Power Resistor Theory

[superflux]

Can I assume that the Power Resistor with the higher wattage will be cooler than a smaller wattage PR OR will they give off the same amount of heat at the same resistance?

I need to find a power resistor that can be out in the open, where it might be touched by human hands, able to handle less than 1 watt and be semi-cool to the touch.

If I use a 20 watt PR, will that be the ticket?

Can I use a 20 watt Thick Film PR without a heat sink?

Thanks,

SF

 

[mneary]

A resistor with a higher power rating usually has a significantly larger surface which would help to cool it.

 

[MrAl]

Hi there,

If you are having a problem with a power resistor getting too hot and being a burn risk for humans or something like that, you can also use more than one power resistor to distribute the heat across a much larger surface area and thus reduce the maximum temperature.
For example, if you need a 2 ohm 10 watt resistor, you can use two 1 ohm 10 watt resistors in series, which will give you twice the surface area which will reduce the temperature rise by about half. In this way if one resistor got up to say 75 degrees C then two resistors would get up to only about 50 degrees C (25 deg C ambient).
Alternately, two resistors in parallel of twice the ohmic value and both of the same original wattage.
Of course more resistors would be even better.

 

[crutschow]

A 20W resistor should stay close to the ambient temperature if it is dissipating less than a watt.

 

[JimB]

I agree with what everyone else has said here, a resistor with a higher power rationg will run cooler than one with a lower rating whne dissipating the same power.

However, I must be pedantic and correct a bit of bad terminology in your question:

Can I assume that the Power Resistor with the higher wattage will be cooler than a smaller wattage PR OR will they give off the same amount of heat at the same resistance?

"Heat" relates to the quantity energy present.
"Cooler" relates to temperature, how concentrated the heat energy is.

For example, a bath of hot water has more heat energy than a soldering iron, but the temperature of the soldering iron is much higher than the bath water.

JimB

 

[Sceadwian]

Keep in mind there's no need to so largely overrate the resistor itself, just use a heatsink, you should be able to find heatsinks that fit over power resistors, or make one yourself.

 

[superflux]

Thanks to all for the comments and suggestions.