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Question about 7.5v from ATX power supply

C

CoogarXR

Jan 1, 1970
0
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
 
C

cpemma

Jan 1, 1970
0
CoogarXR said:
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?

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.
 
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