My battery backup setup

[PoolQ]

Back from a trip to pick up my daughter from school.

The patent says they use the resistor to indicate the 90W power supply verses the smaller wattage one. They simply have a resistor to ground inside the CPAP machine, say for example another 2.7K to ground. If the 90 Watt power supply is plugged in then a comparator will detect that 3.3VDC/2 is on the pin, if the lower wattage supply is powering the CPAP then the pin will have 3.3VDC and don't turn on the humidifier.

IF??? this is all they are doing the unit "should" work without the resistor, but just not with the humidifier turned on. They could also be trying to keep you from using anything but their power supply by using two different resistor values so "some" resistance must be there to work. They could have just used ground center for one wattage and 3.3VDC for the other wattage and not put any resistor in the circuit.

When customers mess with the power you really want to know if the machine burned up using the factory supply or some cobbled together home brew solution. Warranty impact you know.

jumping in here, this is interesting and sorry I did not read it sooner.

Thank you for caing about improviing our knowledge of how the non-standard power-supply-sensing circuitry works!

I asked the time-constant question because it "might" be that the capacitor value is important to the ResMed machine, "if" it affects the ResMed analog resistor-sensing circuitry.

That diagram does not show an RC circuit, you will never pull down that 3.3V regulator through that 2.7K resistor.

Thank you for pointing my error out as it explains something I was confused about.

1. When I first calculated the RC, I used this straight calculator, which only asked for a single voltage value (in addition to the R and C values):
[*] http://www.referencedesigner.com/rfcal/cal_05.php

2. However, when I displayed the graph, I used a different calculator which asked for TWO Voltages as shown below:
[*] http://ladyada.net/library/rccalc.html


This "assumes" that the center pin has 24VDC on it when the device is in use, which may or may not be the case (impossible to know without testing it using a "shunt" of some sort).

Easy way to tell the resistance value is to have another resistor (to ground) inside the CPAP that makes it a resistor divider. the voltage at the center of the two resistors will tell you the value of the resistor with good accuracy.

Thank you for the additional information. All I know (from the patent) is that they:
[*] Use cheap analog circuitry
[*] To sense the resistor value (2.7Kohm indicating a 90W power supply)

My question is whether the 0.1uF capacitor might "affect" that calculation.

[OMyMyOHellYes]

T'would all be so much more simpler if all manufacturers just used a 12 V system (or even 24 V) with standard plug configurations (with center the pwr side and outer barrel the gnd side) on everything. Their unnecessary complications lead my cynical side back to the follow-the-money theory.

OMMOHY

[verbatim1]

The S9 is different from the A10. It has a different input connector and requires a different dc converter.

You can find the A10 dc converter pdf here: google "directhomemedical airsense-cpap-dc-cord-instructions-for-use.pdf"

Thanks for that information that the S9 is different than the S10 even though they're both 24VDC.

I should have realized they'd make each power supply even more non standard, since the basic rule of Marketing is to turn a commodity into a monopoly.

Given there is no (good) reason in the first place for the power supply to be non standard, I should have guessed that would be the case.

Is the discussion past the point, in terms of required effort, of making the expensive resmed power box look attractive?

1. There is never harm in UNDERSTANDING how things work.
2. There is USEFUL INFORMATION in Steve's tests of his equipment
3. When you design things, you learn even more stuff, so, designing both the simple and the idiot-proof system has merit.

However, I do agree that the simpler the system, the more it is likely to be built, so, we can only idiot-proof the system so far.

For example, the Schottky Diodes and 24VDC regulators are more for idiot proofing than they are required, based on Steve's input-voltage tests.

If we assume that the only ones who will actually build the circuit are "tinkerers", then the idiot proofing is more of an intellectual exercise than it is a requirement.

The main requirement is pretty simple:
1. The 2.7K Ohm pull-up resistor & the 3.3VDC "logic 1" sense circuit
2. The 24-volt (nominal) 3.75Amp power supply

Besides, if the simple circuit actually fails, then it makes the otherwise lousy decision of buying the expensive non-standard ResMed equipment easier to swallow.

The patent says they use the resistor to indicate the 90W power supply verses the smaller wattage one.

We don't know yet if ResMed even offers any other power supply other than the 90W power supply (i.e., 2.7K Ohm pull-up resistor), do we?

They simply have a resistor to ground inside the CPAP machine, say for example another 2.7K to ground. If the 90 Watt power supply is plugged in then a comparator will detect that 3.3VDC/2 is on the pin, if the lower wattage supply is powering the CPAP then the pin will have 3.3VDC and won't turn on the humidifier.

