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

Hi,

Yes, I use that idea too, and even less than 50 percent to keep power resistors cooler. I cannot put up with power resistors that I can easily fry an egg on, even if they last 100 years like that.

 

So, do we have a consensus that anything 1/4 watt or bigger is ok?
is this the right calculation for watts through the resistor? P=I squared x R? P=1 x 1 x 0.1
so watts = .1
thanks

 

I used 1/2W resistors 61 years ago with vacuum tooobs.

did they even have 1/4 watt resistors back then?

1962 - For video and audio non-power circuits, 1/2 W was the standard small resistor size in tube gear. 1/4 W quickly became the standard in the up-and-coming solid state gear.

ak

 

So, do we have a consensus that anything 1/4 watt or bigger is ok?
is this the right calculation for watts through the resistor? P=I squared x R? P=1 x 1 x 0.1
so watts = .1
thanks

You need to start using units properly. Just like a length is not just a number (if you have two forms filled out for two different people and there's a spot for "Height" and one says 78 and the other says another says 91, which person is taller).

Physical quantities are almost always composed of a scale factor combined with a unit of measure. So the first form might be 78 inches and the second form might be 91 cm.

P = I²R
P = (1 A)(1 A)(0.1 Ω)

The units of Ω are V/A (by definition), so this is

P = (1 A)(1 A)(0.1 V/A)
P = (1)(1 A)(0.1 V)
P = 0.1 A·V

We can either recognize that volts time amperes results in units of watts, or we can work through the units.

1 V = 1 J/C (by definition)
1 C = 1 A·s (by definition)

P = 0.1 A·V
P = 0.1 A·J/C
P = 0.1 (A·J)/(A·s)
P = 0.1 J/s
P = 1 W 0.1 W

The last line is due to 1 W being defined as 1 joule per second.

If you always properly track your units through your work, you will catch the vast majority of silly mistakes you make (and you WILL make them, we all do) almost immediately, saving you time, embarrassment, possibly your job, possibly damage to valuable equipment, possibly someone's life.

EDIT: Fix typo.

 

You need to start using units properly. Just like a length is not just a number (if you have two forms filled out for two different people and there's a spot for "Height" and one says 78 and the other says another says 91, which person is taller).

Physical quantities are almost always composed of a scale factor combined with a unit of measure. So the first form might be 78 inches and the second form might be 91 cm.

P = I²R
P = (1 A)(1 A)(0.1 Ω)

The units of Ω are V/A (by definition), so this is

P = (1 A)(1 A)(0.1 V/A)
P = (1)(1 A)(0.1 V)
P = 0.1 A·V

We can either recognize that volts time amperes results in units of watts, or we can work through the units.

1 V = 1 J/C (by definition)
1 C = 1 A·s (by definition)

P = 0.1 A·V
P = 0.1 A·J/C
P = 0.1 (A·J)/(A·s)
P = 0.1 J/s
P = 1 W

The last line is due to 1 W being defined as 1 joule per second.

If you always properly track your units through your work, you will catch the vast majority of silly mistakes you make (and you WILL make them, we all do) almost immediately, saving you time, embarrassment, possibly your job, possibly damage to valuable equipment, possibly someone's life.

i appreciate your detailed response but you're talking way over my head.
i'm an old man with no aspirations to be an electrical engineer.
so i need a 1 watt resistor???
thanks

 

i appreciate your detailed response but you're talking way over my head.
i'm an old man with no aspirations to be an electrical engineer.
so i need a 1 watt resistor???
thanks

No, I just don't know how to properly copy a number from one line to the next -- unfortunately, tracking the units won't fix that. I corrected it in the post.

 

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

Hello again,

Since your current is 1 amp and your resistor is 0.1 Ohms, that makes the voltage drop:
V=1*0.1=0.1 volts.
The power is V*A which is volts times current, so the power is:
P=0.1*1=0.1 watts.

A good starting point for the power of a resistor is 2 times the calculated power, so we end up with:
P=0.1*2=0.2 watts.

A quarter watt resistor is 0.25 watts, so that should work as others have pointed out.

There is also the question of what it will be used in, what kind of product. Does the product have to move around a lot, and do you have the room for a 1/2 watt resistor. If you have room for 1/2 watt resistor then you can use that as they are more durable, but 1/4 watt should work in a lot of cases.

There is also the question of what your parts stock will look like. If you think you will use these resistors in other projects and you think 1/4 watt resistors will be used in them too, then it's probably better to just by 1/4 watt resistors. I bring this up because often you can get deals on multiple quantities of resistors rather than just buying one. One resistor could be as much as $1 USD and after shipping it could be $7, so buying one resistor would seem like a waste of money when you might get 10 for $1 and still the total cost would be $7 to $8 or something like that.

 

1962 - For video and audio non-power circuits, 1/2 W was the standard small resistor size in tube gear. 1/4 W quickly became the standard in the up-and-coming solid state gear.

ak

Hi,

Yeah, you know what is funny, I remember some CRT television PC boards as large as 2 feet by 1 foot that's an area of 2 square feet. Amazing how things have changed from the 1960's to now. You can watch TV on your little cell phone.
Going by the volume of that CRT TV and the cell phone, that's like a volumetric ratio of something like 4000 to 1. That's like a ratio change of over 60 to 1 per year on average. Makes you think of what is to come, cell phone on a chip? ha ha.

 

Hello again,

Since your current is 1 amp and your resistor is 0.1 Ohms, that makes the voltage drop:
V=1*0.1=0.1 volts.
The power is V*A which is volts times current, so the power is:
P=0.1*1=0.1 watts.

A good starting point for the power of a resistor is 2 times the calculated power, so we end up with:
P=0.1*2=0.2 watts.

A quarter watt resistor is 0.25 watts, so that should work as others have pointed out.

There is also the question of what it will be used in, what kind of product. Does the product have to move around a lot, and do you have the room for a 1/2 watt resistor. If you have room for 1/2 watt resistor then you can use that as they are more durable, but 1/4 watt should work in a lot of cases.

There is also the question of what your parts stock will look like. If you think you will use these resistors in other projects and you think 1/4 watt resistors will be used in them too, then it's probably better to just by 1/4 watt resistors. I bring this up because often you can get deals on multiple quantities of resistors rather than just buying one. One resistor could be as much as $1 USD and after shipping it could be $7, so buying one resistor would seem like a waste of money when you might get 10 for $1 and still the total cost would be $7 to $8 or something like that.

now i understand
THANK YOU VERY MUCH

 

now i understand
THANK YOU VERY MUCH

Hi,

You're welcome and hope your project works out well. Maybe keep us updated.