Re: Reduce Voltage?
- From: "Homer.Simpson" <Homer.Simpson@xxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: 30 Apr 2005 20:41:49 GMT
mopar said
> I am working on a home surveillance system and I need to do a DC
> voltage reduction. I have a supply that is 12v DC @ 80 ma and
> need to reduce the voltage to a max of 9v @ 80 & 120 ma (variable
> load). As I understand I can do this with a resistor? If so, what
> size and should it be in parallel or in series between the supply
> and device?
You're asking the resistor to drop the excess voltage at the current
of interest. That makes the resistor between (12V-9V)/80mA =~38 OHMS
to (12V-9V)/120mA = ~25 OHMS. The power rating should be (12V-9V)*
120mA=360mW. It's placed in series with the load.
That being said, that is a poor way to drop the voltage. It ignores
things such as surge current which may yield an excessive voltage
drop under transient loads. A better idea might be to series several
diodes (12V-9V)/0.6V= ~5 diodes.
Note you're going to violate the current rating of your 12V PS. I'd
pick up a cheap 9V PS from Rat-Shack or something. They're dirt
cheap.
hth
.
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