Question HP "Smart Pin" PSU. Is there a voltage/resistance chart?

Nov 25, 2020
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I recently got myself a HP Thin Client which I want to use for OpenWrt and bridge it to my ISP modem. I actually have this exact setup running right now, only the PC being used for the router is just overkill for the job, hence the thin client. Tbh, the thin client is also seriously more powerful than it needs to be for a home router but meh. Anyway, the thin client uses a weird power connector (it came without a PSU) which I later found was a "Smart Pin" connector used mainly on laptops. It has an outer ring and a central pin. So I ordered a cable with this connector on the end from e-bay, the outside of the outer ring is ground, the inside of the outering is +19.5v and the centre pin is the "Smart Pin". I have tonnes of laptop power supplies so I figured I'll just modify one with this cable. If I just connect +ve and -ve and leave the smart pin unconnected, the thin client doesn't power on. I tried doing some Googling and found a bunch of conflicting information ranging from 1-Wire ID to transistors/mosfets and the like. But the one that made sense was that the centre pin was just the middle of a resistor divider. So, I tested between ground and the smart pin on the thin client. It reads 73K ohms steady. So I'm thinking it's probably a low precision 75K resistor in the thin client. I'm assuming that different power supplies have different resistors between +ve and the smart pin, when plugged in the resistor in the PSU and the resistor in the device (laptop/thin client, whatever) form a resistor divider between +ve and ground, the device is then able to read the voltage at the middle of the divider to ascertain the PSU's current capacity.

That being the case, which I'm 99% sure it is as putting a 100K resistor between +ve and the smart pin allows the thin client to start up. But, it would be nice to figure out the correct resistor value I need for the PSU's current/power. It's a 19.5V 6.7A 130W PSU. I tried Googling for some kind of chart that lists the voltages at the smart pin. It may not be as simple as that though, as some devices apparently have different resistors to ground, so the voltage at the smart pin would be different for the same PSU, in which case the chart would need to list the resistor values required (as the device would know the voltages)

Cheers
Bolt
 
Nov 25, 2020
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Where R1 is the input voltage and R2 is connected to ground, Vout = Vin * (R2/(R1+R2))

You can set up a spreadsheet and play to your heart's desire. That's why you can't find a "chart"
That isn't what I asked. I know the formula for a resistor divider. What I was asking is for a chart showing what voltages a HP device is expecting to see on the smart pin so it can determine the PSU's power so I can calculate the correct resistor for the PSU I'm using.

For example, if 5V on the smart pin means a 65W psu then the resistor to use on a 65W PSU would be
5 = 19.5 * (75,000/(R1+75,000))
Then solve for R1
((19.5*75,000)/5)-75,000 = R1 = 217.5K (220K nearest common resistor)
My issue is that I don't know what voltages are interpreted by the device to determine the PSU's power. THAT'S what I need the "chart" for.
 
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