Creating a 5v power OK signal for a Dell power supply - Page 1
Forum section: electronics / projects

author: l7777
author_rank: Newbie
timestamp: October 02, 2022, 05:32:38 pm
content: Context. I have a Dell Optiplex 7050 MT with a small 240 watt power supply. I plan to add a GeForce 1660 ti to it. A standard ATX power supply can be used to power the system with an adapter cable, I have this working now. The system appears to only need 12v power. 3.3V and 5V for the external drives come from a MB connector. The down side of course is that you have a full size ATX power supply hanging out somewhere as it does not fit in the case.Now the fun part. Dell makes a 500 watt power supply in the same form factor BUT for some reason it refuses to output the power OK signal when connected to machines older than the 7080 series. The motherboard connector is very simple with only six wires. Two +12v lines, two ground/common lines, a PS ON line, and the power OK line. I want to create a 5v power OK signal from the 12v lines and I'm looking at two options:Use a 7805 ICUse an appropriately sized Zener diode and resistor.What are the pros and cons of each option? Is there something else I should consider?

author: ledtester
author_rank: Super Contributor
timestamp: October 02, 2022, 06:36:08 pm
content: Is the purpose of your "5V power OK" signal the same as the ATX Power-Good signal?From the Wikipedia page:https://en.wikipedia.org/wiki/Power_good_signalQuote

author: l7777
author_rank: Newbie
timestamp: October 02, 2022, 08:17:37 pm
content: Yes. This signal is missing when the 500 watt power supply is connected to the older systems.It is theorized that the older systems only use one 4 pin CPU connector and the newer systems use two. The theory further hypothesizes that the power supply senses that the second four pin connector is not connected and as a result never sends the power-good signal causing the system not to boot even though power is available. The power supplies are made by lite on or Delta, I have a Delta model DPS-500AB-49 A.Seeing that the adapter cable for a standard ATX power supply doesn't do thing special I'm theorizing that if I can get a 5v signal to the power-good pin on the motherboard I could successfully use the newer and more powerful supply with the older system. It would be a hack for sure, but I'm not thinking it would be any more of a hack than a standard ATX power supply sitting outside the case. I was aware that this signal was supposed to wait until power had stabilized but hadn't gotten to figuring out how to properly, or even improperly, do that yet.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 02, 2022, 08:27:08 pm
content: basic resistor divider at 1-2K will do a signal presence

author: l7777
author_rank: Newbie
timestamp: October 02, 2022, 08:33:23 pm
content: Quote from: GigaJoe on October 02, 2022, 08:27:08 pmThank you. Can you expand on this? Maybe with what components are involved and how you came to a 1-2k resistance value? I'm still educating myself on such things.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 02, 2022, 09:23:05 pm
content: that kind of signal sense usually high impedance, means it need fraction of mA to feed, basically voltage potential. so you make voltage divider that will output 5v from 12v and don't dissipate heat too much. i may guess precise 5V also no necessary as 3-5V may work as well , you may start with 2 equal like 30K-50K and see if it will work as 30K resistor high enough to not burn something, and 6V over 30K resistor quite safe ...

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 03, 2022, 12:46:22 am
content: Quote from: l7777 on October 02, 2022, 08:17:37 pmCan you try to figure out the pinout of the second connector? Sounds like it's doing remote sense all the way to the motherboard.

author: l7777
author_rank: Newbie
timestamp: October 03, 2022, 01:32:28 pm
content: Quote from: NiHaoMike on October 03, 2022, 12:46:22 amSure, There are only 5 connectors.P1: 6 pin MB connector, 2x +12v, 2x G, PS-ON, PS-OKP2: 4 pin CPU connector, 2x +12v, 2x GP3: 8 pin GPU connextor, 3x +12v, 5x GP4: 4 pin CPU connector, 2x +12v, 2x G (This would be what I call the "second" connector, though I tried with only this one connected as well and got the same results.)P5: 6 pin GPU connector, 3x +12v, 3x GAnd some pictures of the innards. +12VBDC (Purple wires) go exclusively to the MB 6 pin connector. +12VADC and +12VCDC (Brown wires) go to the CPU and GPU connectors but I didn't trace the exact split yet.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 03, 2022, 02:37:38 pm
content: you have this for an auto power on motherboard, it could be applied for an power good signalthe power good signal has an specific delay, the timing must be calculatedhttps://github.com/pksato/pcautopwon

author: l7777
author_rank: Newbie
timestamp: October 03, 2022, 03:46:53 pm
content: Quote from: coromonadalix on October 03, 2022, 02:37:38 pmThank you, I was reading about that as well and looking at using a 555 timer to get the desired delay.Another thought, there appear to be ICs that actually perform the desired function. Would it make sense to simply implement something like aTPS3510Pthat would actually monitor the voltage and provide the signal?

author: l7777
author_rank: Newbie
timestamp: October 08, 2022, 03:48:51 pm
content: Update. The voltage divider worked perfectly. I didn't realize it at first, but it appears these power supplies output +12v any time they are connected to AC power even if the are not "on". While I almost never turn the system off unless changing hardware, it doesn't seem to care that the power ok/power good signal is present at all times.In the future I may try doing this with a true monitor circuit but for now I'll probably leave it as is. Thank you to all to replied.