Sometimes you can find a good deal on used power supply from various vendors. In my case it was Cisco C3K-PWR-750WAC and RPS 2300. The problem is, that they don’t output any voltage until they are plugged into device. The question is, how to fool them into thinking they are plugged in.
The spec for the supply looks good:
And the interface is following:
You can see, there are some huge terminals which will obviously be power output and then 16 2.54mm pins where the detection will most likely happen. I was thinking how to find out which pins are responsible for the detection and I came up with following idea:
I connected all pins with the RPS and started disconnecting one by one and testing if the supply starts. This way I eliminated all wires, except these two:
Then I thought, well, maybe they just use some fixed resistor to detect the connected load so I measured the resistance and it was 7.5kOhm in both directions, so I connected corresponding resistor to pins 6 and 15 (numbering left to right, top to bottom)
and after connecting the power cord…
Ta-da! Power supply started nicely delivering about 60 Watts, job’s done.
It has quite high reactive power consumption but I guess that is the price for the power capability:
This is the power with 60W car high beam bulb connected:
Activating the RPS
RPS doesn’t output the power by default either but I thought this will be the same story. The main difference is though, that for the RPS we have the pinout.
I don’t have the interconnect cable so I had to improvise a bit and I connected them with jumper wires with the plastic cap removed. I connected a few seemingly important pins but it was not starting. Then I realized, that the documentation shows the connector on a cable, not on RPS so pin 1 is on the bottom right (on my photos). I quickly rewired it and found a working configuration:
Next I measured the resistance and it was about 10kOhm. So I connected a resistor:
and that was it 🙂 .
Bonus: Powering C2960X from an external supply
During my experiments, I also found out that all you need to power a switch is to provide the main 12V supply. It will happily work even without PoE voltage input or any other data pins
Happy hacking!