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Hi y'all

I have a question about resistor and more specifically it's power/watt rating

I know that when resistor in series the total resistance is basically R1 + R2 + Rn..., While on parallel it's something like this

But what about it's power rating?

So far i only found out that when you put a resistor in parallel it "increase" the total power it can handle as in when you have a 2 10ohm 10w resistor in parallel, you basically create something equal to 5ohm resistor capable of handling 20w of power before burning out, is that true?

Also what happen when the resistor is in series, does it increases, decrease, or stay the same regardless of the number of resistor you put? (assuming all of them have the same watt rating)

Edited by Freakwise
Typo

Wilbupdate evercouplmonth

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So the power rating of a resistor is the maximum power that resistor can dissipate before it burns. The power dissipated by a resistor is equal to the voltage dropped across that resistor multiplied by the current flowing through it. Using some Ohm's law substitution, we can also find that the power dissipated is equal to the voltage dropped across the resistor squared divided by the resistance, or the current through the resistor multiplied by the resistance.

In your 10w parallel circuit, assuming the voltage remains constant, the two resistors will each pull the same current and dissipate the same amount of power, so I guess your analysis is practically correct. At first read I was a little confused because it seemed like you were saying the power handling of an individual resistor can change depending on its configuration which is not the case. However, from a modeling perspective you can do this by combining resistors. I hope this didn't create more questions than it answered, but I'll be happy to answer more.

When resistors are in series, you have to be aware of the voltage dropped across each resistance in the series, and the power dissipated by each compared to each individual rating. Say you have a 5W and a 1W resistor in series, you have to check (using P=V^2/R) that the power dissipated by the 1W will not exceed 1W and the same for the 5W.

ASU

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19 hours ago, Freakwise said:

So far i only found out that when you put a resistor in parallel it "increase" the total power it can handle as in when you have a 2 10ohm 10w resistor in parallel, you basically create a 5ohm resistor capable of handling 20w of power before burning out, is that true?

As @Hackentosher has said, it doesn't change the actual power rating of the individual component but you can use several components to make one in a sense.

Note that in some cases one option may be better or worse. Like say in your example, rather than 2x10W ceramic resistors a 20-30W heatsink resistor may better dissipate heat and allow for a bit of overhead or you may just be throwing something together quickly and can just use a bunch of resistors to get the right value in a pinch.

With parallel be aware that if the values aren't the same the current split will be different down each leg.

Ie. Say if I make my overall 5R resistor from a 20R & 6.8R in parallel , the 6.8R will have roughly 3 times the current going through it as the 20R.

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4 minutes ago, artuc said:

heatsink resistor

Can i ask what is heatsink resistor though?

Wilbupdate evercouplmonth

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9 hours ago, Freakwise said:

Can i ask what is heatsink resistor though?

They're probably referring to these. Some power resistors are basically big blocks of cement (for its thermal mass) and some are normal resistors encased in a heatsink to dissipate more heat. The latter are common in automotive LED headlight conversion kits because LEDs don't pull as much current as a normal lightbulb, so you need to add a parallel resistor to make the headlight computer think that there is in fact a light bulb connected.

One last note that seems relevant: Real resistors have tolerance. You will usually see a resistor sold as resistance +/- x%. This specifies the maximum allowable tolerance for that specific resistor. For example if you buy a 100 ohm resistor with 5% tolerance, it can be either 95 ohms or 105 ohms or somewhere in between. Also realize that if you're buying the cheap ones, you wont get exactly the value because the manufacturer will filter out the parts that are closer to the right value and sell those at a premium.

ASU

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3 hours ago, Hackentosher said:

They're probably referring to these. Some power resistors are basically big blocks of cement (for its thermal mass) and some are normal resistors encased in a heatsink to dissipate more heat. The latter are common in automotive LED headlight conversion kits because LEDs don't pull as much current as a normal lightbulb, so you need to add a parallel resistor to make the headlight computer think that there is in fact a light bulb connected.

One last note that seems relevant: Real resistors have tolerance. You will usually see a resistor sold as resistance +/- x%. This specifies the maximum allowable tolerance for that specific resistor. For example if you buy a 100 ohm resistor with 5% tolerance, it can be either 95 ohms or 105 ohms or somewhere in between. Also realize that if you're buying the cheap ones, you wont get exactly the value because the manufacturer will filter out the parts that are closer to the right value and sell those at a premium.

Ah, that make sense, i do actually know those things, just never called it heatsink resistor

Wilbupdate evercouplmonth

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They also make resistors in TO-220 or TO-252 (DPAK) packages which can be screwed to heatsinks or soldered in case of DPAK and there's resistors in larger sizes rated for 25/50/100w peaks which can be screwed down onto something for.better dissipation.

Tolerance ... 5 percent resistors should follow a bell curve, majority should be close to +- 1-1.5 percent with few outliers in the 3-4 percent. They tend to separate the -0.6...+0.6 resistors.for the 1%, and everything else.goes to 5% or in between.

The manufacturing process is refined enough that they'd have to work hard to screw up to get near 5 percent. Often you'll get 1percent resistors. Probably cheap thick carbon resistor are what you can get with 5% these days, or thin film/metal resistors they may not want to sell as 1% due to drift with temperature or other reasons

// Typing on phone, pls excuse mistakes