A L Power Supply Service Sheet 4 - A - Agilent Technologies 4284A Service Manual

4-8. A l POWER SUPPIY SERVICE SHEET
4-8-1. CIRCUIT DESCRIPTION
The Al Power Supply board provides +5 V, +8 V, ±12 V, ±15 V, -21 V, and ±48 V to the A l l
Mother Board. The Al Power Supply board is divided into two sections at transformer Al T3:
the primary circuit and the secondary circuit.
The primary circuit consists the following.
1. Primary rectifier
2. Turn-on surge current limiter
3. Slow start circuit
4. Supply voltage controller
5. Switching circuit
6. Shutdown 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 Al CR1, A l C2, A l C3, A l C4, and A l C5, rectifies the AC
source voltage to supply the unregulated DC voltages. A l CR1 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 100/120 V.
[ Surge Current Limiter ]
The surge current limiter, composed of A l R1, A l K1, and A l FT1, limits the surge current
when the instrument is turned on. A l K1 is activated by the slow start circuit about half a
second after the power switch is turned on to by-pass A l R1 (Surge current limit resistor),
which protects the primary rectifier from current surges at power up. I f A l K1 does not
activate, the heat produced by Al R1 will cause thermal fuse Al FTI to open up.
4-A1-1
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