n00b question about voltage divider

masch · 04-11-2004, 07:18 PM

Hola

Recently someone looked at my setup and asked me why didnt I just use a voltage divider to get the +12, +5, +3.3 etc.

I figured, maybe it could work? The +5 could be tapped from a voltage divider circuit from the +12, and so could the +3.3. The only trick is to regulate the +12 no?

Or is it much more complicated that this?

wizawuza · 04-11-2004, 07:57 PM

much more, cause this would work just fine if the load (carputer in this case) didn't take any current.

If you have 12 volts, and then two 10k resistors, put them in series, you have 6 volts between them right?

What if you put a 10k resistor in parallel with the bottom resistor, you'll only have 4 volts on the middle node.

That last 10k resistor represents your carputer, or anything that sucks up current, which obviously a computer will.

Maybe you're thinking of another way of doing a voltage divider, but I know that a resistor divider won't work 'cause you need the same amount of current to go through all of the resistors to get the expected voltage on the output.

You could use active devices (fets, bjt's, etc), but to get a really good design, you end up pretty much building a power supply from scratch.

Hope this helps.

masch · 04-11-2004, 08:00 PM

oh, great wealth of info. I understood everything you said. Thanks.

But then I got another question, the carputer, the load.. is it variable? Like for a via epia which I think is rated at 60W, does it suck 60W constantly or does it vary between 5W-60W?

Also a related question is the PC speakers. Lets say its rated at 300W, so if I turn it on but switch off the PC (i.e. no sound is coming out from the speakers), does it still suck up 300W?

Ricky327 · 04-11-2004, 10:05 PM

Yup the amount of current changes. When you first boot up the PC, this is when more current is drawn from the power supply. The HDD, CDROM, FDD spin up all its motor and does drain more current than when its already spinning at a constant speed. More current is also drawn when the PC is doing alot of task.

Yes same with the speaker, current is only drawn when its needed. But of course amplifiers or PC do drain some minimum current even if its idle.

frenchnew · 04-12-2004, 12:52 AM

Very nice theory but impossible to implement as you need the following conditions to implement a voltage divider network.

1> A constant regulated voltage source.

2> A constant current draw

While voltage divider network are easy to implement for some simple circuit like a LED light and the dissipation is very low, when it comes to power a computer it is inefficient as the voltage dropped needs to be dissipated as heat.

Take the example of the LED light (Operating Voltage of typical LED 2 Volts at 0.020Amps). If you want to operate it from 12 volts you need to drop 10 Volts at 0.020 Amps so the formula for resitance is R = V/A or 500 Ohms.

The power dissipated by the resitor will be P = V X A or 0.2 Watts.
While the whole circuit would need 0.24 Watts to power a led that consumes 0.04 Watt. This means that you will consume 5 times more power that what is needed to operate the LED, the rest is wasted as heat.

Now lets assume that you need 5 Volts @ 5 Amps from the 12 Volts,

The resitor will need to drop 7 volts at 5 Amps = 1.4 Ohms but the power dissipated by the resistor would be 35 Watts.

With the same calculations, you would use 60 Watts of power on 12 Volts to supply 25 Watts on 5 Volts (5 Volts X 5 Amps). The rest would be wasted as heat in the resistor.

Also, please note that for safety consideration and reliable operation of the resistor you would need to double it's power rating. For your 5 volts circuit at 4 Amps, that 1.4 Ohm resistor would have to be 70 Watts minimum.

Theses factors are the reasons behind switching power supplies which are more efficient then a simple resistor divider. On average most switching supplies have efficiencies of 70% and more.

Best regards

frenchnew

Quote: Originally Posted by masch

Hola

Recently someone looked at my setup and asked me why didnt I just use a voltage divider to get the +12, +5, +3.3 etc.

I figured, maybe it could work? The +5 could be tapped from a voltage divider circuit from the +12, and so could the +3.3. The only trick is to regulate the +12 no?

Or is it much more complicated that this?

masch · 04-12-2004, 10:30 AM

Wow, thanks folks. Didnt expect great answers such as these =)

Keep it up

terrastudios · 04-12-2004, 02:45 PM

hey,
your best getting either a morex-type powersupply as come with cubid pc cases - these are 12v dc in and do your regulation (although the 12v rail isnt regulated - so if you pump 14vs in, youll get 14v down the 12v rail to your mobo). an easy solution would be a 5v, 3.3v step down reg., if your trying to keep a constant 12v input your best bet will be a tank battery (do a search on here lots of circuits) - ive got a tank battery in mine and a morex supply and its a very stable power supply solution for the via boards.

hope this helps

matt

masch · 04-12-2004, 07:39 PM

a question abou tthe tank battery, terra. Does it reboot on cranking?

Do you have a regulator with yours? I'm currently using a morex too, but it wont load windows if I plug the bluetooth dongle in while booting. It will reboot and reboot and 30% of the time it will succeed loading windows. I think its because of the 60W limit of the Morex, this is the same as what happens even if I use the home wall socket with the supplied adapter.

I'm not using a DVD drive or any other accessories, except for one case fan and a notebook HDD.