Current limiting resistor to 12V zener

[siddarth.ghaste]

Hi,
I am giving 220VDC to 12V zener through 22K resistor of 2W. but someone told that I should use double of 2W. How much wattage is good for continuous operation.Please tell me....

 

[nitishn5]

If you calculate the power dissipated by the resistor,

P = V² / R
V = 220 - 12 = 208
R = 22kΩ

P = 1.966W

This is close to the rated value and unless you are using something to dissipate the heat, it will most likely burn up.
You would be better off if you use a higher watt resistor.
It would also be useful to ensure whether the resistor can handle this high voltage. 208V is a big amount. You can just put few resistors in series. This way you can use lower wattage resistors since power dissipation for each would be less.

 

[Tahmid]

Apart from using a sufficiently rated resistor, make sure you have some way of dissipating the heat. For example, ceramic resistors may reach temperatures as high as 200°C under normal operation. While this won't damage the resistor itself, it may be problematic for nearby components and solder connections as well. So, make sure you have some way to dissipate the heat and use a resistor (type) with low thermal resistance.

Hope this helps.
Tahmid.