Hi all!
I have a bunch of Dell 90W power supply’s, and bunch of HP computers. The thing is - HP has it’s own signal line to confirm that a proper OEM supply is being used. I found this sketchy video of how to hack a regular 19.5v supply to work with HP machines. The thing is, I don’t feel fully confident in my electronics know-how to risk something toasting… what do you think about this video, would it hurt to try it out - or would I risk anything with adding a resistor like that?
I have no idea if it’d work or not, but that’s a pretty straightforward and safe modification.
Personally, I probably wouldn’t try to open the case of the charger at all. I’d just cut the wires and reconnect them with crimps or solder + heatshrink, adding the resistor at that point. Wrap the whole thing and you’re away.
There appear to be options from AliExpress for a few $ so perhaps try that one first?
That was my plan. I did look at AliX for adapters/modifications to make it work with HP, but I was unlucky with my search… what keywords are you using?
I think I just googled “Dell to HP Charger Converter” because it was an autocomplete option and it gave me a shopping link.
I don’t know for sure that they’ll have the resistor added as mentioned or the right size plugs/sockets, but it seems like a reasonable place to start.
Ah - yeah, I see what you mean. The barrel jack is the same size, so I don’t need a converter for the plug itself. I just need to trick the HP machine into believing the power supply is OEM, which is done by sending some signals over the signal line (center “spike” in the connector).
Yeah, my presumption is that the adapter may have the resistor added.
If you’ve got a few of them, it’s not likely to hurt anything to try it out. I’d just be careful to check it with a Multimeter to make sure you end up with +V, 0V and the resistor where you expect.
Find a reliable source that shows how it should be connected, with the charger unplugged use the resistance range to check the resistor is correctly positioned (between the pin and +V connection, it seems), then set the multimeter to volts range and check between +V and 0V with the charger powered on.
Most of this stuff is reasonably robust so I wouldn’t personally expect any issues.
Thanks a lot mate. Yeah, I’d have to double check with a multimeter that I’ve got things the right way.
Those adapters that AliExpress are selling does actually seem very useful! Converting old laptop chargers to 100W USB C power supplies is a great case of use. Can’t have too many of those. One beefy charger in each room 
(if I was too troubled, I could get a barrel jack to USB C adapter, and then back to barrel jack again - but I’ll try the other option first)
Yeah, that’s an interesting possibility. I typically avoid anything cheap and questionable when it comes to power supply due to the potential for fire/shock hazards. This would be an interesting mid-point because most of that risk comes from the AC/DC stage. A 19V in to USB-PD out converter is likely to be a lot more benign. Maybe a slight risk of fire/melting, but much less likely.
As one of these adapters on AliX was described: “Doesn’t get hot” 
Hahahaha, yeah, well at least someone else is concerned.
My issue is often more about trusting that there is enough consistent QC that all the products come out the same and perform the same.
I finally found a 220kohm resistor! And the HP machine did actually respond. I cut the signal line before the resistor, to leave out anything coming from the DELL brick.
Usually, when I use the DELL supply(with its own signal) or generic 19.5v supply without any signals - nothing happens.
So - having the HP respond is great. BUT, it won’t start, it gives away a series of beeps. (3 long and 4 short). The manual tells me that it’s a “The system board displays a power failure (crowbar).*” I guess this HP machine is not as easy to “hack” as the one aforementioned in the video.
Next up I’ll get a 470kohm potentiometer and experiment if other values might work out.
That’s unfortunate. It could be that the resistance is signalling a maximum current draw from the adapter and 220k is telling the laptop it can draw more from the charger than it can supply, causing it to immediately brown-out and flag a fault.
Depending on the age, they could also have shifted to some kind of basic digital handshake type scenario. You might need to source an HP supply that works and start probing what’s on that line to know for sure.
I’m not very fluent in electronics, but I’ll try to answer as good as I can. The power adapter is 90W, and the Elitedesk G5 800 mini is supposed to suffice with 65W. I don’t think the power draw on the signal line is significant at all, and the resistor is capable of 0.25W I think.
My hunch is that there’s a handshake, a PWM of some sorts. I have to bring the one HP supply I’ve got to my father in law and borrow his scope I came across a hack where someone used the signal from a HP supply and coupled it with ground and a signal line on a generic supply. But that kinda beats the purpose of doing it without the original HP supply in the first place!
(I have a stack of one-liter HP machines, but the power supplies I got are all DELL (except for one), so I have to figure a way to use them on their own! I’m planning to set up a machine for running the CNC in the shed, a Proxmox server for Home Assistant and other services, and one machine for a opensense router/firewall)
Ah, right. I’d presumed these were for laptops which would potentially have much higher draw for battery charging. If your resistor was saying ‘I’m a 120W supply’ then a laptop could be trying to draw 30W to keep itself running and an extra 90W to charge the battery.
In that case, doesn’t sound like that’s an issue.
It could definitely be a PWM doing a similar job, that’s likely. If that’s the case, it could be recreated quite simply. A scope would show that pretty quickly.
If it’s some kind of comms, that gets a bit trickier and starts either needing broken HP supplies to harvest from or someone else to have undertaken a reasonably significant reverse engineering effort, unfortunately.
It works!! My father-in-law gave me 220ohm, not 220 KILO ohm… using a 470k potmeter, I easily found the right resistance - and voila!
I have always found the multimeter is much better at reading the resistor codes than I am. Just one thing I learned in college: You have to remove the resistor from the circuit before you probe it.
Wow, I guess I would never have known that!
I’m partially color blind, so using the multimeter is an obvious approach. BUT - I’ve found out that my multimeter is not optimal for this use case! Looking for resistors within a specific narrow range is a hassle when they are not labelled and I have to change the measuring range for each try.
Soooo, now I guess I have to find myself a newer and better multimeter with auto range I’m happy to have the cabin, a great excuse for having two of each. It’s also useful with a multimeter that can detect current externally.
The fun part is that you don’t technically need to in a lot of cases, especially if you’re looking to just confirm the correct value is fitted, but knowing what those situations are can be pretty tricky and even depends somewhat on the multi-meter you’re using. It’s kind of a ‘when you understand the rules well enough, you can break them’ kinda scenario.
Good find, and good on you for checking with the pot. That’s not a failure mode I had anticipated at all
Me neither! Especially since my father-in-law is the electric engineer, and I’m the humanities guy! 
