Homebuilt '12V Camping Converter PSU' for Resmed Airsense 10

[TheSnoringMan]

This one is built and tested; it seems to work great. However, it's not cheap, and it's quite big and ugly. If you love building electrical gadgets, then feel free to make a copy! But if you don't like playing with electronic parts - don't bother, just spend the $85 and get the genuine article.

The main reason for using a DC-->DC converter, in comparison to using your at-home AC-->DC converter with a 12VDC -->120VAC "Inverter" in front is efficiency: converting DC to AC wastes a fairly large amount of power (and maybe needs a noisy fan, to dissapate the "waste heat" being generated in the inverter). Turning around and converting 120VAC into Resmed "DC-Out" is also less efficient than the DC-DC conversion done by my box, or the $85 Resmed original. This saves on battery power overnight, and may also eliminate fan noise from the Inverter.

The "strange thing" about ResMed Power supplies is the fact that they have 3 output wires: "+"24VDC, "-" Ground, plus a sort-of-proprietary +3.3VDC "sense pin" in the middle. The +3.3VDC sense pin needs to have an inline resistor , size 2.7k Ohms, between the +3.3VDC power section and the output cord connector. (They did file a patent concerning their clever use of of the "sense pin" within the ResMed machines, which was designed to determine the +24VDC output power of the corresponding PSU.) 2.7k Ohms corresponds to their own "90 Watt" PSU models.

Part #1: A sacrificial AC-DC power supply for Resmed A10. (Refurbs run about $18 on Ebay.) When it arrives, crack it open and cut the Resmed output cord wires (3) from the Power Supply mainboard, preserving the strain relief and as much wire length as possible. Why? Because the Resmed cable includes a couple of large chokes, which help to reduce electronic "noise". (Dell Laptop power supplies from the "PA-12" Series can be bought for a bit less money, and they have the same connector - but the Dell cords include only one such choke, and it's a lot smaller.)

Part #2: A DC-DC "Boost" Converter, creating regulated 24V output from 11-15V input. I used and recommend this one, which costs $18 as I write this: http://www.ebay.com/itm/Waterproof-DC-1 ... 2574605057. Why? Because your "12V" power sources might be providing power from about 11.8V (deeply discharged) all the way up to 14.4V ("bulk" charging from an RV Converter, or Solar Charge Controller). Most "Variable" and "Doubler" DC converters will vary the output Voltage when the input Voltage changes, we need a "regulator" type.

Part #3: A DC-DC Variable "Buck" Converter, to create 3.3V output from 24V input. I used and recommend one like this, which costs barely $2: http://www.ebay.com/itm/Step-Down-LM259 ... 2405283762. Why? The middle pin of the PSU is required to supply 3.3V, through a 2.7k Ohm resistor. This converter can be an unregulated "variable" model, because it's input will always be regulated 24V.
Part #4: A 2.7k Ohm Resistor, 5% tolerance. Lower quality might also work.

Part #5: A 1000uF Electrostatic Capacitor, rated at 50V Why? I think it helps to protect the Voltage Converter from low-frequency noise emanating from the Resmed VFD. (The cable chokes also play a big role.) This might not be necessary, because the Boost Converter definitely contains a similar "Filter Cap" inside the sealed unit. But I can't ascertain the size within the Boost Converter, and Resmed used a big one across the 24VDC/Ground output legs of their AC-DC Power supply. Note added in 2021: This part appears to be unnecessary, several builders have had success with boxes which don't include it. The Boost Converter filtering is sufficient, all by itself.

Part #6: A "Project Box" capable of containing the components, connectors, and cable stress reliefs. I used this one: http://www.ebay.com/itm/158mm-x-90mm-x- ... 2278707309 With this plastic one, it's easy to drill and cut slots for the strain reliefs on your two cords.

You also need some wire connectors; input wire (I used Low-Voltage landscape cord; 14-2 would be fine); an input cable stress relief for the box entry; and some wire for internal wiring (#16 stranded THHN is fine for this). Here's a picture of my unit:

Ignore all the wire Colors (I didn't have "proper colors" on hand). We have, from the left, two red wire nuts connecting external "+12V" and "Ground" to the input leads of the silver Boost Converter (underneath). Boost Converter +24V output (yellow wire) goes into 5-way push-down wire connector, and Boost Converter "Ground" (black wire) goes into the second connector. Between the two Connectors, we have the Filter Capacitor (with correct Polarity, of course). The small "Buck Converter" (the blue circuit board on top) inputs also connect one wire into each of the 5-way connectors ("+24V", "Boost Converter "-Ground").

