# Current sense resistor wattage requirement?

#### joewales44

Joined Oct 8, 2017
218
I'm driving LEDs with a PAM2861 with 12 volts running at 1 amp.
Current sense resistor value is 0.1 ohm.
I've been using 10 - 1 ohm 1/4w resistors in parallel with no problems but due to new design with limited board space, i need to go to a single resistor.
What wattage size does the sense resistor need to be?
Thanks

https://www.diodes.com/assets/Datasheets/PAM2861.pdf

Joined Feb 20, 2016
4,426
Have a go at working out the actual dissipation, then go for a resistor with a higher power rating than the calculated value.
You have the current and the resistance.
Here is a handy Ohms law wheel.

#### BobTPH

Joined Jun 5, 2013
8,659
Seriously? You are designing a circuit without knowing how to compute the power in a resistor?

#### Dodgydave

Joined Jun 22, 2012
11,234
0.1 ohms 100mW, so anything over 1/4 W will do.

#### Audioguru again

Joined Oct 21, 2019
6,606
Why not use a 0.1 ohms 1/4W resistor? It will be fine and get a little warm.

#### WBahn

Joined Mar 31, 2012
29,844
The title says that you are using this as a "current sense" resistor.

Be aware that resistance is a function of temperature, and hence as your resistor heats, the resistance will change and your current sense readings will be inaccurate. This may or may not matter for your application.

The first step is to consider how accurate/precise you need your current readings to be. Nominally, your voltage would be 100 mV at 1 A through your sense resistor.

Would it be a significant issue if, at 100 mV, the actual current was 1.1 A or 0.9 A? If not, then you are probably pretty safe ignoring this issue. But if that's very important, then you need to consider things deeper.

First, you have to ask yourself whether accuracy or precision is most important. Perhaps both are, but usually, one or the other is more critical.

#### BobTPH

Joined Jun 5, 2013
8,659
Current through LEDs is not typically critical.

#### ronsimpson

Joined Oct 7, 2019
2,886
Your eye can't see a 10% change in light.
When building a volt/amp meter the value verses temperature is important.
When building a flashlight not so much.
At digikey.com I searched for 0.1 ohm 1/4 watt surface mount resistors. They have 25 different types in stock. On the first page there are parts with 75parts/million/C. Some at 100ppp, 200, 300 and some at 600ppm. If money is not a problem, they have parts at 2ppm.
I think you will have to look long and hard to find a modern resistor that will change enough for the brightness to be affected noticeability.

#### bertus

Joined Apr 5, 2008
22,264
Hello,

The reference voltage for the current sens seems to be between 95 and 105 mV:

As for the resistor this is given in the datasheet:

There is a Nominal current given.

Bertus

#### joewales44

Joined Oct 8, 2017
218
0.1 ohms 100mW, so anything over 1/4 W will do.
that's the answer i was looking for. thank you very much.

#### joewales44

Joined Oct 8, 2017
218
The title says that you are using this as a "current sense" resistor.

Be aware that resistance is a function of temperature, and hence as your resistor heats, the resistance will change and your current sense readings will be inaccurate. This may or may not matter for your application.

The first step is to consider how accurate/precise you need your current readings to be. Nominally, your voltage would be 100 mV at 1 A through your sense resistor.

Would it be a significant issue if, at 100 mV, the actual current was 1.1 A or 0.9 A? If not, then you are probably pretty safe ignoring this issue. But if that's very important, then you need to consider things deeper.

First, you have to ask yourself whether accuracy or precision is most important. Perhaps both are, but usually, one or the other is more critical.
accuracy is not critical and i plan to use a .011 resistor to give a little safeguard to stay below 1 amp.
THANK YOU

#### joewales44

Joined Oct 8, 2017
218
Seriously? You are designing a circuit without knowing how to compute the power in a resistor?
i'm a mechanical engineer, not electrical as you can tell but i try to understand what i can.
i'm using the circuit provided by the chip maker.
when i look at the circuit, it seems all the power to the LEDs has to go through the sense resistor?
what am i missing?
thanks

#### joewales44

Joined Oct 8, 2017
218
Why not use a 0.1 ohms 1/4W resistor? It will be fine and get a little warm.
THANK YOU

#### MrAl

Joined Jun 17, 2014
11,256
Hi,

1/2 watt resistors are more durable physically than 1/4 watters.

#### AnalogKid

Joined Aug 1, 2013
10,943
A starting point rule of thumb for reliable electronic circuits is to run the parts at 50% or less of their ratings. 24 V circuit - use transistors and capacitors rated for at least 50 V. 0.1 W power in a resistor - use a quarter-watt part. Some companies have strict mandatory rules about this. Of course there are exceptions, but the 50% thing is a common number.

ak

#### WBahn

Joined Mar 31, 2012
29,844
accuracy is not critical and i plan to use a .011 resistor to give a little safeguard to stay below 1 amp.
THANK YOU
Is this resistor there to sense the current, or to limit the current?

#### bertus

Joined Apr 5, 2008
22,264
Hello,

@WBahn , It is a current sense resistor. It is placed between Vin and the Isense connections of the chip:

Bertus

#### joewales44

Joined Oct 8, 2017
218
Is this resistor there to sense the current, or to limit the current?
to sense the current.
but it looks like the current goes through it to the LEDs but others say a 1/4 watt is enough.
i don't know.

#### Audioguru again

Joined Oct 21, 2019
6,606
Hi,

1/2 watt resistors are more durable physically than 1/4 watters.
I used 1/2W resistors 61 years ago with vacuum tooobs.

#### MrAl

Joined Jun 17, 2014
11,256
I used 1/2W resistors 61 years ago with vacuum tooobs.
Hi there,

Oh yeah, and did they even have 1/4 watt resistors back then? I don't remember now but I don't remember seeing any.
I had a number of tube stuff but they are all long gone now. I neighbor wanted to play around with a VT device so he bought a two tube audio amplifier kit off the web, I think Amazon. They still sell that stuff for the novelty of it I guess.
I used to have a huge collection of vacuum tubes when i was around 15 years old. Not sure what happened to them now.

I often bring up 1/2 watt resistors simply because of the heavier construction of many of the models being sold today. In devices that have to vibrate around a lot or subject to mishandling they seem to work out better, and in some military applications 1/2 watters are preferred.

I don't even have that many 1/2 watters around anymore. Mainly 1/4 watters and even 1/8 watters, as well as a bunch of ceramic power resistors. That's the through hole type. Of course SMD types are pretty good too.