Question about 7.5v from ATX power supply

Discussion in 'Electronic Basics' started by CoogarXR, Jan 19, 2004.

1. CoogarXRGuest

Hello,

I am desigining a PC-based DVR security system. The cameras run at 8.5vdc @
700ma. I would like to run 2 of them off of a drive controller lead from the
ATX power supply running the PC. Here is my math, feel free to double-check it,
as I am just getting back into electronics, and I haven't used my electrical
math skills in quite some time
........
V / I = R (Ohms law)

Voltage Difference 12v -8.5 = 3.5v Divided by .7a (700ma) = 5 Ohm
700ma x 4.5v = 2.45w
........

Now, provided I did my math correctly, could I just use 2, 10ohm 10watt
resistors in parallel? I only can get 1ohm or 10 ohm in that high of wattage.
Does this all sound right? Thanks!

CoogarXR

2. cpemmaGuest

Don't use resistors to drop the voltage, very dodgy practice if the load
varies at all. Use a couple of 317T adjustable regulators, one for each
camera. One *will* take 1.5A but what you save on the extra reg, you'll
spend on the bigger HS needed.

I make 470R and 2k7 about right for the resistors or use a preset and get it
on the nose.