JimL said:
I found the PS in the blister pack for 50 cents. I need a 5v PS for a 2.5"
USB external drive case and I'm NOT going to pay RS $35 for one. I figured
it was probably not adequate, but hey...
Thanks
To make 5 volts from 12 volts, this is what you use.
"+5V Fixed-Voltage Regulator 7805" Output Voltage: +5VDC @ 1A
http://www.radioshack.com/product/index.jsp?productId=2062599
A 2.5" hard drive will draw on the order of 500mA when up to
speed. It can draw up to 1000mA (1 amp) when first starting
up. So the 7805 might handle it OK.
Linear regulators kick out heat, and the 7805 is most
practical for smaller loads. At this current flow level, it
should have a decent sized heatsink. Immediately, the price
of our project is going up, unless it is possible to
reuse a heatsink from one of your processors, that you're no
longer using.
The regulator will be dropping 7 volts at 500mA, or a total of
3.5 watts. That 3.5 watts will be present as heat, on the tab of the
TO-220 casing. Using thermal paste and a nut and bolt, you
strap the LM7805 to a good sized heatsink. That helps keep the
temperature of the LM7805 lower. If the LM7805 gets too hot,
it will shut off the output. The LM7805 is fully protected,
and about the only thing it doesn't like, is when someone
connects up the power to the wrong pins.
In terms of a circuit, they have some examples here. They
add a couple small capacitors to the device, to improve
stability. (Always download the exact datasheet for the
product you buy. If the brand of regulator is Fairchild,
go to the Fairchild web site and get their datasheet.)
http://www.national.com/ds/LM/LM340.pdf
Notice, in that datasheet, that the device also comes in a
TO-3 metal can, which might marginally couple heat into
the heatsink a bit better. But the TO-220 style, only
requires a single bolt, so if you're drilling a hole in
an aluminum heatsink, it would be easier to prepare for
a TO-220, than for the TO-3. You can buy heatsinks
specifically for the TO-3, but then your project cost
will be too high. (We need to get a heatsink "for free",
to make this project worth while.)
In Figure 1 on page 11 of that datasheet, shows another
form of protection you can add to the LM7805. If the
source of input power (the thing providing the 12V),
gets shorted to ground (as a means of turning it off or
whatever), current can flow backwards through the
regulator, damaging it. (That current flows from any
large value of capacitor on the output side of the
circuit). Typically, that would happen, if the output
capacitor had too many microfarads of capacitance. Now,
when you're building your circuit, chances are you would
use a small cap on the output. But when connecting
it to a load like the disk drive, they may have their own
capacitor inside which is larger than yours. Since
your input power source is not likely to short directly to
ground, as shown in the figure, you can probably dispense
with that kind of protection.
I keep one of those three terminal regulator circuits, right
next to my desk. It actually runs an external fan on one
of my computers, using a wall wart with a higher output
voltage. The two capacitors for stability, are soldered
right to the regulator. The project isn't in a fancy housing,
and simply lies on an insulated surface next to the
computer. In my case, the heat output (volts dropped times
current) is low enough, that the heatsink only gets mildly
warm. So I don't need to keep a fan pointed at it.
The very best kind of regulator is a switching regulator.
They use those in a lot of the wall warts now, and they're
pretty efficient. Notice they don't have fancy cooling.
Switching regulators don't kick out heat like linear
regulators do. The problem is, trying to buy a
switching regulator at retail, one finished and ready to
go. Nice "black boxes" do exist, for converting from
one voltage to another. At work, we used to pay slightly
over $100 a piece for them, in power ranges from 50W to
200W perhaps. Those needed a heatsink, and our equipment
had substantial forced air cooling, so temperature was not
a problem. But your "three terminal regulator" can be
a good solution, as long as you're not paying for a
fancy heatsink.
If you need capacitors, these are an example. These
are non-polarized, so you don't have to worry about
which end is (+) and which end is (-). The LM7805
application note, says no output capacitor is required,
other than to improve transient stability. So you could
rely on the capacitor inside the disk drive for that.
You could use the two of these, placed in parallel,
as your input bypass capacitor. That helps your LM7805,
when the cable coming from the wall wart is long. It is a
form of local energy store for very short transients.
Two of these in parallel, is the minimum recommended value.
You could use a higher value of capacitor if you wanted.
If you purchase a polarized capacitor, you have to remember
to connect the (+) end to the more positive voltage.
http://www.radioshack.com/product/index.jsp?productId=2062365
And this page will show you more than you care to know.
http://www.sparkfun.com/commerce/tutorial_info.php?tutorials_id=57&page=1
You'll notice in the picture here, the LM7805 is running
without a heatsink. If it is not called on to deliver
very much current, then what they're doing here is fine. But
when you run a 500mA 2.5" disk drive, you need a heatsink
bolted to the tab of the TO-220. Otherwise, your
LM7805 will shut itself off (at 150C ?) .
http://www.sparkfun.com/commerce/im...ngEmbedded/1-PowerSupply/BB-PowerSupply-1.jpg
So, yes, you can build a nice circuit for a few bucks,
as long as the heatsink is free. If you have to buy a
heatsink, it might be cheaper to buy a 5V wall wart...
If you purchase an external 2.5" enclosure kit, like the one
in this example, it comes with a power supply. This one
costs $23 for the whole kit. The only thing we can't tell
from this distance, is whether the output power connector
is the right type.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817106099
Paul
