Current Limiter ]
The surge cu rrent limiter, composed of A1 R 1 , A1 K 1 , and A1 FT1 , limits the surge cu rrent
when the instrument is turned on. A1 K1 is activated by the slow start circuit about half a
second after the power switch is turned on to by-pass A 1 R 1 (Surge current l imit resistor),
which protects the primary rectifier from current surges at power up.
activate, the heat produced by A1 R 1 will cause thermal fuse A1 FT1 to o pen u p .
4-A 1 - 1
If A1 K1 does not