[Equipment] Alternate power supply

[Ralph199]

Hi Everyone,
My wife has recently started using a ResMed A10 and we like to travel in our caravan. When on a powered site - no problem. When free camping - problem.
I love to tinker and build things so I purchased a 12v-24v 10amp DC converter ($23 ex China) and a Dell/HP laptop plug & lead ($2.50 ex China). I checked the output from the ResMed power pack  as 24.46v with 3.33v on the centre pin. After building my alternate power supply I found I had 24.3v.
   
To mimic the signal voltage I neatly inserted 2 x AAA batteries.
   
This gave me 3.12v on the centre pin.
Plugging my new lead into the A10, the machine's green LED flashes then nothing.
Back to the genuine power supply - all ok. Try my supply again - still nothing.

Can anyone please explain what I am missing here?
I would not have thought the very, very slight under-voltage on both the 24v & 3v could be a problem.
I am stumped!

BTW, the laptop lead appears to be an exact match in all respects.

[trish6hundred]

Hi Ralph199,
WELCOME! to the forum.!
Hang in there for answers to your question.
Good luck to your wife with CPAP therapy.

[Sleeprider]

Resmed sells a 12-24 converter that works with the machine for about $80. They have designed their proprietary system to very tight specifications to keep us from using alternatives. Battery Power Solutions and BixPower both sell non-Resmed alternatives for $56 to $67 on Amazon, and BixPower offers a Li-Ion battery with converter that is about $250 for the package.

[Ralph199]

Resmed sells a 12-24 converter that works with the machine for about $80.  They have designed their proprietary system to very tight specifications to keep us from using alternatives.  Battery Power Solutions and BixPower both sell non-Resmed alternatives for $56 to $67 on Amazon, and BixPower offers a Li-Ion battery with converter that is about $250 for the package.

When we picked up the unit in Perth I enquired about using it in the caravan. We were quoted at the time a figure of $150 (AUD) for the 12v-24v adapter they supply. Being a gadget making guy I thought that figure was a bit steep and knew I could make something a lot cheaper. Of course I didn't know at the time just how tight their specs were or that I would have such trouble mimicking their adapter.
I also know that ResMed equipment is a lot cheaper in the USA although it is made in Perth.  I asked (in Perth) if we could buy the wife's machine from the USA to save money. We were told we could, but the Perth arm of the business would not help in setting up or any on-going issues we may have with the equipment. We would be on own own. So we bought locally.  Hence my idea to make our own adapter.

[srlevine1]

With all due respect, this is a medical device and surely your life is worth more than a paltry few dollars -- especially when amortized over a few years. I suggest you get ResMed's DC-DC Converter from the states and pair it with a deep-cycle marine battery and go with that. If you want to play, use some of the stackpole connectors, fuses, and leads to make an ultra-clean installation. Then add solar recharging for the hell of it.

Best of luck.

[Waz1]

The other way I use a 300 watt 12-240 converter and plug the standard power supply into that. A standard car battery ran my airsense 10 for about 14 hours before the inverter low voltage alarm came on at 11 volt that was with heated hose and humidifier both on if you have plenty of solar might be an inefficient way but can get you out of trouble.

[Sleep2Snore]

It might be easy enough to build a power supply to convert 12 to 24vDC, however, they use a strange plug just to make it difficult to get a plug for te machine.  I have not gone to the bother of working out how the plug connects to the machine, but it is not just + and - so any info would be helpful.
Here in the UK the cost of the 12 to 24vDC converter is crazy.  ResMed ask far to much, but they dropped the price on the ResMed s9 converter.  I have not studied if they are the same, though they might be.  £110 for the power adapter is a bit much, that is around USD $154.
Not found an alternative supply here, searching Amazon UK only comes up with power supplies supplied by ResMed, but I bet they are not, just the heavy price tag.
However, I have not done a search for at least a month, so they might be there now.
I will not be paying ResMed's price for a power supply, that is for sure.

This is one draw back to ResMed machines, if you go off grid, it becomes very expensive, here in the UK anyway.

If anyone knows otherwise, please let me know as I want to go off grid as well.

[Sleep2Snore]

Ralph199, Welcome,
I know where you are coming from, prices in Australia can be a bit hefty as well.
I would have still bought my cpap from America to save on price.
however I didn't need to buy one thanks to the NHS but if I had to I would have bought from there rather than pay the prices here.

[sylvan]

A problem sometimes seen with power supplies is found in the output waveform. To keep the price down, by using fewer components, the DC
output is not true, pure DC. It is pulsating DC. Pulsating DC is the rectified upper portion of the AC sine wave. Pulsating DC is OK for many
appliances - think electrical, not electronic. It seems that your CPAP machine is rejecting the new input power.

You are on the right path.

Off the top of my head I can only think of 2 ways to determine what you have.
1. Data from the company you got the power supply or wiring diagram from.
2. Checking the output of the power supply with an oscilloscope.

I hope this works.

[Ralph199]

Thanks for the various replies.
First off my wife doesn't need or use her machine every night so my issue isn't urgent. If it was I would certainly buy the ResMed adapter and not try to save "a paltry few dollars".
Being a gadget man this is a project and now a challenge. The suggestion to check the output with an oscilloscope is a good one and I will. However the mains power adapter is, like most things these days, made in China. Although the Chinese stuff is improving all the time, to me it looks and feels like any other power adapter used for laptops etc. So I don't think it would be unduly complicated.
I have no wiring diagram as I am just cobbling together bits that should work. It is a bit "Heath Robinson" and I was , years ago, an Aircraft Electrical Fitter in the RAF and back then we did do some "experimenting".
So going from a DC battery through a dc to dc converter you wouldn't think there would be any ripples or transient waveforms.....however the oscilloscope will settle that. Also I do enjoy a challenge and as we won't be doing any free camping in the near future, there is no time pressure for me and my little project.