# Power dissipation in a resistance

#### Tesla86

Joined Oct 25, 2016
34
Dear friends,

I've developed an electronic design in which I've included a current sensor. The input current range is 4mA - 20mA. To measure this current I've designed the simply system depicted in the image attached (Capture.JPG), based on a resistance of 250ohm connected to ground, so a (4mA, 20mA) corresponds to (1V, 5V) range, which is registered in one ADC channel of an Arduino.

The question is I've solder a 250 ohm SMD 0603 resistance with this characteristic:

- Max dissipation @ Tamb=70ºC ------- 1/10W (0.1W)

If the input current is the maximum in my range (20mA), the dissipated power in the resistance will be 0.1W. Notice that Tamb in my system will be rooom temperature aprox.

The point is that I'm not sure if this margin is a poor margin and consequently I've a mistake in my design (the resistance is already soldered to the pcb...).

Do you think this design is OK or I shoud resolder another kind of resistance?

Thanks!

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#### Dodgydave

Joined Jun 22, 2012
8,294
I would go with a bigger wattage like 1/8, 0.125 or 1/4 0.25, to be safe...

#### thumb2

Joined Oct 4, 2015
122
Never never never reach the absolute maximum ratings.
If the power dissipation is 0.1 W, chose a resistor with power dissipation greater than 0.1 W.

#### kubeek

Joined Sep 20, 2005
5,587
I´d go with 0805 as well, you can usually solder a one size bigger package on the smaller pads without a lot of hassle.

#### mcgyvr

Joined Oct 15, 2009
5,394
ALWAYS go 2-3 times calculated max.. No less ever unless that resistor is intended to actually be a heater..
So if you calculate 1W then use a 2 or 3W resistor..

#### #12

Joined Nov 30, 2010
18,076
The problem with an exact wattage match is that the resistor will heat to its design max temperature. I don't think you want a tiny spot at 100 degrees C on your circuit board. That's why people use at least double the actual watts for their resistor rating.

#### wayneh

Joined Sep 9, 2010
16,102
One more vote for at least a 2X safety factor if at all possible.

#12

#### EM Fields

Joined Jun 8, 2016
583
Most resistors are parametric, so when the resistor heats up, its resistance will change - the change depending on the tempco of the resistor - and, for a given tempco, the smaller the package the more its resistance will change as the power through it increases.

That'll affect the accuracy of the signal sent to the ADC, so it would be prudent to select a resistor with a physical volume which would take that into consideration .

There are also lifetime considerations in that the hotter the resistor runs, the shorter its time to failure will be.

#### Tesla86

Joined Oct 25, 2016
34
OK, so after your advice I'll go for one of 0.5W (0805), just to be sure everything works fine (5X safety, I know maybe it's too much but it's cheaper than 0.25W 0603 (2.5 safety)). Thank you very much!

#### mcgyvr

Joined Oct 15, 2009
5,394
As long as it fits in your design and is appropriately priced then go for it.. And "overkill" is really not an issue here..

#12

#### Tesla86

Joined Oct 25, 2016
34
As long as it fits in your design and is appropriately priced then go for it.. And "overkill" is really not an issue here..
Do you mean the bigger the safety factor the better, don't you?

#### #12

Joined Nov 30, 2010
18,076
Do you mean the bigger the safety factor the better, don't you?
Up to a point. Nobody would install a 50 watt resistor for a 0.1 watt need, but plenty of designs use the cheapest over-size part to get the safety factor nice and generous. Consider schematics from 50 years ago. They say, "All resistors 1/2 watt unless specified." That's because no manufacturer wanted to also stock the more expensive 1/4 watt resistors when 1/2 watt will do the job.

#### EM Fields

Joined Jun 8, 2016
583
OK, so after your advice I'll go for one of 0.5W (0805), just to be sure everything works fine (5X safety, I know maybe it's too much but it's cheaper than 0.25W 0603 (2.5 safety)). Thank you very much!
My pleasure.

If I buy one of your widgets and it doesn't blow up because you weren't chinchy with that resistor, you'll have made my life less onerous.