# resistor wattage curiosity

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I haven't looked at a resistor in 6 or 7 million years, much less use one.
But I have one in front of me now that I'm wondering what the wattage is,
and I have no clue how to locate such info.

The resistor is about 3.5 mm long and 1.7 mm in diameter.  Does that tell
anything?  (It takes a small microscope to read the color bands - orange,
orange, orange, gold)

The resistor drops a power supply voltage from 12v to 5v.  Just guessing I'd
say, considering the resistor size, the PS probably wouldn't run a freight
train...

Thanks

--
JimL

The purpose of computers is to complicate life for complicated people;
simple people can go suck eggs. Winstun Churchhill

## Re: resistor wattage curiosity

JimL wrote:

a 33K ohm resistor. Scroll down to the "colored bars" table.

http://en.wikipedia.org/wiki/Resistor

Next, I headed over to Digikey. A 33K 5% resistor.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=33KH-ND

Your resistor could be a 1/4 watt.

Paul

## Re: resistor wattage curiosity

In the upper left photo in the PDF the smallest resistors are very nearly
actual size.  If I got it right, the math says about a 360 ma (12v/33ohms)
so 1/4 watt would seem a tad on the light side.  Not surprising for "Dollar
Store" stuff I guess.

Thanks

--
JimL

The purpose of computers is to complicate life for complicated people;
simple people can go suck eggs. Winstun Churchhill

## Re: resistor wattage curiosity

JimL wrote:

I think three orange bars on a resistor, is 33 *thousand* ohms, of which the
shorthand is 33K. If you stick that across 12V, not a lot of power will be
dissipated. If we use

P = V**2 = 12*12 = 0.004 watts
----   -----
R    33000

Double check the band colors again on the resistor. Maybe the third
band is a color other than orange. A lower resistance value,
will help get you into an area where you can "cook" that 1/4 watt
resistor.

What exactly are you attempting to do ? Describe your application
a little bit.

Paul

## Re: resistor wattage curiosity

It's a dual voltage thing with the resistor in series to another pin.

Just looking for a cheap 5v 1a supply for an external enclosure.  RS has one
for \$35.  Online I can get them for \$4 or \$5 if I want to put my credit card
in the public domain.

--
JimL

The purpose of computers is to complicate life for complicated people;
simple people can go suck eggs. Winstun Churchhill

## Re: resistor wattage curiosity

JimL wrote:
JimL wrote:

Not 12 volts.  You are dropping seven volts across that resistor (12V - 5V).

Theoretical current dissipated in that resistor is then given by 7V/33R =
0.21 W.

If you were building something to last and not catch fire, you would want to
use a 1/2 watt resistor in that application.

Jon

## Re: resistor wattage curiosity

Which is part of the reason I gave up on electronics engineering.  (The
other bigger reason was a stroke.)

Given the state of todays actual engineering I'd guess that the "cable" on
the wall wart would melt down at anything other than about 1/16 watt.

Thanks

--
JimL

The purpose of computers is to complicate life for complicated people;
simple people can go suck eggs. Winstun Churchhill

## Re: resistor wattage curiosity

Jon Danniken wrote:

Sorry, but power equals voltage *squared* divided by resistance. So for 7v
and 33 ohms, it would be 49/33, or 1.48 watts.

## Re: resistor wattage curiosity

UCLAN wrote:

Meh.  You are, of course, correct.

/reminds self not to post anything math related before breakfast.

Jon

## Re: resistor wattage curiosity

My rule is not to post anything math related before tomorrow.

--
JimL

The purpose of computers is to complicate life for complicated people;
simple people can go suck eggs. Winstun Churchhill

## Re: resistor wattage curiosity

I would say it's a 1/8 watt resistor.  I have a complete set
of all 1/4 watt 5% resistor values, manufactured by IRC
and Ohmite, and they are all 7 mm long by 2mm diameter.

## Re: resistor wattage curiosity

Makes sense to me.

Thanks.

--
JimL

The purpose of computers is to complicate life for complicated people;
simple people can go suck eggs. Winstun Churchhill

## Re: resistor wattage curiosity

On Wed, 16 Sep 2009 21:42:22 -0400, "JimL"

Resistor wattage is a matter of size, heat sheding.

At that size it is 1/4 w at most but more important is why
you ask, what you need of it.

A resistor drops a PSU voltage based on the current,
regardless of it's wattage rating till it fails.  OHM's law
applies.

## Re: resistor wattage curiosity

Agree with Kony
sounds like a 1/4 watt...  unless of course it's a metal film type,
these can be deceivingly small for their wattage, standard types are
usually of a carbon composition, the better quality ones being metal
film.

Here is a resistor 'colour code calculator '
(http://tinyurl.com/n5helv )if you are interested

davy

## Re: resistor wattage curiosity

Thanks

--
JimL

The purpose of computers is to complicate life for complicated people;
simple people can go suck eggs. Winstun Churchhill

## Re: resistor wattage curiosity

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

--
JimL

The purpose of computers is to complicate life for complicated people;
simple people can go suck eggs. Winstun Churchhill

## Re: resistor wattage curiosity

JimL wrote:

To make 5 volts from 12 volts, this is what you use.

"+5V Fixed-Voltage Regulator 7805"  Output Voltage: +5VDC @ 1A

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
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
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,
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.

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/images/tutorials/BeginningEmbedded/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

## Re: resistor wattage curiosity

<huge snip of stuff I would have understood once upon a time)

That one's nice, but this is an IDE enclosure.  In two days the only wall
wart I found that even ALLOWED non-USB power was the one I have.  I guess I
don't have enough appreciation for the current tendency to power everything
but the cat off USB ports.  I know you can get powered USB hubs, but this is
a Poverty Level Computer Center (\$100 T42 on a \$10 desk by a \$5 window -
literally) and there are even more important things that won't be had.

Thanks

--
JimL

The purpose of computers is to complicate life for complicated people;
simple people can go suck eggs. Winstun Churchhill

## Re: resistor wattage curiosity

JimL wrote:

The purpose of my post, was to hint that a voltage
regulator provides a smooth 5V output from a 12V source,
and does so no matter what the current flow. You could
not connect a resistor to a 12V supply like this, and
have it work. This.. is.. bad.

+12V ------ resistor ------- (+5V) ----- 2.5" disk drive ---- GND

If you did that, as the current drawn by the 2.5" disk drive
varies, the voltage level at the drive would also be
changing all over the place. You could get enough of a shift
to damage the drive. In short -- don't do it with a
single resistor like that.

The regulator concept delivers exactly 5 volts, at any current draw
from 0 milliamps to 1000 milliamps. That is why I described
the regulator solution.

+12V ------ LM7805 ------- (+5V) ----- 2.5" disk drive ---- GND
|
GND

A wall wart, such as the one on that enclosure kit, is also
a regulated source. If you buy an enclosure, with an
where the USB bus has insufficient power for the drive.

So the most efficient solution is to buy a raw drive
mechanism, an external enclosure including its own adapter,
and assemble it. That way, the adapter connector and the
enclosure match, and you're all set.

Paul

## Re: resistor wattage curiosity

Agreed, a cheap 7805 regulator is the way to go, then there's no need to
calculate or know anything.

That resistor is the same size as every 1/8w I've used.