Hi,

I want to place a resistor between my main battery terminal and the rest of the PCB, this resistor will be a 0 ohm 0.25w resistor and is used like a link, the project will run at 3.9v with a maximum of 2A in short bursts, will a 0.25w 0R resistor be OK

Thx

Steve

 

Because of the fact that a zero-ohm resistor is zero ohms, it will have a negligible voltage drop and dissipate very little power. That "0.25 watt" thing is more a physical description of the size than anything. Yes, it may be rated for 0.25 watt, but you'd have to run a pile of current through it to ever dissipate that much heat.

Dean

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Thanks Dean

Always helpfull on this site

 

Just to empasise the point, do some maths.

W = I x I x R

so, if I = 0 or R = 0, the W must be zero.

If no power is dissipated in the "resistor", then a 0.25w "rating" is fine.

JimB

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Use a fuse rated slightly above the average DC current draw.

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In case of 0R resistors we caluclate the wattage with the maximum tolerance of the resistor
If you check the datasheet there will be some maximum value given for the resistor
use that resistor value and current to caluclate the power dissipation in the resistor

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Here is part of a table from Vishay. It looks like the smallest part that meets your requirements is an 0805, which, as you know, is pretty small. As you can see, power dissipation doesn't tell the whole story, if you want to stick to the specs. I suspect you'll find similar specs from other mfrs.

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Ron

 

The obvious question is "why use a zero ohm resistor?" - why not just use a wire link?. The only 'advantage' of zero ohm resistors is that they can be fitted by automatic component insertion machines - if the board is being built by hand a wire link may as well be used.

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I have seen these fitted ( or other very low Ohm units ) as a sort of "sacrificial" component like a fuse. Often where it has been used is where space is limited, and there is a very low likelyhood of it needing to do it's job. Cost is less than a pico fuse or similar ( I can buy a thousand 10R for about 10 dollars, way less than any fuse ). Modems come to mind, where there are often a pair of 6R8 or similar resistors on the incoming phoneline. A surge on the line will quickly open a metal film resistor, but in use it has very little effect on the circuit.

We often do the same at work, placing 10R metal film resistor on the gate leads of paralleled FET's, improves the operation and also prevents a shorted FET from doing damge to the rest of the circuitry.