Hi
I have a system with an RC filter in a 6V power rail.
It is 5R and 100uF.
During normal operation the power rating of the 0805 0.125W resistor is fine.
However at power on there is an inrush current of 0.4A for 2ms.
During this 2ms the power in the 0.125W resistor is 0.78W.
As its such a short time -2ms- I am assuming it would just be fine.
Am I correct to assume this?
How could I quantify this in design documentation to confirm it would be fine? (e.g. relating a datasheet specification etc)
I would really appreciate any thoughts anyone has.
Why don't you change your resistor for a 1Watt resistor?
Otherwise you can put resistor in series or parallel to reduce power dissipation per resistor.
I dont want to respin the PCB. I am just looking for a way to quantify that the resistor wont be damaged. (As the inrush only happens at power on and is incredibly brief (2ms))
So how often does this power on current occur and what is the normal current? From these you can work out what is the average dissipation over a reasonable time. i.e. if you get the surge every 5 mS and the normal dissipation is 110 mW, then it looks bad. if the surge occurs every day and the running power is 50 mW it looks OK. Perhaps you could find the thermal time constant from the manufacturer. You also set up a test jig to switch the unit on/off every 10 S. This would give you sufficient data over a week end to guess the life of the resistor.
Frank
* Many resistor datasheets give the pulse load rating, often in a chart.
* select thick film carbon resistors, they are more rugged against pulse load.
* There are special pulse load resistors, also called as surge proof resistors.
Any metal film (or wire wound) resistor the explosive pulse of heat is generated in the thin metal layer which expands. Over time the metal will fatigue and fail open circuit.
Thick film carbon resistors generate the heat all the way through the body of the resistor, not just on the surface, and therefore be much less prone to failure.