Sounds simple.
The patent did imply it was simple analog circuitry that detected the value of the pull-up resistor, and the patent did say that they prioritize power to the blower during the inspiration cycle based on the perceived power available.

They could also be trying to keep you from using anything but their power supply by using two different resistor values so "some" resistance must be there to work. They could have just used ground center for one wattage and 3.3VDC for the other wattage and not put any resistor in the circuit.

Yup.
They will never say it, but they're happy to have a non-standard power supply because that means you have to purchase your 24 volts from them.

ResMed marketing managed to turn a 12volt commodity into a 24-volt monopoly.

Kudos to their marketing team; they did their job admirably well.

When customers mess with the power you really want to know if the machine burned up using the factory supply or some cobbled together home brew solution. Warranty impact you know.

First off, it's just a 24VDC power supply, for heaven's sake.
And it's just a simple 3.3Volt (2.7K Ohm) sense circuit, for heaven's sake.

As long as the voltage and current and resistance is within acceptable ranges, the equipment will not be harmed. (Of course, the acceptable range limits are what we strive to better understand!)

Resmed simply tried to put a patent on a basic power source, that's all.
Their marketing guys managed to turn a 12V commodity into a monopoly.

That's their job; and they did it well. I admire that they were smart enough to pull it off.

However, if anyone is really worried about using this circuit, then they should not use this circuit. It's a pretty simple decision.

If someone doesn't trust that 24 volts is 24 volts (or that the range is too great for the equipment to handle), then they shouldn't even THINK about using this circuit!

Remember, we're just talking about 24 volts, 3.75 amps, and 2.7K ohms, where ResMed doesn't own the rights to those values.

If someone is so worried about this circuit, then either they don't understand volts, amps, and ohms, or, they don't understand what the equipment does - or - they aren't the right person for this circuit.

Let's be clear. MOST PEOPLE ARE NOT the correct people for this circuit.
Maybe 1 in 1,000 are the correct people for this circuit.

a. Most people don't understand volts, ohms, and amps.
b. Most people don't build circuits
c. Most people would be AFRAID of this circuit

And, all of us are worried about the RANGE that the equipment can handle (since the input voltage from two batteries WILL vary).

So, this circuit was never intended for MOST PEOPLE.
It's for "tinkerers".

That's 1 out of 1,000, at best, don't you think?

T'would all be so much more simpler if all manufacturers just used a 12 V system (or even 24 V) with standard plug configurations (with center the pwr side and outer barrel the gnd side) on everything. Their unnecessary complications lead my cynical side back to the follow-the-money theory.

But then they couldn't charge you about $100 ($85 + $15 for cables) for a simple power supply!

Simple MARKETING 101.

You need to turn a commodity into a monopoly.

They've essentially turned a 12Volt commodity into a 24-volt monopoly.

That's great marketing.

And, like with all manufactured monopolies, the people who lose are the consumers and the people who win are the manufacturers.

[surferdude2]

Just to play the devil's advocate, you must realize that medical equipment is subjected to very stringent specifications. Given the litigious society we live in, risk management often dictates how carefully and fool proof some thing are manufactured. E.g., A typical household convenience receptacle cost 69¢ whereas a hospital grade version costs north of $5 and often far north. It's the quality, testing and liability insurance that adds to the cost.

Dude

[verbatim1]

Just to play the devil's advocate, you must realize that medical equipment is subjected to very stringent specifications. Given the litigious society we live in, risk management often dictates how carefully and fool proof some thing are manufactured. E.g., A typical household convenience receptacle cost 69¢ whereas a hospital grade version costs north of $5 and often far north. It's the quality, testing and liability insurance that adds to the cost.

Dude

Nothing wrong with that argument, but my counter argument is simply WHY did they use a non-standard power supply when EVERYONE ELSE makes similar medical equipment with a standard (12VDC) power supply.

Everyone else is subject to the same litigious society, right?
So what's different with ResMed over, say, Philips?

Given that they could have chosen ANY voltage, we must assume that everyone else chose 12VDC BECAUSE it was standard; and then we must then assume that ResMed either had a good reason for 24VDC, or they had a marketing reason.

Since the equipment is simply a 3-phase motor (essentially), I can't see any "good reason" (other than good marketing reasons) for the non-standard supply voltage ... can you?

If you can come up with a "Good Reason" for the non-standard 24VDC, then you can BEGIN to make your argument. Otherwise, the fact that EVERYONE ELSE is using a standard voltage makes your argument moot, don't you think?

Again, I have nothing against your argument; I'm simply applying basic logic to your argument. What's different about ResMed's liability when it comes to choosing 12VDC versus 24VDC when everyone else has the 12VDC selection?

