What's the purpose of series resistance and how to calculate it?

[saurav_sdpl]

Generally in power supply design we place a 30 ohm/4W resistance in series with mains supply, before giving it to bridge, wat is the purpose of this resistance and how to calculate this value

 

[yaqoob]

series resistance

this resestance is for voltage drop and current limiter
you should know the voltage you need and the currrent you need
if you know power you can calculate simply using Ohms law this resistance buy voltage divider

 

[hamdard]

Re: series resistance

Mr. Yaqoob has given correct answer.
If you know the output voltage and current, the ressistance=(Vout-Vin)/Iout

 

[Btrend]

Re: series resistance

Generally in power supply design we place a 30 ohm/4W resistance in series with mains supply, before giving it to bridge, wat is the purpose of this resistance and how to calculate this value

Are u sure it is a "30 ohm/4W " resistance ?
a 30 ohm will burn down when current flow larger than 365mA.

 

[Kral]

Re: series resistance

saurav_sdpl,
It's purpose is probably to limit the inrush current. Its value would be a function of the load current. Often PTC thermistors are used for this purpose. The initial resistance is high, but as the PTC heats up, its value decreases dramatically, reducing the power loss and voltage drop across the PTC.
Regards,
Kral

 

[cameo_2007]

series resistance

i too have come across an SMPS circuit...where there are series resistors at each R,Y,B inputs and neutral.
its not an NTC..just wire wound resistors..
can some one give a clear idea on that..