The Buck Converter "+ output" goes into the Resistor, and then to the "Middle Pin" wire of the Resmed Cable. (Buck Converter "- Output" is left unconnected, it is wired in common with "- Input" internal to the mini-board.) A wire from the +24V Connector is connected to the Resmed Cable "Inner Cylinder" wire (yellow wire nut), and a wire from the "Ground" Connector is connected to the Resmed Cable "Outer Cylinder" wire.
- - - -
Before connecting a Resmed A10 CPAP machine to this Converter Unit, the internal Buck Converter adjustment screw needs to be "tuned" to create 3.3V output, and all 3 voltage values should be verified. You might, as I did, take the unit out to a car (not turned on), and connect the input cable ends to the battery with a jumper cable. You will probably find that 8-12 turns of the adjuster screw are required to bring the output down to 3.3V. After adjusting at the mini-board output contacts, you should also verify that the Resmed connector pins are correct: zero voltage "Outer Cylinder" (i.e., continuity with the vehicle's frame); +24V "Inner Cylinder", and +3.3V "Center Pin".

It's done. Disconnect the jumper cable and close up the box, take it on your next camping trip.

[VinnieBarbarino]

While I certainly applaud your enterprise and can-do, hacker spirit, I'm a bit curious why you decided to put your precious Airsense 10 and your automobile at risk for this project. And I'm saying this as someone who loves to buy cheapo electronic components from China on Ebay.

[TheSnoringMan]

Mostly I did it for the challenge. I did a similar "HomeBuilt" for another machine in the past, and because I have a close friend with the exact same requirement: A Resmed A10, with a need for use in camping trips (with a Travel Trailer which is exactly like mine). Since we canp without plugins, it's also nice to avoid the 40-50% power loss on the Inverter Path. (DC to AC and back to DC is ~ 75% efficient at each step; 12VDC to 24VDC is ~93% efficient.)

When I bought our other machine, Respironics 'PR System One', they didn't even offer a compatible DC Adapter Cord. So, I had three basic choice when camping in my RV Trailer at that time:

• #1, use the AC-DC adapter, along with a somewhat noisy little Inverter. Or;
• #2, presume 12V from the batteries to be OK (even though I knew l, already, that Trailer 12V systems can present anything from 11.8V up to 14.4V to "12V" consumer appliances.) Or;
• #3, build a 12V->12 Regulator device.

Having Low confidence in the "Whatever Voltage the trailer battery delivers" alternative I built the 12V -> 12V regulator, and I've had great results. (Upon it's release, it turned out that Respironics "Official Cord" is straight-through, ignoring Voltage changes - and I still don't like that idea.)

So this time, I was feeling only slightly "challenged", and pretty confident about the result. My "12V - 24V Resmed HomeBuilt" Converter definitely creates zero risk to the SUV and Trailer 12V systems. The only possible failure outcomes were:

• My Converter would fail, releasing "magic blue smoke" from itself. (This was tested before attaching the Resmed CPAP.) Or;
• My Converter would somehow fail to satisfy the Resmed A10's "verification" testing process (and fail harmlessly). Or;
• My Converter would somehow "blow up" the Resmed A10 system.

But this last failure, catastrophic for the Resmed CPAP, seemed nearly impossible when providing the correct input voltage values on the tri-axial Resmed connector plug. I considered the second failure mode to be most likely - and it did not occur.

[palerider]

As a side note, here's the back of the PCB on in the resmed s9 converter:



the top had a heat sink that was silastic'd in place, and I didn't feel like trying to get that off.

[CapnLoki]

This seems like an exceptionally silly thread. In particular, I question:

#2, presume 12V from the batteries to be OK (even though I knew l, already, that Trailer 12V systems can present anything from 11.8V up to 14.4V to "12V" consumer appliances.)\

Having used a PRS1 560 on 12v power for the last 4 summers, I can say that there's no problem running at 11.8 Volts. I'm pretty sure I've been at 11.6 on occasion, and I expect the PRS1 would run at 11.5 or maybe lower. I've also run on a charger, probably at 14 or more. It would probably handle 14.4 just fine, but you must know that batteries do not spontaneously present 14.4 volts, this only comes from a charge source, such as a generator or solar panel, neither of which should happen in the middle of the night. If your RV is plugged into the grid you could use the AC power supply.

Also, any device designed to run from a "cigarette socket" should handle voltages from 11.5 to 14.5. Similarly, AC voltage has an acceptable range,about 106 to 127 volts for most equipment. The only thing I worry about on my boat is odd spikes, such as from starting the engines, so I make sure the pump is unplugged if I'm not sleeping.

[TheSnoringMan]

This Thread is primarily about Resmed, supporting 24V and 3.3V output lines with an inline resistor on the 3V. Respironics is completely different, and much more simple.

