[Equipment] Looking at S8 circuit board

[justMongo]

Hi Mongo,
I connected the S8 to 12 volts to try it out because it has a normally 12 volt plug pictured on the Resmed adapter. I learned later that the center pin is negative unlike most automotive accessories. May I ask where or how you determined that the S8 DC input is 24 volts?
Thanks

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Good Morning
Thanks for the reminder, I guess I forgot my basics.
But now I see another way to post a picture of the board in question, because of your reply.
Thanks for your effort.

I'd like to see the component location your referring to.
An S9 runs off 24 VDC for both the blower unit and the humidifier.

The S8 uses the AC voltage to heat the humidifier. It has an SCR that is pulse modulated to control the heater plate.

The designer may have relied on the plug to protect against reverse polarity.
I would have protected it with a pair of Schottky diodes -- low forward drop.

[Sumwhatinterested]

With the AC cord plugged in, I do get a 12vdc signal coming from the power pins of the supply. This is coming through the terminals at the main board.

As I look at my bottom label, I recall now why I used 12v, it's because it does take 12v! There must be a few different versions of the S8? I would like to post images, however, I have not reached that magic 8 post cap limit.

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This is a ResMed AutoSet Vantage BTW.

[trek4ward]

Hi everyone, I know this is an old post but here's the solution if anyone needs it. I did the same thing. We had a cyclone come through our area and we lost power for 9 days. I figured I would run my machine on DC since I was not sleeping so well and it has an input for it.
Here is the stupid part: Res Med have reversed the polarity for the 12V DC input compared to the standard for just about everything else on the planet. I fail to understand why!!! Anyway, if you make up your own lead with centre pin positive you will damage your unit. That was me.
I am lost by the reference to 24v in this thread. The Res Med S8 is a 12v unit all the way through. No 24v at all.
So, I damaged my unit by hooking up to 12v with the centre pin positive. That killed it and after that it did not work at all. First I tested the power supply. All good and producing a perfect 12v DC to the main PCB. So now it was time to tackle the top PCB. Although it looks rather daunting, the fix is very easy.
There is a white ceramic fuse situated between where the six 12v DC pins engage with the PCB and the back lit screen. The position label is F1A (if I remember correctly). This is a 5amp fuse and will have been blown by the reversed polarity. If you place a pair of pointy nose pliers across the two ends of this fuse, with the unit plugged into power, it should light up and come to life. If it does, you have confirmed your problem.
I left the current fuse in place and soldered and replacement over top and a bit off to the side so as to not get in the road of the buttons. My machine now works again on both 240v AC and 12v DC as it should.
Hope that helps someone. I reckon a few people will have made this mistake.
Cheers, Craig

[justMongo]

S8 VPAPs are 24 Volt. Perhaps because they can put out a max pressure of 25 cm-water. Or, perhaps because the wider possible split between EPAP and IPAP requires faster response from the multi-stator electronically controlled motor. (Higher Voltage overcomes inductive reactance resulting in faster current change in the coils; and the H field is proportional to the current.)

Bottom of S8 VPAP S. Note Voltage lower left.

[trek4ward]

That is interesting. I understand Res Med is an Australian company but your label has USA only on the label.
As mentioned by Sumwhatinterested, there must be variations in different countries.
My S8 is 12v and this is what the post was relating to. Interestingly, the humidifier on our models only run on AC. Thus we can only use our CPAP on a car battery or for camping without a humidifier unless using an inverter.
Anyway, thanks for your comments but this is definitely the fix for a CPAP that has an option for a 12v input. These S8's are 100% 12v internally.
Cheers, Craig

[OMyMyOHellYes]

I don't know, but I'd bet that the CPAP/APAP S-8s outnumber the VPAP by a wide, wide margin. Drawing from that, S-8s, in the generic sense are 12 V .... S-8 VPAPs OHTH are 24 V. Go figure. Engineers at Respironics are smart enough to be able to make do with 12 V ....

As to the coaxial plug's polarity being reversed from the standard, I suspect it has more to do with selling proprietary cords/plugs to folks at exceptionally inflated prices and even new machines to others that wired the plug up in the conventional manner.... That there be one reason that I usually hold the ResMed company in low regard.

OMMOHY