The Power General FLU3-40-1 Power Supply
It uses a Unitrode UC3844 switch mode controller controlling a MOSFET switch device. Feedback loop is optically isolated, and is attached to the 5V output circuitry via a R-pot-R divider that's fed into a TL431 reference. There is a datasheet from Power General.
Pinout, looking at top side, left to right, plastic tab to 'bottom' of pins:
AC connector: Line, Neutral (Line is connected to a slo-blo fuse
Flipping around, DC connector:
| Pin | Volts | Max Amperes, notes |
|---|---|---|
| 1 | +12V | 1.5A cont, 3A max, has 80Ω ballast |
| 2 | +5V | 3A cont, 5A max Feedback control tapped from this winding. |
| 3 | ||
| 4 | GND | 40W max total! Do not draw 25W from +5 and 36W from +12 at the same time! |
| 5 | ||
| 6 | -12V | 0.7A max, linear regulation (on own isolated winding, using 7812 with output conected to GND, GND connected to -12V output, not a 7912) |
It has a measured efficiency about 70% at 120VAC inlet, suspecting similar with 220V. It uses a 100KΩ bootstrap (R5), dissipating 160V (filtered rectified RMS) - 11V (approximate running voltage) = 149V, P=V^2/R = 149V^2/100KΩ = 0.22W. At 220V, almost 1W is dissipated.
Other big resistors: 3.9KΩ (R6) in series across flyback snubber
diode, unsure of real dissipation here. I dared not to touch that high voltage
to see how warm it gets...
80Ω resistor from the 12V output to the 5V rail: (12V-5V)^2/80Ω = 0.6W
So in total, without the 5W resistor, least 2W wasted here... that's not including inductor and switch losses.
This specimen was made in the early 1990s. It seems fairly reliable. So far the key failure I've seen is the switch control/startup filter capacitor failing, causing the power supply to take a long time (a minute?), oscillate on and off when cold, or never turn on. Replacing this capacitor restores operation back to normal. I did this repair to two of these power supplies, and a third one that also used a similar design. This particular supply uses a 39µF, 35V capacitor to filter. Using a larger capacitor will slow down turn on time. I used a 47µF capacitor as I didn't have any 39µF capacitors on hand and did not notice much change in behavior. This needs to be a low ESR type (less than 200mΩ) - I see 1Ω ESR capacitors fail to work properly, as it is subject to high frequency. (This capacitor that seems to frequently fail is C24 - the black capacitor marked 105°C next to the off white 6-pin optoisolator. Hint: it's different color than the rest of the capacitors for a reason...) I've also seen one of these capacitors blow its guts all over the board, so watch out, it is a cap that fails frequently.
The unit I have had all Nichicon or Nippon Chemicon capacitors,
including the capacitor that failed.
Now has a no-name to take place of the defective capacitor.
Also though the screws in the heat
sinks appear to be threaded and have hex key bodies, they seem to actually be
rivets not screws... Rotating them did not free the semiconductors from the
heat sinks. After trying with some other hex keys, it appears that
a 2mm METRIC hex key was able to free these screws. The SAE hex keys do not
work.
Table of devices
Since people probably searched for this PSU are most likely doing so to
repair this device, here's a list of devices on the board as they
are requested:
| Device | Value | Notes |
|---|---|---|
| C6 | 0.022µF | Polypropylene |
| C15 | 0.15µF | Monolithic Ceramic |
| C24 | 39µF | positions for three capacitors in parallel on board, This device fails often |
| R1 | 820K | ½W |
| R5 | 100K | 2W - This is the bootstrap resistor |
| R6 | 3.9K | 5W |
| R19 | 80Ω | 2W - This is the anti-overshoot resistor |
| R33 | 620K | ¼W |
| R34 | 620K | ¼W, identifier underneath |
| Q1 | IRF830 | Enhancement MOSFET - this is the flyback switch |