80486 PC: Power Supply

Power Supply

This particular PC power supply is rated 230W: 5V@23A, 12V@9A, -5V@0.5A and -12V@0.5A.  There is no 3.3V output.

It takes 120V AC (with a switch for 220VAC).  The AC color code is usually black for line, white for neutral and green for ground.  The earth ground is tied to the chassis and is tied to the circuit ground.  In this power supply, the line and the neutral are brought to a switch at the front panel.  The switch connects them to a connector on the board (brown and blue wires) and to an AC socket to optionally power a monitor, a feature that disappeared later.  It does not have a way to turn the power off electronically by software. The line is fused (5A/250V), but the fuse is soldered to the board, so not easily replaceable.  A thermistor is used to limit the inrush current; it starts at 5 Ohms and drops to 0.2 Ohm at the full load of 3A. The AC input is filtered by capacitors and a common-mode choke.  Then it is rectified by a diode bridge and filtered by two large capacitors (330uF 200V).   These two capacitors are connected in series; depending on the 120/220V switch, they are either charged in series with the full wave rectifier or individually charged by half wave rectifier (a voltage doubler).  This is how both 120V and 220V are accommodated.   The maximum voltage is 2*120*sqrt(2)=340V.  If the switch is closed (intended for 120V) but 220V is applied, the voltage would exceed the capacitors' rating.  I wonder if any power supply is destroyed this way.

Now we have a high voltage DC to work with.  The switching regulator is in the half-bridge configuration.  Two power NPN 2SC3040 transistors (400V/8A) mounted on a heat sink drive a transformer primary.   The secondary is diode rectified and filtered; the diodes are mounted on a heat sink.  (For some reason, the heat sink is tied to the cathode of the 12V rectifier diode.)  The circuit board is one-side; all components are on one side and all are through-hole.  The only IC DBL494 is the pulse width modulation controller and is resided on the secondary.  The base drive comes through a transformer.  The frequency is about 32KHz.  DBL494 has a 5V reference.  The feedback is on 5V; the other voltages are not regulated and do vary with the load on 5V.   When the 5V is unloaded, the 12V is only 10V.  When 5A is drawn from the 5V, the 12V is 12V.  DBL494 is powered from the 12V.  An interesting question is, how does the secondary bootstrap itself?

The fan is often the source of irritating noise.  The size of the fan is 3" x 3".  The fan (12V/0.14A) is powered from the 12V.  The fan is brushless and an outrunner (the magnet is out side); a Hall switch inside the fan does the communication.

The motherboard power connector is as follows, (note the color code)

Pin	Signal	Description
	P8.1	Power Good
	P8.2	+5 V
	P8.3	+12 V
	P8.4	−12 V
	P8.5	Ground
	P8.6	Ground
	P9.1	Ground
	P9.2	Ground
	P9.3	−5 V
	P9.4	+5 V
	P9.5	+5 V
	P9.6	+5 V

There are other power connectors for floppy drive and hard drive.

[I accidentally shorted something and the fuse was blown.  The two power BJT were destroyed.  Not sure how it happened.   I might have touched the heat sink to the chassis ground.  The fuse is not there to protect the components.]