Series resistor in power supply protection?

[blapcb]

For my power supply in automotive project, I understand that I should have a series input resistor to help protect against high current. But, I am not sure how to figure out the value and wattage (have read anywhere from 3ohm to 10ohm). Also have read that it should not be SMD type but axial (and high wattage, with no tracks below, not sure why). Can someone explain please? Thanks.

 

[Engr Saeed]

first you should know the total current required by your circut.divide it by 1.25 to get the factor of safty.now apply


V=IR..............................................to get the resistor value.

eg, supply voltage is 12v ...... and the current required is 4 amps.......
the series resistor should be of value


12/4=3 ohm.



now the wattage of this resistance should be about 5w or 7w and it should not be of smd type coz high curent flow resul heat generation and to desipate heat there should be proper air gap b/w res and pcb..........

 

[ea.arun]

There are high wattage resistor made of separate process to withstand high inrush/surge/pulse input current.....and many are with lead.......this is to protect the PCB from damaging from overheating....

 

[blapcb]

The basic current used is say 100-200mA but this is to protect against a load dump and/or transient voltage in automotive project.... so not sure what current to use.

first you should know the total current required by your circut.divide it by 1.25 to get the factor of safty.now apply


V=IR..............................................to get the resistor value.

eg, supply voltage is 12v ...... and the current required is 4 amps.......
the series resistor should be of value


12/4=3 ohm.



now the wattage of this resistance should be about 5w or 7w and it should not be of smd type coz high curent flow resul heat generation and to desipate heat there should be proper air gap b/w res and pcb..........

 

[ea.arun]

can you please post the configuration you are using??

 

[blapcb]

can you please post the configuration you are using??

This is what I am planning to protect against:
- Reverse polarity
- Load dumps
- Transients
And I am not sure if it is good enough

 

[xaccto]

I use Polyswitch resistor devices these days for my 12V automotive type projects. Check em out.

And you indicate using a switching supply chip after it, why overly waste heat in a resistor then ? Polyswitch, choose your average current, and the hold/fault current you want to choose your selection.

 

[blapcb]

My understanding is that it is proper to use a series resistor to protect against inrush currents (it's an automotive application). See figure 8 on last page here (for example):
https://www.vishay.com/docs/88490/tvs.pdf

I use Polyswitch resistor devices these days for my 12V automotive type projects. Check em out.

And you indicate using a switching supply chip after it, why overly waste heat in a resistor then ? Polyswitch, choose your average current, and the hold/fault current you want to choose your selection.

 

[xaccto]

then combine Polyswitch with TVS then for ultimate/best protection.

I actually usually use reverse protection diode and a 18V zener diode. The polyswitch
then does the job of limiting to the fault current the diodes can handle.

Hmm...TVS is kinda like a zener diode......

 

[leo_o2]

Exact value depends on application. Too big resistance will limit sourcing capability.
Too small resistance can't limit the current for the zener diode.