Resistor Wattage: Find Out If You Need Bigger Resistor

[david90]

I have a circuit that uses a resistor to drop the input voltage. From my calculation, the power dissipation thru the resistor is less than it's wattage rating but yet the resistor is hot to the touch. Should I use a bigger resistor because it's too hot to the touch or should I keep using it because the power dissipation is within spec? The resistor is the ceramic wire wound type.

 

[0xDEADBEEF]

I admit I have little experience with this. But you realize that another resistor will get as hot, because it needs to transport the dissipated energy, unless it is bigger (more surface) or has cooling ribs. I would start by checking that the voltage across the resistor is correct. You could check the temperature specs and see if the resistor should be cooled. Maybe you could show how us you use the resistor, and how you calculate the power dissipation and maybe tell us what size and rating it has so we can see if it looks right. Also: what type of voltages/currents are we talking about?

Otherwise I don't think anyone here would tell you that you cannot use the resistor if the specs say you can.

 

[berkeman]

I have a circuit that uses a resistor to drop the input voltage. From my calculation, the power dissipation thru the resistor is less than it's wattage rating but yet the resistor is hot to the touch. Should I use a bigger resistor because it's too hot to the touch or should I keep using it because the power dissipation is within spec? The resistor is the ceramic wire wound type.

What is the application? Why are you burning up all that power? What voltages and current are we talking about here?

Depending on the application, it might be better just to use a buck DC-DC stage there, to drop the voltage and dissipate very little power...

 

[Carl Pugh]

Most ceramic wire wound power resistors:
If you can touch resistor and not burn yourself, then resistor is running cool.
Check resistor manufacture's data sheet for how hot resistor should be at maximum power.

 

[sophiecentaur]

Even a Watt or two can get an object pretty hot - take a lightbulb filament, for instance. There's nothing wrong with a bit of 'brute force and ignorance'. A resistive dropper may well be the most convenient way to lose some volts. If it gets hot and does not damage then that may be quite OK. A hefty transistor in series may achieve the same thing at greater cost!

 

[TurtleMeister]

It is not necessarily unsafe for a power resistor to be hot to the touch. However, in most cases it is not safe to operate a power resistor at or close to it's power rating. From Wikipedia:

Note that the nominal power rating of a resistor is not the same as the power that it can safely dissipate in practical use. Air circulation and proximity to a circuit board, ambient temperature, and other factors can reduce acceptable dissipation significantly. Rated power dissipation may be given for an ambient temperature of 25 °C in free air. Inside an equipment case at 60 °C, rated dissipation will be significantly less; if we are dissipating a bit less than the maximum figure given by the manufacturer we may still be outside the safe operating area, and courting premature failure.

The general rule of thumb is to select a power resistor whose power rating is at least double the power it will be expected to handle.