HP Elite Desk Motherboard As Gaming PC

There are a few similar instructions on this site to this topic. In my opinion they are a bit too complicated. So I tried to figure something out on my own.

I had a HP EliteDesk 800 G4 Tower PC with a motherboard with an Intel I7 8th gen processor and wanted to integrate an Nvidia graphics card into this PC with the HP motherboard. The main issue was, that HP uses their own power supplys with their own connectors which had only 250W, which was not enough for the big GPU. The HP motherboard checks, if a HP supply is connected. If not, it will not turn on. To turn it on, you have to trick the motherboard, that the right supply is available.

It doesn't look good and is not well completed, but it is cheap and works.

HP EliteDesk 800 G4 Tower PC with the standard motherboard

GPU (in my case Nvidia GeForve RTX 3070)

ATX power supply with a 20 or 24 pin connector (in my case Tacens Anima APIII850, ATX 850W)

Some small wires

Solder iron + supplies

Any kind of resistor with 220kOhm

Small switch

Disconnect and get rid of the HP power supply. Find a way to install the new power supply. (Either it fits into the case or it has to be kept external). In my case the new supply fitted quite snuggly where the harddisk holder and the DVD-drive holder was (I got rid of the whole thing). The power cable for the new supply finds it way to the supply through the DVD-driver cutout (this plastic thing is removable, so there is a hole in the frontcover, which also acts as ventilation opening).

To create a mounting platform for the new supply I made a baseplate out of wood above the motherboard. I used the screw holes of the board to attach the wooden plate with an insulated crimp spade connector. I bent the insulated part as in the picture, so I got the spade which screws to the motherboard holes and the insulated part which acts as spacer and screw hole for some wood screws. I desgined two shelf pieces which hook on to the baseplate and are holding the power supply. I printed them on my 3d printer, the STL-file is attached. You can use the drawing in the PDF as reference for the plate but you will have to do a bit of filing to make it fit and align with the holes. The position is not the best but it holds really well in the upright an in the lie down position of the pc case.

To attach the front piece of the pc case where the DVD-drive holer was, I simply used some zip ties.

To connect the wires directly to the 24pin connector, I used some bigger wires with 1,5mm². It doesn't look good and doesn't hold that much force, but for me it works.

Attach an external switch to the big connector of the power supply (20 or 24 pin) to power it actually on. (The on/off switch on the supply only connects it to the grid and there is usually no output at the power connectors, when switched to on). Therefore you need the external switch, because the motherboard doesn't send a "switch on" signal to this supply.

Next connect GND of the 24pin connector with the HP Powercmd connector on the motherboard (usually a white plug with 7 pins which goes to the HP power supply). I connected both GNDs of the Powercmd connector. To make it more elegant, you can cut the plug of the HP power supply and solder the wires directly to it. I didn't do that, i solderd the wires to the motherboard connector.

The PWR ID pin has to be connected with a resistor to the new power supply so that the motherboard recognizes the new power supply. I figured out, that you have to use one of the 3,3V pins on the 24pin connector and a 220kOhm resistor in series with the Power ID pin on the motherboard. Otherwise it won't boot correctly.

The FAN Tach pin on the motherboard checks, if the fan of the original power supply runs. Because we removed this signal of the old supply, there will always be a error message when booting, which you have to confirm and which is annoying. In order to remove this problem, you have to use a speed reading signal from one of the other fans on the mainboard (ideally the main CPU fan). I connected the tacho-pin of the CPU main fan with FAN Tach pin on the motherboard Powercmd connector.

The last step is the connection of the power supply power delivery cables. For the motherboard you need 2 pieces 2x2 4pin connectors of the new supply which are basically 12v for the cpu and motherboard. For the GPU you need one or two pieces (depends on your GPU) of the 2x3 6pin connectors. Then you are done and can boot the pc.

I know that the cable management doesn't look good and the SSDs are falling around but I'm ok with that :)

Additionally it would probably be nice to cut a hole (or a few small holes) in the removable pc cover to let more air through the GPU-fans, because it is getting a bit warm in the case sometimes and the fans are working quite hard.