What wattage resistor should I use?

nevermind1281

Joined Dec 4, 2007
14
Hello. I am going to try and make a surface mount version of the project listed below. What wattage resistors should I choose? Thanks

beenthere

Joined Apr 20, 2004
15,808
With SMT components, sometimes handling is a necessary consideration. You may find that anything below 805 is just too hard to handle. That's 1/8 watt. If power is beyond that, use 1206.

For an exercise, figure current through the resistors. Multiply that by the resistance to get power dissipated.

nevermind1281

Joined Dec 4, 2007
14
Well, I forgot to mention earlier that I am putting this in a wireless Xbox 360 controller so the voltage is going to be 3v max. So Im guessing 1/8 watt would be fine?

scubasteve_911

Joined Dec 27, 2007
1,202
In your case, I would think 1/10W resistors should do. Whenever SMT resistors are used, they are readily paralleled in order to increase power handling. It is common to see many 1W SMT resistors in parallel on a PCB.

You cannot say that it is low voltage, so it will dissipate little power. For example, a 3V supply with a 3 Ohm resistor will draw one ampere, and dissipate I^2*R watts, or 3W.

Steve

nevermind1281

Joined Dec 4, 2007
14
Ok thanks. I am trying to get all of the parts on mouser, but I cant find the 1n914 diode. What diode can I replace it with? I decided to get the 1/8 0805 resistors. Thanks for the help

scubasteve_911

Joined Dec 27, 2007
1,202

nevermind1281

Joined Dec 4, 2007
14
Now the only thing I cant find in SMT is the transistors. Are there any equivilants of the 2n2905 and 2n3053 for SMT that I can use? And I know it says it pulses between 3.5 and 6.8 times per second, if I wanted to increase that what would I do?

thingmaker3

Joined May 16, 2005
5,073
To change the pulse frequency, you'll need to change the RC time constant:

f = 1/(.693 x C1 x (R1 + 2 x R2))

nevermind1281

Joined Dec 4, 2007
14
I did the formula with C1 as .00001, R1 as 10,000, and R2 as 100,000. I got 2.079. The circuit says it pulses between 3.5 and 6.8 timer per second. Did I do someting wrong?

thingmaker3

Joined May 16, 2005
5,073
According to the list, C1 is .000001 rather than .00001

I get: 1/(.693 x 0.000001 x 210000) = 6.8Hz.

I don't know why Jared lists the range. Perhaps one of the resistors is supposed to have been drawn as a potentiometer.