Metal Oxide resistors...whats the difference?

[Illum]

Looking down the long rack of resistors in Radioshack...I feel the familiar rows of tan colored ceremic resistors and then this little grayish blue little thingamajigger labeled "Metal Oxide resistors"

Whats the difference between this and the regular ceremic resistors?

PS, I did a search on it, results inconclusive

um..another question [too small to need another thread], where can I find 6.8 ohm 1 Watt resistors?
can I create this value with resistors I own?
only resistors I have: [1/2 watt]: 33, 22, 15, 10 ohm; [1/4 watt]: 10 ohm; [1 watt] 10 ohm

:thanks:

 

[SpiritLed]

Materials are selected for power handling capability, precision of resistance value required etc.
Metal Oxide applications are typically higher power and pulses of power.
Metal oxide film can be laser cut for precision values.
Those materials are often used in surge suppressors i.e. "MOV" in Vari(able)stor format.

Resistor pairs - reverse engineered calculator (and others):

http://www.computertorture.com/noncompliant/parallel/index.html

 

[LumenHound]

resistor info

parallel resistance calculator

 

[MatajumotorS]

As I think, they have lower noise, or i am wrong?

 

[LumenHound]

Connect 2 33Ω resistors in series to make a 66Ω resistor.
Connect 3 of your 22Ω resistors and your 66Ω resistor in parallel to get a total resistance of 6.6Ω. Close enough.

Alternately, connect 3 of the 33Ω resistors in series to make a 99Ω resistor and then connect the 99Ω resistor in parallel with 3 of your 22Ω resistors to get a total resistance of 6.8276Ω.

 

[Illum]

Thanks for the reply on the Metal Oxide question. so theres really no difference in application other than different materials used

Connect 2 33Ω resistors in series to make a 66Ω resistor.
Connect 3 of your 22Ω resistors and your 66Ω resistor in parallel to get a total resistance of 6.6Ω. Close enough.

Alternately, connect 3 of the 33Ω resistors in series to make a 99Ω resistor and then connect the 99Ω resistor in parallel with 3 of your 22Ω resistors to get a total resistance of 6.8276Ω.

Thanks for the link to the calculator, the resultant resistance may equal to 6.8276, but is it 1 watt?

heres what Im planning to do...create some type of centralized emergency lighting by running the entire assembly on a jumpstarter
Sounds funny already...since I have used CPU heatsinks I figured why not...wire the fans in parallel

 

[MrAl]

Hi there,

The total resistance RT of two resistors in parallel is:
RT=R1*R2/(R1+R2)
Making R1=10 ohms and R2=22 ohms you get 6.875 ohms, which is pretty
close to what you want (6.8 ohms).

The problem, as you guessed, is that the two resistors *also* have to be
able to put up with the power...this means they both have to have at
least a minimum power rating, although it may be different for the two
resistors R1 and R2.

The power in R1 is:
pR1=pT*R2/(R1+R2)
and the power in R2 is:
pR2=pT*R1/(R1+R2)
where
pT is the total power.

Since you need a total power of 1 watt, pT=1 for your application.

Plugging all the values into both equations we get:

pR1=1*22/(10+22)
pR2=1*10/(10+22)

Doing the math results in:
pR1=0.6875 watts
pR2=0.3125 watts

This means the 10 ohm resistor has to be rated for at least 0.6875 watts
and the 22 ohm resistor has to be at least rated for 0.3125 watts.
Since it appears that you do have these resistors on hand, you should be
able to make a 6.875 ohm resistor that has a rating of at least 1 watt
by connecting the 22 ohm 0.5 watt resistor in parallel with the 10 ohm 1 watt
resistor. In fact, the resulting resistor will actually be rated for 1.45 watts
because they both have a rating a little higher than what is actually needed
to make a 1 watt resistor.

 

[Illum]

mmm

wasnt as complicated as I previously thought...and this would save me alot of resistors