[palerider]

This Thread is primarily about Respironics, supporting 24V and 3.3V output lines with an inline resistor on the 3V. Respironics is completely different, and much more simple.

you might want to edit that.

[CapnLoki]

This Thread is primarily about Resmed, supporting 24V and 3.3V output lines with an inline resistor on the 3V. Respironics is completely different, and much more simple.

I understand. What I find silly is the implication that the device serves any purpose or is in any way necessary.

On the other hand, there are a number of cases where device firmware has been "open sourced" to the benefit of the community - the various open source router projects come to mind. It would be interesting to speculate what change would be made if we source access to cpap firmware.

[tinkerer1]

TheSnoringMan, thanks for your posting of the 12V converter.
I was thinking about playing with a converter design also. Could you do me a favor and measure the voltage drop across the 2.7K resistor when attach to the cpap?
If I come up with another working design, I'll let you know.

P.S. Laughed out loud when I read about the "magic blue smoke". I find it so hard to get it back in once it gets out.

[D.H.]

They are selling one for under $20 on Amazon. It's not the ResMed brand one, but it should work. It is no more likely to void your warranty than the homemade one.

[TheSnoringMan]

They are selling one for under $20 on Amazon. It's not the ResMed brand one, but it should work. It is no more likely to void your warranty than the homemade one.

D, I don't see it in a quickie search - just the original Resmed for about $80, and a BiX clone for about $60. There are plenty of cheap AC-to-DC converters. Are you sure that you're looking at a DC-to-DC converter? (Should be something like "10-18 VDC input, dual 24V +3.3V output for Airsense 9 and Airsense 10").

If so, please give some search keywords that I might use. I know some other people who need one, and are afraid of connecting little electrical thingies together for themselves. Anything less than $40 would make my effort totally "obsolete".

[TheSnoringMan]

TheSnoringMan, thanks for your posting of the 12V converter.
I was thinking about playing with a converter design also. Could you do me a favor and measure the voltage drop across the 2.7K resistor when attach to the cpap?

IIRC, the Resmed CPAP performs a quick "sweep" on the wire - verifying both the presence of 3.3V total voltage, and the amount of Voltage drop across a CPAP resistance. Voltage Drop across the 2.7K resistor varies during the sweep. There probably isn't a simpler way to pass the "Genuine Resmed?" test than using both components (fixed voltage source and fixed resistor, exactly as they provide in their own power supplies ). A bigger resistor, combined with the original 24V voltage, woudn't pass.

Nearly half of my "objective" was to show that: 'Even though some greedy bstrds put this "Resmed Verification" garbage in there, I can emulate their unnecessary trick and screw them out being rewarded with MY money for making it unnecessarily complicated.'

[palerider]

]'Even though some greedy bstrds put this "Resmed Verification" garbage in there, I can emulate their unnecessary trick and screw them out being rewarded with MY money for making it unnecessarily complicated.'[/i]

What would you suggest as a way to tell the unit what kind of power supply was attached, and thus what capabilities it should use?

I'm also amused by the rants of "GREED" about Resmed, (when respironics is doing it too on the dreamstation), given that *every* notebook manufacturer comes up with their own connections.

Heck my thinkpad has a 170 watt power brick... it's got an almost the same connector as the slightly lesser model with it's 90 watt brick... and the 90 watt brick will plug into this notebook... and the notebook powers on... *but it refuses to charge the battery* even if it's pulling considerably less than the 90 watts the lesser power supply is capable of.

Have I ever seen anybody ranting about GREED? No.

[tinkerer1]

D.H. Is that a DC -> DC converter you saw on amazon? Or maybe just the AC brick? I've seen those for about that price.

TheSnoringMan Hmmm, a 'sweep'. I can understand a voltage sense with no load and then a voltage sense with cpap load. But as 'sweep', something like tracking a curve (an A/D and all that), seems over designed. But if that isn't it, I'm a little lost (ok more than a little. lol).
Well, anyway, I figure the best place to start would is to duplicate your design, and then play from there. Ordering parts today.
Got a couple of old Dell bricks that look like the same connector.
Wish me luck!
Thanks again for sharing!

I've previously looked at the converter at Bix, but they have been out of it. They tell me they should be in at the end of April. Time will tell.
And their's looks like it has a fan in it, which I would rather not have. Extra load, noise, and point of failure.

[Goofproof]

Nearly half of my "objective" was to show that: 'Even though some greedy bstrds put this "Resmed Verification" garbage in there, I can emulate their unnecessary trick and screw them out being rewarded with MY money for making it unnecessarily complicated.'

I would think of that as a worthy goal, I wouldn't have gave them the profit of buying their XPAP. I didn't buy one and never would, mainly for that reason. Jim