If we move on to the 3.3VDC 2.7KOhm "sense circuit", that's a DIFFERENT argument, which I'll take up in the next post.

[verbatim1]

Let's apply basic logic to the sense circuit now.

The argument on the 2.7K Ohm sense circuit is a DIFFERENT argument that that for the non-standard power supply voltage. This is simply because there could "possibly" be a "good reason" for the sense circuit that the other manufacturers didn't think about (yet).

So what is that "good reason"?

As far as I've read, there are only two possible reasons for the sense circuit, both of which may apply at the same time:
A. It's 'really' used as a sense circuit, or,
B. It's 'actually' just a marketing gimmick to turn a commodity into a monopoly.

Let's assume, for this post, that it's really used as a sense circuit.
Well then, what does the sense circuit do?

The 2.7Kohm pull-up resistor "tells" ResMed that there is a 90Watt power supply attached.

OK. So that's nice. Now the equipment "knows" there's a 90W power supply.

But, what good is knowing that a 90W power supply is attached?

Based on what the patent says, the ResMed equipment can prioritize inspiration power over expiration power, given that knowledge of the power supply wattage.

But wait. There's something potentially fishy here...

Since the main power supply IS a 90W power supply, then, um, well, then we can PRESUME that there is no need to prioritize power under full-power circumstances, so, we might risk the assumption that, at 90W, the equipment doesn't NEED to prioritize power for inspiration needs.

We don't know this for a fact, but, IF the 90W power supply is the "full power" version, then we can assume that the power prioritization starts with the 60W power supply.

But wait. That doesn't jive with what we know.
As far as "I" know, there is no ResMed 60W power supply.

Huh? So, it was all just a gimmick?
The resistor tells the equipment that it has a full-power power supply, yet, there is no other sized power supply in existence?

Can it all just have been a marketing gimmick?
Naaaah. They wouldn't stoop that low, would they?

Of course, the every faithful would simply assume that ResMed just didn't get around to making that 60W power supply yet. I guess.

So, this argument on the pull-up resistor hinges on whether or not ResMed makes any other power supply for the A10 than a 90Watter.

Do they?

[justMongo]

Consider that each motor field coil is an inductor.
Inductors resist changes in current -- they buck.
The faster one can slew the current (and the H field is proportional to current) the faster you can run the motor; and the better speed control you have. That motor has to speed up and slow with every breath.
I argue that 24 Volts makes for a better motor design.

[verbatim1]

The argument on the 3.3VDC "logic 1" is far more vacuous.

As far as I can tell from the patent, there is only one reason for the "logic 1" and that's to tell the ResMed equipment that there is either a logic 1 or a logic 0 on the sense line.

Huh?
Really?

In the patent, they simply say it's interpreted as a logic 1. They don't seem to say any more than that. So, it's just there.

What does it do?

Well, again, in the absence of any other power supply from ResMed, the logic 1 can't do anything useful other than tell the ResMed equipment that it's connected to a ResMed 24 volts.

But, how is a ResMed 24 volts any different from a non-ResMed 24 volts?
24 volts is 24 volts.

Of course, to your argument, this "logic 1" ensures that a "ResMed 24 Volts" are attached; but, then my argument that Philips doesn't need the logic 1 for "their 12 volts" comes into play, with respect to the litigious society we live in.

But I'm always open to reasonable argument.
Maybe my logic is wrong? Or missing something?

[verbatim1]

Consider that each motor field coil is an inductor.
Inductors resist changes in current -- they buck.
The faster one can slew the current (and the H field is proportional to current) the faster you can run the motor; and the better speed control you have. That motor has to speed up and slow with every breath.
I argue that 24 Volts makes for a better motor design.

Thanks Mongo for bringing up the voltage advantages.

When I looked up WHY ResMed used 24 VDC, the fact that they use a 3-phase brushless motor came up.

While I googled for the advantages of the 3-phase motor, I didn't have enough information on the competitor's motors to make any reasonable assumptions as to whether a 24-volt 3-phase motor is better or not than whatever it is that Philips (or anyone else) uses.

Your argument that the 24-volt motor is inherently better than a 12-volt motor makes sense if the ResMed equipment is (somehow) better/faster/cheaper/etc than the Philips equipment.

Does the 24VDC ResMed equipment have better motor specs than the 12VDC Philips equipment?

[Sleeprider]

Oh crap! I've stumbled into an electrical engineer's convention.

Don't get me wrong, I love it when you guys can de-solder a microscopic chip on a board, but I happen to know you're pedantic about circuits and still have a messy garage.