A few weeks ago I was looking around our stock room when I came across a non-functioning CPAP machine sitting on a shelf. It was an old REMstar Pro with an E-24 error, usually associated with some sort of water damage (figure 1). With the same level of curiosity I had with machines (radios, tvs, video games, etc.) when I was a kid, I found a screw driver and opened up the machine. We don't repair CPAP machines here, so this was the first time I'd ever seen the inside of a CPAP machine. Not a big surprise, really, except for the fact that the CPAP machine case was filled with mostly sound-absorbing foam. Besides that, of course, there was a circuit board with buttons and an LCD display screen, a blower motor and some tubing connecting the blower to the air outlet port on the CPAP machine.

I've taken pictures of the components of this CPAP machine and I'm going to share them here so that you know a little bit more about your unit, even if it's not the exact same CPAP machine. After all, every CPAP machine works in pretty much the same way. These photos should also prevent you from feeling any temptation to open your CPAP machine out of curiosity. If your CPAP machine is within it's warranty period and you open it, it's likely that your warranty will be voided. So, sit back and enjoy my broken CPAP machine.

Opening the Respironics REMstar Pro CPAP machine only requires the removal of three philips head screws. Once those screws are removed the top enclosure of the CPAP machine pops right off (figure 2). Some machines are a bit trickier to open, such as the Puritan Bennett 420G. The Puritan Bennett 420G CPAP machine only has four screws, but they're weird ones that appear to require some sort of star-shaped allen-wrench-type of tool for removal. The Puritan Bennett 420G is made in France, which likely explains the unusual type of fastener.

Once the top enclosure is removed from the CPAP machine the fun can begin. The circuit board - also known as the main printed circuit assembly or Main PCA (figure 3) is attached to the top enclosure and can easily be removed from the top enclosure. Two wires connect the blower assembly to the circuit board. Removing these two wires allows you to completely remove the top enclosure and circuit board from the main body of the CPAP machine. Figure 4 shows the CPAP machine completely separated into two parts.

The Main PCA is the control center of the REMstar Pro CPAP System. The Main PCA utilizes a microcontroller chip which reads from and writes to various I/O devices (control pad, LCD, memory, pressure sensor, motor control circuitry, RS-232 interface, etc.). With all of this information, the microcontroller makes appropriate decisions so the CPAP machine blower is properly controlled to deliver the desired CPAP therapy in a safe and reliable manner. The serial number of the CPAP machine is programmed into the memory of the Main PCA during the manufacturing process.

Part of the Main PCA consists of the slot for the Encore SmartCard. More sophisticated CPAP machines like the REMstar Pro offer SmartCard functionality. On the Respironics REMstar series of CPAP machines, the SmartCard is a plastic card similar in size and shape to a credit card. Instead of holding information on a magnetic strip, it holds it in a small silicon chip embedded into the card.

When installed in the REMstar Pro CPAP machine, the SmartCard records the date, time at pressure, pressure levels, and duration of each use. Using the Encore software, the usage data can be downloaded and viewed. You can see the Encore SmartCard in figure 4.

Notice that the bulk of the interior of the CPAP machine consists of sound- and vibration-absorbing foam (figure 5). My estimate is that about 65% of the interior of the CPAP machine is filled with foam. Surrounded by the foam is the blower assembly, which is obviously a key component of the CPAP machine - the part of the CPAP machine that actually generates air pressure (figure 6). The blower is a three phase brushless DC motor with a permanent magnet rotor and stationary coils that are conenected in a ""Y"" configuration and serve as its stator coils. The microcontroller sets the motor speed - and this in turn determines the output pressure - by varying the voltage which is delivered to the motor.

In figure 6 you can see that the CPAP blower assembly is a fairly simple design. The motor makes a plastic ""fan"" spin really fast inside the blower assembly and this spinning fan generates air pressure and pushes the air out of the blower assembly and through the pressure outlet port. This particular REMstar Pro CPAP machine features C-Flex which is a technology which lowers the pressure setting when the CPAP user exhales. When exhalation is sensed the micro-processor sends a signal to the motor (in the form of a lower voltage) in order to briefly slow the motor down. Upon inhalation the voltage to the motor is increased and the motor speeds up to deliver a higher inhalation pressure. This happens with each breath you take - thousands of times per night.

That's all there is to a CPAP machine! Micro-processing technology and a motor hooked up to a fan. This should satisfy your curiosity about the inner workings of your CPAP machine. If you do ever take apart your CPAP machine, be sure not to plug it in. It'll work just fine, and that's the problem. There's a risk of electric shock, so be careful.

Andrew Senske

Note: this article includes some technical information copied from the REMstar Pro Service Manual.

CPAP-Supply.com is a leading online retailer of CPAP equipment. Located in Spokane, WA CPAP-Supply.com has been serving thousands of customers around the world since 2001. Founded on a belief that patients are their own best primary care providers, CPAP-Supply.com understands the importance of educating patients and customers on both the effects of and treatment for obstructive sleep apnea. For more information visit CPAP-Supply.com or call toll free 1-888-955-2727.

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