When a power supply has DRM in it - Page 1
Forum section: electronics / repair

author: SeanB
author_rank: Super Contributor
timestamp: November 12, 2024, 04:23:31 pm
content: Went away for the weekend with friends, and there was a mains surge, associated with the lovely rain Friday night. Killed a phone charger and the PSU for a CPAP machine. Well the power supply, after a few minutes work with a hacksaw blade, is open, mains fuses are fine (surprise surprise, lovely input filtering, 2 stages of common mode filtering, and NTC, plus a LC noise filter, with 2 fuses, line and neutral, but no MOV at all, you would think they only ever tested with pure sine wave filtered mains) but input side PFC smells somewhat crispy. well, it is a 24v 3A supply, something you should be able to get right. Look at the 3 wires leading to the cable, and there is a red wire, marked ground, a white wire, marked +24V, and a third black wire marked ResMedBus, vanishing into the secondary side to a 8 pin device with no visible part numbers, and a few transistors and resistors.So Saturday morning go to the town, hard to find ant info, because typing the name brings you up bacon, the main factory in the town, or "adult service" places. hit the China mall, and there is a single universal laptop power supply, switchable from anything from 12V to 24V. Test equipment to hand is very limited, a 12v SLA battery, and a panel mount 3 digit DVM module, and some fishing kit. PSU grafted onto the cable, and then plug in and test. Screen powers on, followed by a error message that there is a lack of communication with the PSU. DRM via that black lead strikes.So a solution is needed, secondary side seems fine, so spliced the 3 wires back in, so the old PSU is now also connected, but no primary side, and power is once again applied. CPAP powers on, goes through the self test, and is happy with power supply now. so now it has a mid cable wart on it, to provide the DRM to make it work, with now an aftermarket supply, which cost $25 AUD, instead of the original, which is close to $200AUD, and a week to get. We were not going to drive for 4 hours to fetch the home power supply for it.Then got hit by a typhoon that night.....

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 12, 2024, 06:41:28 pm
content: Wow! Life sounds fun over there... I'm almost feeling good about boring daily life thanks to you.So you could pull the mystery device out and house it in the CPAP itself, solve the problem for good.I say this requires some bus sniffing to find out what's going on and how to keep the machine alive if the mystery device fails one day.Have you looked at device's pinout? It could be as simple as a 1 wire EEPROM that could be copied even without understanding the communications or something more complex relying on a small MCU.

author: amyk
author_rank: Super Contributor
timestamp: November 13, 2024, 12:02:50 am
content: Certain laptops have had this for a while:https://hackaday.com/2014/03/03/hacking-dell-laptop-charger-identification/

author: coromonadalix
author_rank: Super Contributor
timestamp: November 13, 2024, 05:23:27 pm
content: yep could be 1 wire devices sneakly hidden on the pcb, somewhereyes the Dell ones ... and some others ...i was able to buy china cloned ones, they never failed, and they had the good ID'S .... to be detected by the lappy and charge ...

author: tooki
author_rank: Super Contributor
timestamp: November 13, 2024, 05:56:05 pm
content: Given that medical devices are used on the body, and that poorly designed power supplies canand havekilled people, I can sorta understand a medical device manufacturer enforcing their power supplies on a device used at home. You don’t want someone running their CPAP machine on some random AC adapter that, even if it isn’t deadly, is of unknown quality and whose failure is potentially life-threatening.

author: SeanB
author_rank: Super Contributor
timestamp: November 14, 2024, 12:34:44 pm
content: Yes it is a mystery 8 pin microcontroller, with interface logic to allow it to have a 1 wire at 24V swing. Hard to fit in the case, as the power supply is around double the volume, and the secondary side of the PSU is a third of the board, so making it an in cable wart was the thing to do out there on the dam, with minimal tools, and a roll of insulation tape. done here because a replacement PSU would take a week to arrive, and at home there is another PSU, with UPS function, as power delivery from Eskom can be less than reliable at times. Next time we will take the entire battery back up though, so there is redundancy. Replacement is around $150 AUD, and a new unit is around $1000AUD, and you still need to have an appointment with the specialist to program it as well, which you will also pay for as well, along with a 2 weeks or more wait.