basic resistor/capacitor question

[carlcarl2]

if you have 9 volt power supply (basic 9v battery) and I have like a R 6.8k 1% (all resistors are 1%) and C 1pf 2% (all capacitors are 2%) when I buy them, am i buying 1/4,1/2,....? Watts? Thanks

 

[v_c]

The power rating, etc. should be in the part specifications. What power rating you need depends on your application. Are you asking what you *need* to buy based on your application? The percent tolerance that you are stating does not necessarily have anything to do with the power rating.

Best regards,
\[v_c\]

 

[carlcarl2]

Thanks for the reply. I guess based on using a standard 9v battery, do i need to buy 1/8 , 1/4, 1/2watt ones or more to be safe and work??? I think i will have an output max of 1 watt.

 

[btbass]

The power dissipated in the resistor can easily be worked out. It depends on the current passing hrough it and the voltage across it.

Ohms law states: Volts = current(Amps) times Resistance (ohms), or

V = I * R;

Power in Watts = V * I;

If you have 9 Volts across 6k8.

I = V/R
I = 9/6800
I = ~ .001

Watts = V * I;

Watts = 9 * 9 / 6800;
Watts = (V squared) / 6800;

The reciprical being

Watts = (I squared) * R;

Power can be worked out using the current or the voltage

W = (I squared) * R;
W = (V squared) / R;

Your 6k8 resistor with 9 volts across it will be dissapating less than 10 milli Watt or less than 1/100 of a Watt.

As far as the caps go, for small signal purposes, you just need to worry that they meet or exceed the working voltage.

 

[xulfee]

it is power rating,check out ur specification n get it