So everyone that has done the 330W power supply mod that I posted earlier has experienced the power supply shutting down at 240 watts of power draw. That is pretty counterproductive since the M17x ships with a 240W power supply. I did some reverse engineering and some load testing and figured out what the problem is.
I built a simple dynamic load from a few resistors, two op-amps, and an IGBT that I salvaged from an old motor drive. I attached the schematic at the bottom for anyone that wants to build a similar device. I didn’t have a small enough current sense shunt resistor to handle the current, so I used feedback from the gate-emitter voltage since it is roughly proportional to collector-emitter current after about 10 volts. I also used an MC34072 op-amp since it’s what I had laying around. It’s a bit crude but it works.
The dynamic load let me test the power supply and confirm that it was shutting down at 240W. It did, with the highest power I could get at the output being 19.6V @ 12.5A, roughly 245 watts. I also noticed quite a bit of buzzing.
It is pretty unlikely that Dell would make a power supply that badly, and it successfully powers the M18x so I took a closer look at the only thing that could have any effect on the power supply: the ID wire. When I figured out how to put the 240W 1-wire ID chip in place of the 330W ID chip, I found out that the M17x couldn’t drive the 1-wire bus. Something was loading it down farther down the line. Cutting the ID trace after the 1-wire PROM fixed the issue and allowed the M17x to drive the bus high and charge the PROM so it would work (the PROM is parasitically powered). The only thing that could have an effect was whatever was behind that trace.
I had to remove a bunch of the white thermal stuff and follow the trace, where I found it goes through a buffer and then a comparator. With the power supply powered up, I found the signal was pulled high to nearly the rail voltage. Grounding the trace that I cut drove the comparator output low. With this figured out, I tried a load test with the trace grounded. Since it was cut, the M17x could still interrogate the power supply and accept it, while the rest of the line could be held low.
Load testing the power supply with the signal grounded resulted in a maximum output of 19.5V @ 22.5A before shutting down. It was also nice and quiet. That’s roughly 440 watts. Pretty amazing for a little sealed brick, and 33% more output than rated.
Apparently the M18x interrogates the power supply, and when it sees it is a 330W supply pulls the ID line low to enable the higher power level. Since it is convention to leave a 1-wire bus high, any other computer is only going to get 240 watts. So ground the signal trace that you cut on the other side of the 1-wire PROM. Enjoy.
Here is the schematic for the dynamic load. Components are just examples, I was trying out CircuitLab and there wasn’t a huge choice of components. Any op-amp and IGBT or MOSFET should work, but you may have to adjust the resistor values.
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