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Resistor power rating

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Bsr

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Hi ,

I have a doubt related to Resistor power rating , How much it is important to consider the power rating for an Resistor ?

Generally people saying that the power rating for a Resistor should be greater than the power dissipates
through it . Is it a serious thing to consider practically when we are designing a ckt.

Ex. 1.PNG

If is the case that a simple ckt has a supply of 24v and 2A then the power dissipates across the resistor is 48w , so Is it necessary to consider 48W power rating resistor ?
 

Yes.

Although in theory you could use a resistor rated at 48W, it would be considered bad practice and one with at least 10% higher rating should be used and preferably 25% or more.

Think of it like a car engine speed, if the manufacturer said 10,000 RPM maximum, you wouldn't run it at 10,000 RPM all day every day and expect it to last long.

Brian.

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    Bsr

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people saying that the power rating for a Resistor should be greater than the power dissipates
Also should note that whenever dealing with high dissipating devices it is always important to consider forced ventilation, which could relieve a little the need to over rate its power specifications.
 
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    Bsr

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I have a doubt related to Resistor power rating , How much it is important to consider the power rating for an Resistor ?

It is important to get to know the idea behind it.

If you pass 2A at 24V via a resistor, the 48W power has to be dissipated as heat. The resistor becomes first warm and then hot and as the temp increases, more heat is lost (by conduction, convection and radiation). As some temp, after some time, a steady state is obtained and the heat produced by the resistor becomes equal to the heat lost by the resistor. We reach a steady state.

But if you pass 20A at 24V only for a 1ms, the total energy is only 480 mJ. If the pulse is repeated only once a second, then the total power, registered over one second, will be 0.480mW.

You can also put a heat sink on the resistor to increase the power dissipation- but the basic idea is that a resistor running at a total power less than its rated capacity runs cooler and hopefully lasts longer.
 
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    Bsr

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Resistor Power Ratings can be very Misleading.

I Bought some 2 watt resistors one time, and when dissipating 2 watts, they were EXTREMELY HOT, even when dissipating just 1 Watt
In an Enclosed container, this heat would destroy most surrounding parts.
While some other 2 watt resistors I have and with the same Resistance and Current, were just a bit warm to the touch.

So it is important to know the Temperature rating for the given wattage rating.
 
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    Bsr

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Hi,

you also should look at what conditions a resistor´s power is rated.

I´ve seen identical size resistors but the one´s resistor power rating was alomst twice of the other.
One was rated with ambient temperature of 25°C the other with ambient temperature of up to 70°C.

The chart for temperature vs max dissipating power was almost identical.

Klaus
 

I Bought some 2 watt resistors one time, and when dissipating 2 watts, they were EXTREMELY HOT, even when dissipating just 1 Watt ( ... ) While some other 2 watt resistors I have and with the same Resistance and Current, were just a bit warm to the touch.

The higher the surface of the resistor, the lower the amount of heat per unit space for the same amount of energy per unit time, so the temperature will be lower if both the the resistors were rated for the same power, but having different sizes.
 

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