Need VALUE for resistor..

[Kasamiko]

Hi All,
Please suggest a good value for this resistor (Ohm, Wattage)
Thanks..

rhonn

 

[Yevgenip]

What is the resistance of the other components? or can it be neglected?

 

[audioguru2]

Hi Rhonn,
The 12V output won't stay at 12V with either a 110V mains or a 220V mains unless it has a 5W zener diode across it.

Drop the voltage with a 1K resistor.
With a 110VAC mains, the 12V load's current is 100mA and the zener's current is 42mA.
With a 220VAC mains, the load's current is still 100mA but the zener's current is 198mA. If the load becomes disconnected, the zener's current will be 298mA and it will dissipate 3.6W.

With a 110V mains, the 1K resistor will dissipate 20.2W.
With a 220V mains the 1K resistor will dissipate 88.8W.

If you use a 1.5K resistor, the output voltage will be less than 12V and the zener won't be regulating with a 110V mains. With a 220V mains, the zener will dissipate 2.4W without a load (1.2W with the 100mA load) and the resistor will dissipate 59.6W.

You know how hot resistors get at their rating. You will need a resistor rated for 100W or 200W!
A little transformer would be much smaller, cheaper and safer. ;D

 

[Kevin Weddle]

There is a better way to accomplish this task. Take the output of the rectifier and add a series capacitor at 1000uF, a series resistor at 20 ohms, and a capacitor to ground at 1000uF. Tap the output across the capacitor to ground. This should give you the DC your require and extra filtering because of the resistor.

The voltage of the first capacitor needs to be about 150.

 

[Kasamiko]

@ALL
I think a small transformer, about 200 mA will do the job safer and cooler.. ;D
Thanks for all the replies..

rhonn