Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Power-on inrush current and resistor power rating

Status
Not open for further replies.
J

J_M_B

Junior Member level 2
Joined
Jul 24, 2015
Messages
23
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Activity points
239
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.

Thanks
Jamie
 

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.
 

Hi

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))

Thanks
 

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
 

Hi,

* 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.

Klaus
 

KlausST said:
Hi,
* select thick film carbon resistors, they are more rugged against pulse load.
Click to expand...

This is pretty important for reliability.

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.
 

Sorry have been out of office...
Thankyou everyone for the suggestions I have a few things to work with now.

Cheers
Jamie
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top