VeriFone 9V-4A power supply (model Au-7992n

Hello everyone, and I wish you good health.
Today I decided to write a short post. It’s mainly for fellow power supply enthusiasts like me (I’m no pro). This is about a used VeriFone power supply (9V-4A).
Today I’m focusing on model Au-7992n. There are two revisions; the only difference is the shielding gasket.
Revision "A" has a gasket only underneath the circuit board (under the insulation). Revision "B" has a gasket all the way around (in two halves). The board is first inserted into the insulation, then the gaskets are fitted.
Revision "A"


Revision "B"


I came across these power supplies by chance. I went into our little shop to place an order, and there were some huge boxes. I asked about them and was told they were used power supplies, complete with their cables and wires, salvaged from equipment.
I picked up a couple of them; they’re all from 2006–2008. The more compact VeriFone power supplies (around 2009 and later) use a different circuit design.
But at first I didn’t even know there were different models in the boxes. I picked up a couple of VeriFone 9V-4A power supplies (model Au-7992n)
. Specs from the case (main ones)
1. Input voltage: AC 100V-240V 50-60Hz
2. Output voltage: 9V, output current: 4A.
3. Size: 125 x 50 x 30 mm
The power supply has a standard 220V input connector (computer-style). I was really surprised when I opened them up: why were they different? One had the board under the cover, while the other had a metal plate under the cover. They seemed to be the same model, but I figured it out quickly.
As for the boards themselves, they’re a quality product, built the old-fashioned way. The filtering is comprehensive. They didn’t skimp anywhere.
All the filter components are large and generously rated (all the values are higher than in the common budget power supplies).
The input electrolytic capacitor is 100uF. These are the only power supplies where I’ve seen bulging electrolytic capacitors (the usual two, before the inductor).
The board looks great; no complaints at all.

At no load, the voltage is around 9.4V. But the cables on these power supplies need replacing (many are chewed up). Revision "B" also has a thicker cable. You can even tell this revision by the cable. My guess is that there was too much voltage drop (the original cables are long). They had two options: raise the voltage or use a thicker wire. They went with both: I have one of their power supplies from the boxes that’s marked 9.3V.
The usual setup: a round fuse (4A), thermistor, varistor, one small inductor, an X capacitor, and a large inductor.


In their newer power supplies, they started using 16AWG wire (excellent). All their power supplies use a 100uF/400V high-voltage capacitor (18/30mm). There’s a huge amount of headroom.
I usually replace all the electrolytic capacitors with Samwha RD, and make sure to replace the small ones too. There are 47uF/25V and 33 or 22 uF/50V capacitors. I leave only the original high-voltage capacitor in place.

Components: P10NK60ZF, SG6841SZ PWM controller, MBRF20100CT diode assembly. High-voltage capacitor: 400V/100uF (18/30mm). On the output: three 16V/1000uF capacitors (10mm) and a filter inductor.


Now for the question I was also interested in.
I checked the resistors around the TL431 and decided to modify a couple of power supplies for 12V. But it didn’t work out. They stopped working under any load, and squealed loudly with no load.
Modifying the power supply for 12V.
Usually, all you need to do is change the resistors around the TL431. The original resistors that set the voltage are 1% tolerance: (R32) 1002 (10 kΩ) and (R39) 3651 (3.65 kΩ). According to the TL431 calculator, that gives 9.35V. This power supply has both overvoltage and undervoltage protection on the output. Most inexpensive power supplies don’t have this protection.
Here’s what you need to do to modify the power supply for 12V.
1) Either remove the original Zener diode or replace it with an 18V or 20V one. An 18V Zener diode worked for me. I’ve marked it in the photo.



2) Change the resistor around the TL431. Solder an 8.2 kΩ (0805) SMD resistor on top of the original SMD resistor (R39, 3.65 kΩ). This gives an output voltage of 12.4V. I deliberately used an SMD (0805) resistor that was available locally (someone asked us for one).


I really liked this power supply, and there’s a good reason for that. All the electronic components for it are available locally. And it’s a pleasure to do a bit of soldering. If I just wanted a power supply, I’d get the VeriFone 9V-4A (model AU1370933g) instead; it has a synchronous rectifier. I tested it under a load of around 5A, and everything was fine. I’ll have to put them side by side on the bench, test them head-to-head, and get out the oscilloscope.
More photos.

Links to lots more photos.
VeriFone 9V-4A power supply (model Au-7992n) - CLICK.
VeriFone 9V-4A power supply (model AU1370933g)