How hot do resistors get?

[PaperclipGod]

Is there a formula to determine how hot a resistor of a given size will get? e.g., 1/4 watt resistor will raise the ambient temp by 2 degrees?

 

[Billb2]

So much fail.

 

[bobdole369]

Basic physics and algebra apply here. OMG fail.

 

[gorcorps]

What those 2 mean is that they don't know how to do it either, but rather than say it they pretend they do and then not help you out.

 

[tatteredpotato]

I don't know the exact formula, but the power dissipated in a resistor is turned into heat. You would have to measure the power over time to figure out the amount energy dissipated in the resistor, and it would all depend on the environment and the thermal properties of the resistor's material for cooling.

Of course maybe there's an easier way?

 

[Demon-Xanth]

There are too many variables here. Such as resistor type, mounting, and air flow. Some datasheets do have charts regarding the film temperatures and such though. (Dale CRCW line being one I know of)

 

[DesiPower]

maybe this will help

 

[CycloWizard]

It's very easy to compute the heat generated by the resistor, but not so easy to compute the rate at which that heat is removed by its surroundings. It is the latter that will dictate the surface temperature, and it is governed by how easily the air can move away from the resistor, whether a fan is blowing over its surface, and a lot of other things. I used a simple resistive heater design in a project a while ago, but this is OT, not HT, so I'm not going to go into the details of natural and forced convection analysis.

 

[PaperclipGod]

Is there a way to compute just a raw BTU figure and ignore environmental variables?

 

[Demon-Xanth]

You can figure out watts easily enough simply by current times voltage drop. Then to get BTUs: 1 BTU = 0.2930711 Watt-Hour.

 

[shortylickens]

Yes.
Thanks Demon-Xanth for not shitting on a guy with a legit question.

 

[deadlyapp]

sigh heat transfer. I'd help you but it is too early for me to get into Re and Nu numbers and such bullshit.

by any means, typically the power rating of the resistor is the maximum amount of power it can dissipate, and the majority of resisitors are 1/4 or 1/2 watt. This is typically nearly all converted to heat, as thats how resistors work.

 

[PaperclipGod]

Thanks, m8's

 

[bobdole369]

I'd help you but it is too early for me to get into Re and Nu numbers and such bullshit.

this.

for back of the envelope calcs figure 80% of watt -> btu calc (as the wattage rating is typically the flame point vs the amount of safe dissipation)

 

[Evadman]

you can weld steel with a 4 ohm resister plugged into the wall.

 

[Rubycon]

you can weld steel with a 4 ohm resister plugged into the wall.

You've reminded me of something...

http://www.youtube.com/watch?v=IiF_CCbHl_0&

 

[bobdole369]

Dunno from where teh 92 Amp figure came , but since a speaker is a complex system, it isn't exactly 4 ohms. It varies significantly based on a number of variables. I recall my brother did that back in the day to get a new 15" Kicker. It took about 10 seconds to blow the house fuse - 25A. So I would put the impedance of that particular speaker (my brothers) around 4.8 ohms at 60 hz with 25A running through it - while gyrating at 60hz in and out of the magnetic field.

 

[Rubycon]

Dunno from where teh 92 Amp figure came , but since a speaker is a complex system, it isn't exactly 4 ohms. It varies significantly based on a number of variables. I recall my brother did that back in the day to get a new 15" Kicker. It took about 10 seconds to blow the house fuse - 25A. So I would put the impedance of that particular speaker (my brothers) around 4.8 ohms at 60 hz with 25A running through it - while gyrating at 60hz in and out of the magnetic field.

Yes there is an impedance curve since it's a motor not a resistive load. People are doubting the 92A figure but it is very brief and the excursion makes it believable although most of the 20kW would be ballasted into the voice coil. If it were allowed to continue for more than a few seconds it would heat up FAST!

The fact that it just didn't blow up is good though. Never in professional audio do we subject that much power to a single bass driver. Perhaps 1/10th of that and only momentarily. It's not uncommon to use larger amps (10-20kW) into arrays of drivers where each driver is operated in its optimal range of thermal power handling and power compression.

Connecting a large horn system to 240V 50Hz line is scary because it's close to the box tuning frequency and will draw lots of power and produce a LOUD tone. (50Hz at 145+ dB) Wall plugging is the "poor man's" way of destructive testing as it affords a decent source of potential multiple kilowatt sinus power at 50-60Hz.