Resistor wattage.  - Page 1
Forum section: electronics / beginners

author: --Oz--
author_rank: Regular Contributor
timestamp: June 27, 2023, 04:34:28 pm
content: If you have a 0.24 ohm resistor and it's got 1.24v across it with 5 amps flowing, what is the wattage?That should be simple, buy why does this revised 20+ year old LM338 datasheet show 2W?

author: Zero999
author_rank: Super Contributor
timestamp: June 27, 2023, 05:01:59 pm
content: P = V2/R.It isn't unusual for a data sheet to contain errors. Always check.

author: mariush
author_rank: Super Contributor
timestamp: June 27, 2023, 05:13:57 pm
content: Voltage = Current x ResistancePower = I2x ResistanceP = 5 x 5 x 0.24 = 6 watts , so you'd want a resistor rated for at least 8-10 watts.They chose 0.24 ohm because the regulator has a 1.24v internal reference and you set the current limit using formula I = Vref/R ... in this case I = 1.24 / 0.24 = ~ 5.16AAs for the 2w rating, yeah, I can only assume they're wrong, it seems low

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 27, 2023, 05:21:32 pm
content: I assume the datasheet was copied from a LM317, (with probably a 1Ohm resistor and 1.25A) and they did change the resistor value, but overlooked the power dissipation.To check this, verify it with other datasheets from the same manufacturer.

author: --Oz--
author_rank: Regular Contributor
timestamp: June 27, 2023, 11:22:46 pm
content: ST and Ti show identical 2W errors. LT does not show that schematic in it's datasheet.But it makes sense 2W is for the smaller 1.5A max part. Thx guys!

author: radiolistener
author_rank: Super Contributor
timestamp: June 28, 2023, 12:37:09 am
content: Quote from: --Oz-- on June 27, 2023, 04:34:28 pmEven if you use resistor with high enough power rating it still getting hot and it leads to change it's resistance. In order to reduce that effect, you're needs to use resistor with more high power rating, it will be more cold.Regarding to power rating for 5 Amps and 1.24 V across resistor, the dissipated power on resistor will be:P = U * I = 1.24 * 5 = 6.2 WSo, you're needs resistor with at least 10 W power rating

author: MrAl
author_rank: Super Contributor
timestamp: June 28, 2023, 06:36:54 am
content: Quote from: --Oz-- on June 27, 2023, 04:34:28 pmHi,My guess is that it could be just a typo, which happens when non-technical people write out technical papers sometimes, in fact often.Since the approximate power is 6 watts and it is a usual practice to use a resistor with twice the power rating, it is entirely possible that they left off the '1' in "12 watts" and so wrote "2 watts". That's just another possibility.The reason for the 2x power rating is to keep the resistor cool. Power resistors that run at their full power rating get super hot, way over the boiling point of water, and can cause all sorts of problems. The usual practice is to use one that is rated for twice the required power unless it's a huge production run. In fact, three or four times the power rating is even better.You can try a standard 10 watt resistor if you can find that value, or go to a 20 watt resistor which would be even better.On some power resistors you can use a heat sink, and some power resistors come with fins that help keep it cool.

author: gbaddeley
author_rank: Regular Contributor
timestamp: June 28, 2023, 09:46:36 am
content: 0.24 Ohm is not a common value, I’d use 2 x 0.47 5W WW in parallel.Also consider the power dissipation in the regulator IC !

author: Terry Bites
author_rank: Super Contributor
timestamp: June 28, 2023, 12:12:13 pm
content: pretty common, E24 no?

