Power question

**cyber** · 08-03-2004, 01:45 PM

I have a self-powered usb hub that operates at 5 volts, obviously I can't wire this up directly to my car's battery. I was wondering what I would have to do to drop the input power from 12volts to 5 so it is usable by my hub. Thanks for the insight

**pip** · 08-03-2004, 01:52 PM

Run 5v from a molex connector?

Ratshack cigar lighter adapter set to 4.5V might work.

**binary.h4x** · 08-03-2004, 01:58 PM

a voltage regulator. 7805 is the chip number. Has 3 pins, input, output, and ground. You can run these in parallel for higher amperage, and may require a heatsink.

**cyber** · 08-03-2004, 02:30 PM

the 7805 yields 5 volts at 1amp so if I needed 5volts at 2amps I would have to wire two 7805s in parellel?

Code:

(assuming the pins are in, ground, out from left to right 
& that the middle pins are grounded)
  ___      ___
 |   |    |   |
 |_ _|    |_ _|
 | | |    | | |
 |   `----)---`------(+5v to usb hub)
_|________|
^
|___(+12v from battery)

**trancified** · 08-03-2004, 02:36 PM

You'd also want to use some caps to even out the power. Don't forget an inline fuse.

**cyber** · 08-03-2004, 02:52 PM

what size capacitor(s) would I be needing then? Also would a 5amp fuse suffice or wouldn't that be effective since I'll only be drawing 1 or 2amps (I don't remember the number of amps my hub requires...I'll need to check on that when I get home)

**binary.h4x** · 08-03-2004, 03:47 PM

for 2 amps i would run 4 chips maybe 6. They are fairly cheap and small so you can get away with it. Yes, your drawing looks good. As far as the caps go, put one large cap and one smaller cap across the input source and the output source. A 5A fuse should work, just make sure your wire is rated for more than 5 amps.

**cyber** · 08-03-2004, 11:02 PM

how large and how small should those caps be? my ac adapter for my hub states the following:
input: 100-240V~0.5A 47-63Hz
output: 4.0~5.5V = 2.4A max

then further down it lists:

input: AC 100-240V 32-43VA,50-60Hz
output: DC 5.0V,2.1A

so I'm assuming I need to present 5.0volts at 2.1amps for my hub to operate properly

**choyak** · 08-04-2004, 12:38 AM

I don't know if I would do that with the 7805. I have seen instances where they do not share current that well, and one hogs more than the others. It is better to build a switching regulator or use this mondo regulator
http://www.national.com/pf/LM/LM323.html

**cyber** · 08-04-2004, 01:28 AM

so if I went with this regulator I'd still need the capacitors on the input and output?

No external components are required for operation of the LM123. If the device is more than 4 inches from the filter capacitor, however, a 1 µF solid tantalum capacitor should be used on the input. A 0.1 µF or larger capacitor may be used on the output to reduce load transient spikes created by fast switching digital logic, or to swamp out stray load capacitance.

sorry for all the questions but my field of knowledge dealing with circuits is very basic