Cpap and camping

rickst29 wrote:
You guys are in luck - me and the DW own both of the machines, Respironics and Resmed. And I'm pretty capable with electric things. ;)

- - - The efficiency issue - - -
Using a 12V regulator (Phillips "Respironics") or 24V converter (Resmed "Airsense") creates a big advantage over using the "120V Brick" with an Inverter: Efficiency!

With a typical "cheapo" Inverter, you will only get about 70% "output power" on 120V, in comparison to the 12VDC input power consumed. Following that power loss, the brick itself is only about 85% efficient. About 40% of your Trailer battery power gets wasted (85% times 70% = 59.5% efficient). In other words, you consume 40% more battery capacity to run the Inverter --> Power Brick Combination. That's why Inverters have fans on them: they're inefficient, creating a lot of waste heat.

In contrast, the DC-to-DC power supplies described here are about 93% efficient. No fans, maybe just a few small air holes. (My own Respironics power supply/regulator doesn't have any holes and stays cool - but I built it differently from my recommendation. It uses a "salvage" Vicor regulator module, list price > $200.)

- - - Respironics - - -
Running Respironcs "12V" from the 12V electrical system is fairly easy, and operates at 92-100% efficiency. You just need a much better plug and socket (instead of the "cigarette lighter" connectors). I use 15A twist locks, type ML1. They have the advantage of not pulling out when the sleeper rolls over. ;)

But you should probably use a Voltage Regulator as well. I did. (Trailer power varies from 11.9V up to 14.4V, depending on both the State-of-Charge and the operating mode of any active Converter/Charger device.) The CPAP machine doesn't face that much variance when running through the at-home "Brick"cConverter, and I feel that you should improve the regulation to meet or exceed the quality of the 120VAC Brick converter.

I'll SWAG this one to be suitable, although the seller's description doesn't make sense: http://www.ebay.com/itm/Waterproof-DC-8V-40V-To-DC-12V-10A-120W-Step-Down-Converter-Power-Regulator-/272770715494. (He says that it's "Step Down" Buck Converter, but I note that the Voltage Specification describes both "Boost" operation, 8-12V, and "Buck" operation, 13V-40V. You DO need both functions.)

Worst case - If this device isn't actually a dual function converter/regulator, you can use separate "Boost" and "Buck" Converters back to back, within your power supply project box: The first unit does "Boost" conversion to 24V (same as the Resmed converter/regulator), and the second unit does "Buck Conversion" back to regulated 12V. Efficiency will be a bit less, wasting 8-15% of the input power.

- - - Resmed - - -
This one is a lot more complicated than you might expect: Resmed uses a three-pin connector, with a specific requirement for Voltage and Resistance on the tiny pin "in the middle". Your first step, therefore, is to buy a cheap Resmed "Airsense-Compatible" 120VAC power supply from EBay, Amazon, or elsewhere: You're going to remove and use the output cord, throwing the rest of the power brick away.

You need 3 other parts (and a "project box" to hold the parts): A Regulating Boost Converter: 9V-12V in, 24V out, at least 120W (so it runs at half power or less, staying cool). We will use this one: http://www.ebay.com/itm/Waterproof-DC-12V-to-24V-10A-240W-Step-up-DC-DC-Converter-Regulator-Boost-in-US/122582903601

You also need a tiny step-down converter to create a 3.3V supply on Resmed's weird test wire. If you want to do surface-mount on a board, you can buy an individual chip to perform this job. I chose a cheap "tunable" Buck Converter with an input/output Voltage Display built-in (to help with tuning it properly), this one: http://www.ebay.com/itm/NEW-BUCK-STEP-DOWN-LM2596-POWER-CONVERTER-MODULE-DC-4-0-40-TO-1-3-37V-VOLTMETER/351486640764

+12V comes into the Converter from your high-current plug, creating 24V output. The 24V output goes into a "Y", with a "thicker" wire going into the +24V wire of the Resmed cord, and a thin +24V wire going into the small Buck Converter. Buck Converter Output must be routed through a 2.7K Ohm resistor, and then into the Resmed Sensor wire. (For example, the one I bought: http://www.ebay.com/itm/2-7K-Ohm-2w-2-Watts-1-Tolerance-Metal-Film-Resistor-10-Pieces/282286525128 The 24V ground is wired to the Resmed ground wire, and the 12V input ground wire goes back to your high-current plug.

As with Phillips/Respironics, this one should operate around 93% efficient, and it won't require require a fan. I'm waiting on parts and haven't actually built it yet, but I have high confidence that it will work great.

or for the resmed you can buy a complete ready to use 12-24 DC to DC converter either from resmed, or from battery power solutions. BPS converter when I bought it was just over $50. works on the airsense 9 or airsense 10. comes complete with the correct plug.