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 28, 2023, 01:23:27 pm
content: Quote from: Terry Bites on June 28, 2023, 12:12:13 pmWell, yes but not all E24 (or even E12) are necessarily common below 1 Ohm. Due to their most common use as current shunt, a nicely round or fractional values are usually more readily available.10, 15, 20, 25, 30, 33, 50, 75 are common. 24 and 47 are both fairly uncommon. They still exist, but not exactly at the same amount of selection or price range as the previous numbers.

author: mariush
author_rank: Super Contributor
timestamp: June 28, 2023, 06:38:46 pm
content: You could just parallel 4 1 ohm resistors to get 0.25 ohm and you have the added benefit of spreading the heat across more area.Close enough to 0.24 ohm, and means just slightly less current at around 4.96A ( 1.24v reference / 0.25 = 4.96)In reality, the reference voltage will change as the regulator gets hot, the resistors will also drift a tiny bit as they get hot and so on.

author: TimFox
author_rank: Super Contributor
timestamp: June 28, 2023, 06:47:43 pm
content: 0.25\$\Omega\$ is just within the +/- 5% tolerance band for 0.24\$\Omega\$, which error band is typical of E24 values, and comparable to the +/- 4% tolerance on the 1.25 V reference voltage.

author: Zero999
author_rank: Super Contributor
timestamp: June 28, 2023, 08:28:12 pm
content: Digikey sell more 0R24 resistors, than 0R25 because it's a standard E24 value.I had a quick search and found this one. Make sure it's elevates above the PCB, because it will burn the board is it's too close.https://www.digikey.co.uk/en/products/detail/vishay-beyschlag-draloric-bc-components/AC07000002407JAC00/595586

author: MrAl
author_rank: Super Contributor
timestamp: June 28, 2023, 08:32:24 pm
content: Quote from: Zero999 on June 28, 2023, 08:28:12 pmHello,You don't want to use a 7 watt resistor, 10 watt min. Read the other posts.

author: Zero999
author_rank: Super Contributor
timestamp: June 28, 2023, 09:43:07 pm
content: It's a cement coated high power resistor, which is designed to handle high temperatures, with no problem, just keep it away from anything which is sensitive to heat, or flammable. If it's a big deal, then of course use a larger resistor to spread the heat further. It doesn't have to have a higher power rating, just a lower temperature rise.

author: MrAl
author_rank: Super Contributor
timestamp: June 29, 2023, 11:11:31 pm
content: Hi,Actually, 2x the power rating is a good choice but only if it gets free air flow. I prefer even higher than that for one-off projects.Yes, power resistors are about to withstand a high temperature because of the materials they are made of, but that doesn't mean we necessarily want to or are forced to use it at that power just for the sake of using it at the fullest potential. The only time we really might want to do that is in a large production run or when space is limited.Let me quote a real-world example. Installing LED lights in an automobile where incandescent bulbs are normally used.The car in question has bulbs mounted inside a plastic case. The LED bulb would be in there and the resistor might be in there too.The bulb gets hot, the resistor might get hot. The bulb has a special way of mounting that keeps the glass part away from the plastic at all times. The resistor does not have such a mount, it lays on the plastic case or close to it. To keep the temperature lower on the resistor we want to use a resistor with a body as large as we can get, or at least something that does not let it get too, too hot.Measuring a 10 Ohm 10 watt ceramic resistor at 2.15 watts, the temperature rise takes it up to about 170 degrees F, with a 10 Ohm 15 watt ceramic resistor the temperature rise takes it up to about 150 degrees F. Now a 20 degree F is not a huge difference, but it helps when the ambient temperature goes up too, and in the automotive application that can be significant. The difference in price is not huge, so which one to go with. The bigger one is better because it will help keep the temperature down lower. In fact, I wasn't even satisfied with that, but I may settle for that size. If I can use a 25 watt I would prefer that just to help guarantee the longevity.The other parameter to think about is the cost. Since I only need 2 of these and I will most likely never need any more for the life of the car, the cost is almost insignificant even if it was twice the price. Is it $2 for one or $4 for one. That comes to either $4 total or $8 total, but I only have to do this one time only.Now if this was a production run, I'd have to think about that cost spread out over thousands or even millions of units. That's an entirely different story. Instead of 2 million dollars it would be 4 million dollars, which is significant I think.So that's the reasoning behind my methodology in a nutshell.