eBay Chinese Chassis Power Supply S-400-12 400W 12v 33A

S-400-12 PSU
S-400-12 PSU

Here’s a cheap PSU from the treasure trove of junk that is eBay, rated at a rather beefy 400W of output at 12v – 33A! These industrial-type PSUs from name brands like TDK-Lambda or Puls are usually rather expensive, so I was interested to find out how much of a punishment these cheap Chinese versions will take before grenading. In my case this PSU is to be pushed into float charging a large lead acid battery bank, which when in a discharged state will try to pull as many amps from the charger as can be provided.

Rating Label
Rating Label

These PSUs are universal input, voltage adjustable by a switch on the other side of the PSU, below. The output voltage is also trimmable from the factory, an important thing for battery charging, as the output voltage needs to be sustained at 13.8v rather than the flat 12v from the factory.

Input Voltage Selector
Input Voltage Selector
Main Terminal Block
Main Terminal Block

Mains connections & the low voltage outputs are on beefy screw terminals. The output voltage adjustment potentiometer & output indicator LED are on the left side.

Cooling Fan
Cooling Fan

The cooling fan for the unit, which pulls air through the casing instead of blowing into the casing is a cheap sleeve bearing 60mm fan. No surprises here. I’ll probably replace this with a high-quality ball-bearing fan, to save the PSU from inevitable fan failure & overheating.

PCB Bottom
PCB Bottom

The PCB tracks are generously laid out on the high current output side, but there are some primary/secondary clearance issues in a couple of places. Lindsay Wilson over at Imajeenyus.com did a pretty thorough work-up on the fineries of these PSUs, so I’ll leave most of the in-depth stuff via a linky. There’s also a modification of this PSU for a wider voltage range, which I haven’t done in this case as the existing adjustment is plenty wide enough for battery charging duty.

Bare PCB
Bare PCB

The PCB is laid out in the usual fashion for these PSUs, with the power path taking a U-route across the board. Mains input is lower left, with some filtering. Main diode bridge in the centre, with the voltage selection switch & then the main filter caps. Power is then switched into the transformer by the pair of large transistors on the right before being rectified & smoothed on the top left.

Main Switching Transistors
Main Switching Transistors

The pair of main switching devices are mounted to the casing with thermal compound & an insulating pad. To bridge the gap there’s a chunk of aluminium which also provides some extra heatsinking.

SMPS Drive IC & Base Drive Transformer
SMPS Drive IC & Base Drive Transformer

The PSU is controlled by a jelly-bean TL494 PWM controller IC. No active PFC in this cheap supply so the power factor is going to be very poor indeed.

Input Protection
Input Protection

Input protection & filtering is rather simple with the usual fuse, MOV filter capacitor & common mode choke.

Main Output Rectifiers
Main Output Rectifiers

Beefy 30A dual diodes on the DC output side, mounted in the same fashion as the main switching transistors.

Output Current Shunt
Output Current Shunt

Current measurement is done by these large wire links in the current path, selectable for different models with different output ratings.

Hot Glue Support
Hot Glue Support

The output capacitors were just floating around in the breeze, with one of them already having broken the solder joints in shipping! After reflowing the pads on all the capacitors some hot glue as flowed around them to stop any further movement.

This supply has now been in service for a couple of weeks at a constant 50% load, with the occasional hammering to recharge the battery bank after a power failure. at 13A the supply barely even gets warm, while at a load high enough to make 40A rated cable get uncomfortably warm (I didn’t manage to get a current reading, as my instruments don’t currently go high enough), the PSU was hot in the power semiconductor areas, but seemed to cope at full load perfectly well.

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