component package size 0805 and 0402

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#1
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Hi All

Other then size, are there any futher differnce between two differnt package sizes.

I am using a refernce design that require 0402 package for all its caps,resistor,inductors.

Now i previously i made various other circuits usinf 0805, now as i am joing two circuits togeather, if i used 0402 instead of the 0805 package, would i nessary see any change.

I presume not, please do advice.

DJ

Thanks

Regards

DJ

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You shouldn't have any problems using 0402 instead of 0805. You need to check voltage and power ratings, of course.

Leon

Leon Heller G1HSM

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0402 resistors might not have text on them

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djoshi wrote:
if i used 0402 instead of the 0805 package, would i nessary see any change.

0402 has lower power and voltage ratings than 0805. Refer to the 0402 datasheets to see if any volatage and power exceeds the maximum rating of your components.
A rule of thumb for reliability reasons is to always pick resistors with a maximum power rating of at least twice the maximum power that will dissipated in them at any given time.

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Well the max power i will have is 5V with 1A, and that for my Li-ion charger,

Rest will be 6V 400mA, that for a DC-DC which very small pulse.

other then that it just the avr.

So i think i will be ok

DJ

Thanks

Regards

DJ

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Quote:

Well the max power i will have is 5V with 1A

Quote:

i think i will be ok

Your 0402 resistor can handle 5W continuous? DigiKey listings indicate like 1/8W to 1/32W.

(And I'm a bit confused--are you trying to put an 0402 resistor onto 0805 pads? Will it reach?)

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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No this is fresh pcb. So all pads will be 0402.

The 5W does not go through any resistor, the resistor are used on compoent to ether set the current, feedback voltage, or set freq speed.

DJ

Thanks

Regards

DJ

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There's a quick way to see if all resistors can handle the power dissipation if you know the maximum voltage anywhere in the circuit is 6 V.

I don't know what the maximum power rating is for your 0402 resistors, but let's say it's 1/16 W which is the typical power rating for standard 0402 resistors.
If you decide to never dissipate more than half the resistors maximum power rating in the resistor for reliability reasons, then you must newer dissipate more than 1/32 W in any resistor.

Then you can calculate the minimum 0402 resistor value (Rmin) allowed in the circuit if it's exposed to the max voltage of 6 V.
Rmin = Vmax^2 / Pmax
Rmin = 6V^2 / (1/32)W = 1152 ohms

Then check if you have any resitors in your circut below 1152 ohms. If no you are fine, if yes then you have to check the maximum voltage across this resistor. If it's the full voltage of 6V then you have exceeded the Pmax of 1/32W you decided earlier and need to change the circuit. But maybe there's newer the full voltage of 6V across this resistor so you might not have to do anything if the power dissipation is below your Pmax.

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Would i be correct in saying the only time an issue with 1A of current would be if resistor is in series.

The Li-ion charge IC Lm3658, use it a current limit set, i am sure that current should of high values does not go via this, so i think i should be ok.

What i will do is design my PCB for 0805, but if i find i am requiing more space then i will decrese it to 0402, as i am limited to a pcb size.

Dj

Thanks

Regards

DJ

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You could try the intermediate size 0603.

For manual assembly I find 0805 is easy to handle, 0603 is quite more difficult and I won't dare to handsolder 0402 unless there's absolutely no alternative.