Newbie voltage divider question

[RickieSalad]

Hey guys and gals.  I was hoping for a little help. 

My brother uses a CPAP air machine when he sleeps.  He likes to go camping and would like to setup a battery solution for the times he's away from home. Luckily the machine uses a laptop style power brick, so if I can put something together that'll run off a car battery that would work perfectly.   

There is already an item that can be bought for exactly this but it's $85-$90 and we're trying to do it cheaper.

On the power brick it says it puts out 24v @ 3.75A.  I did some poking around with my meter and determined it isn't outputting JUST 24v, there is also 21v & 3v.

I bought one of these 12v to 24v DC converters so it can be run off one car battery... http://www.amazon.com/gp/product/B00VRAQZVK?psc=1&redirect=true&ref_=oh_aui_detailpage_o05_s00

So I'm thinking if I put together a voltage divider like the one in the attachment that should give me the 24v, 21v, & 3v nicely. Then I started thinking about power handling.  24v @ 3.75A is 90watts!  So those resistors would have to be MASSIVE. Then I thought that the CPAP machine itself would actually be the load pulling the current and dissipating all that power.  Except the current still has to flow through those resistors to get to the CPAP machine. So maybe they will have to be huge?  So will I need huge resistors, or is the CPAP going to be dissipating all that power and the resistors are just dividing up the voltage?  I have a feeling this is going to be a very "DOH!" moment when I get an answer, maybe I'm thinking too much, or not enough.

It could also be that I might be barking up the wrong tree and maybe a voltage divider isn't even the best solution?

Sorry in advance if I'm just being really stupid on this. 

[MarkF]

Your circuit will NOT provide the voltages you want.  The lower value voltage outputs from your circuit will depend on the current being used.  You will only get your desired voltages if the current load on each is very very low.  The voltage drop across each resistor depends on the current through it ( remember Volt = Current * Resistance ).

You would be better off with a 12VDC to 120VAC inverter but your current load on the battery would be pretty high.   I'd use a separate deep cycle battery to power the inverter.  The other option would be a small portable generator.

[Connoiseur]

What are the current draws in those two voltages? If the currents are under a few hundred miliamps you can get the job done with an LDO like LM317. Otherwise you mave have to use additional switching power supplies.

So I'm thinking if I put together a voltage divider like the one in the attachment that should give me the 24v, 21v, & 3v nicely. Then I started thinking about power handling.  24v @ 3.75A is 90watts!  So those resistors would have to be MASSIVE. Then I thought that the CPAP machine itself would actually be the load pulling the current and dissipating all that power.  Except the current still has to flow through those resistors to get to the CPAP machine. So maybe they will have to be huge?  So will I need huge resistors, or is the CPAP going to be dissipating all that power and the resistors are just dividing up the voltage?  I have a feeling this is going to be a very "DOH!" moment when I get an answer, maybe I'm thinking too much, or not enough.

Voltage divider is not a good option if the device really consumes more than a few miliamps. However if the lines just send a voltage signal like power good or something alike then voltage divider will work.
And those 3.75 A will not flow into the resistor network unless you select such low value resistors. Remember "Ohm's law" and "current division rule" and you'd be good to go.

The power supply seems to be potted. So nothing can be done to it. However I would suggest you to obtain 3.3V from 12V input rather than from 24V. This will drastically cut down your losses.

[RickieSalad]

Thanks guys.  I wasn't sure of the voltage divider was the best option. It's just the first thing I thought of.  Hmmmm, back to the drawing board, so to speak.