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Thin Clients: HP PSUs 


 


 

 



HP Power Supplies

PSU label The more modern HP Thin Clients such as the t520 and t620 use the same 19V power supplies as used by models from the their range of laptops. The coaxial connectors on these power supplies actually have three connections:

  1. The outer barrel - (earth)
  2. The inner barrel - (+19V)
  3. A centre pin - ('signalling')

People have reported that, where they have attempted to use non-HP PSUs with their HP t520s or t620s, the Thin Client has not powered on.

Some Background

I first met this style of connector many years ago with a DELL laptop where suddenly the laptop would no longer charge the battery but would still run quite happily off the PSU. It transpired that the centre pin carried a simple 1-wire serial protocol that let the PSU signal to the laptop the power rating of the PSU. The laptop then adjusted the battery charge rate to suit the capability of the charger (eg 65W or 90W). It was this signalling function that had failed in the charger, and no signalling from the PSU equalled no charging of the laptop battery.

I guess the default of 'no charging' was a marketing decision to persuade customers to buy/use genuine DELL chargers. The engineering approach would have been to fall back to a safe slow charge rate.

Non HP PSU

It looks like the HP power supplies use a similar approach but use a voltage level and/or resistance on the centre pin as its signalling method. No 'signalling' on the centre pin, no power up. My first question is "Why?". I can understand the laptop scenario described above, but on a mains powered box such as a Thin Client? Anyway, ignoring the 'why', here's the solution:

In January 2020 I heard from Jeroen who had tried to use a DELL power supply with his t520.

"I bought the device without a power adapter. Unfortunately, the HP didn't accept a Dell adapter. But after some research, I found out that soldering a 240k resistor (I only had 480k, so 2 in parallel) between the center pin and the 19 volts pin does the job."

It is a similar situation with the t620 where I've seen a recommendation for a 330K resistor between the centre pin and +19V.

More Info

For the curious techie: http://nerdipedia.com/wiki/tiki-index.php?page=Smart+laptop+charger may (or may not) help you.

 


Any comments? email me. Added April 2020