Reanimating Old Server Cases with HP Common Slot Power Supplies

We at HOSTKEY have been using HP Common Slot power supplies from 460W to 1400W for years. They are efficient, reliable and easy to integrate with servers from different manufacturers. If you have a bunch of old server cases piled up, use them to save about €500 to €600 on new systems.

Standard Common Slot power supplies first appeared around 2010 in the Proliant G6 and most other HP servers at the time. These mass-produced devices are extremely inexpensive on eBay/AliExpress (€20 to €50 each). As a rule, the power supply units are operational: cases of failure when switching them on are extremely rare. There are no problems with bulk shipments either: we buy more than 100 at a time.

Common Slot power supplies are quiet: at low and medium load their noise is lost behind the noise of a normal computer. At full load, the small fan makes a characteristic loud squealing noise of 65 dB (however, it is not audible in a data center). The HP site contains little information on the specifications: there is no diagram or pinout available, but there is a suspiciously similar Murata Power Solution device from another vendor. On their website, we found the documentation we needed, including detailed manuals for 460W and 1600W power supplies. There are also some research results from enthusiasts available on the web, so HP Common Slot seems to be more than just our cup of tea.

What do the hackers say?

• The efficiency of the HP Common Slot at medium load is 92 - 94%.
• To turn it on, connect pins 36 (Power Supply Present Signal - short pin) and 37 (12V Standby Output) on the bus through a resistor of about 22 kohms to make the PSU think it is inserted in the slot, and connect pin 33 (Power Supply on/off control signal - far right) to the ground wire.
• Up to eight HP Common Slots can operate in parallel.
• No extra steps are required to run the PSU in parallel with the load: just run the main load and the Vsb lines separately (see table in Murata Power Solution documentation).
• The PSU has an I2C interface and an analog power indication output (60.15 mV/A).

How to use it

The easiest option is to buy a ready-made distribution board like this on AliExpress or another online store:

Or a slightly different one:

Note that the first board has a power button - it would be inconvenient to remove it from the server enclosure. The second board has a switch that you can turn on and leave alone. The price of the item is about €9.

If you don't like ready-made boards, you can make your own. You will need a Wingtat model 2.54 EDGE SLOT DIP 180° SINGLE LEAF TYPE WITHOUT EAR High Power S-64M-2.54-5 slots. The company responds quickly to inquiries and ships orders for 50+ connectors for about $2 apiece. They are available at retail on AliExpress. If you intend to use a full 1200W power supply, it is best to order the board with thicker copper. The connector on it should be soldered with a hair dryer and a thin solder with flux: this is the fastest way to do it; a regular soldering iron does not work well on such boards.

We use boards of this design:
The schematic is available in our repository on GitHub. At connector J1 you can measure the voltage and understand the current load on the power supply - 60.15 mV for every amp of power on the 12V bus.

A finished example:

What does it look like in a server?

We often use universal Supermicro enclosures and many of them no longer have the original power supplies, which have been discontinued or upgraded for new motherboards. Looking for native PSUs is impractical because they are expensive and just as worn out.

In this case, the native PSU had fallen into disrepair and was replaced with a universal 750W HP Common Slot. We cut off the excess and attached the unit to the case with double-sided adhesive tape. To convert 12V to the required ATX24 voltage we used native PicoPSUs or their Chinese analogs. The server in the photo is a Supermicro H11DSi with two AMD Epyc processors and about 400W power consumption. It absolutely refused to work with Chinese analogs, and the native PicoPSU didn't fit to the height and was plugged in via an adapter. The power is connected directly to the processors; it doesn't interfere with anything.

Back view. The power supply doesn't fit very well, but it doesn't affect speed and usability.

Why is this necessary?

We save about €500 to €600 on new systems. This allows us to keep the cost of renting dedicated servers at market level, plus we don't throw away old cases with all the fans and backplanes, so as not to unnecessarily pollute the environment.

Regarding reliability: The power supply usually fails the first time you turn it on. We have a special test rig with a category A 5A circuit breaker for this purpose. If the power supply turns on, it will continue to work for years. PSUs of this type in our data center with several thousand machines break down about once every six months, usually when upgrading the equipment. PicoPSUs in regular servers hardly ever fail.

Let's try using power supplies in pairs and build a test rig for this on our server.

Here the two power supplies with our adapter boards are connected in parallel. The boards are connected to each other with straight GPU extension cables. A pair of cables of 3 wires each provide a jumper for just 400 watts (at the limit). It is important that the connecting cables are of good quality: of the right cross-section and well crimped (we have just such cables). For greater reliability, 400W requires three cables with 3 wires each. The processors are each plugged into their own power supply.

Does this really work?

to be sure. We run Linpack on the server with both power supplies connected and take turns disconnecting them under load, then reconnecting them after a while. We ensure the voltage remains normal and the server does not reboot under stress test. The load according to the server is about 400W under stress test on the processors, so about 500W is flowing through the circuit.

As you can see, the setup works: The cables and connectors do not heat up, and everything works as it should and without the server's native power supplies. Since the HP Common Slots are designed to work in pairs, they have all the necessary safeguards.

Why use two PSUs?

Many people think that redundancy is needed for fault tolerance: if one power supply breaks down while operating a server in a data center, the second one will keep it running. This opinion is partly correct, but there are other nuances.

Often, when a power supply fails, a circuit breaker in the PDU or in the breaker box blows out (by short circuit or overload). If both units are plugged into the same PDU, redundancy will not help. Data centers are usually set up so that there are two non-overlapping power lines coming into the rack. They have different distribution cabinets, they go to different UPSs, and those UPSs are connected to different transformers and generators. This all gets switched with some delay, and the data center ensures that the power is always on one of the lines, but not both at the same time.

It is essential that the server have two power supplies, or you must use a rack ATS, a fairly expensive device with a solid-state relay that can quickly and seamlessly switch between asynchronous phase inputs.

Bottom line

If you have empty server cases lying around, don't rush to throw them away: old hardware is not difficult to restore. The solution described in this article will allow you to organize a setup with a guaranteed or redundant power supply quickly and without significant costs.

The HP Common Slot at HOSTKEY powers dedicated GPU servers, modular systems with ITX boards, and general purpose servers. This approach allows us to combine reliability of operation, ease of assembly and maintenance, and of course, our main goal: low prices for our customers.