Current rating and Heat

Hi! Are higher current rated components don't heat as fast as lower rated with same packaging? ex. LM317 and LM338 both running at .5A. Is LM317 hotter?

With the same voltage drop and the same current, they generate exactly the same heat. The heating can be slowed down by a heatsink or a larger component.

To get less heat, use a DC-DC converter.

Thanks for the reply! I thought I don't need heatsink if Im using high current rated component LOL.

Thanks for the reply! I thought I don’t need heatsink if Im using high current rated component LOL.

It gets complicated because you can overheat the device with less than maximum current.

Heat is directly related to power, and power is calculated as Voltage x Current.

So for example, you can have a MOSFET or transistor that’s rated for 30 Amps and 100 Volts, but it can’t handle both at the same time… You can safely switch 100V and 30A because when the device is switched-on, there is current with almost no voltage dropped-across the device, or it’s switched-off where all of the voltage is dropped across the device with almost no current. Similarly, a wall-switch in your house doesn’t have to dissipate power when a 100W (or more) light bulb is plugged-in, and the switch doesn’t generate heat. (Light dimmers do tend to get warm, but they are also designed as “switching” circuits to minimize heat.)

But the same 30A/100V device might die with 10 Amps and 10 Volts at the same time (100W dissipated).

Most of these voltage regulator ICs have built-in over-temperature protection, and they’ll just shut-down when they overheat, but MOSFETs & transistors don’t and they can be permanently fried.

There is usually a power spec, temperature spec, or a power spec with an “ideal” or infinite heatsink at room temperature. This can get really tricky and it’s difficult to know if you have a big-enough heatsink (and/or low-enough ambient temperature).

Ambient temperature is a factor too… A device with a heatsink might be OK at normal room temperature, but the heat generated in the device is added to the ambient temperature and the device might not survive in the desert in the summer.

Tamulmol: Hi! Are higher current rated components don't heat as fast as lower rated with same packaging? ex. LM317 and LM338 both running at .5A. Is LM317 hotter?

The amount of heat dissipated by a device is solely dependent on the power.

Any part with the same physical configuration and same power dissipation will get hot as fast as the other and reach the same temperature as the other.

Now, the case design DOES influence how much and how well you can CARRY AWAY the heat (using a heatsink).

For example, there are LM-317 regulators in TO-220 packages and TO-3 packages. The TO-3 package, being larger surface area will more effectively transfer it's heat to a heatsink (if it's mounted properly), but aside from that there's no difference.

Power dissipated = Heat generated. Period.

What you’re talking about is SAFE OPERATING AREA.

In your example, a 30 amp, 100 volt device can handle up to 30 amps or 100 volts, but not both.

If you take the maximum POWER DISSIPATION allowed for the device, then iterate through voltage or current (to derive the max of the other), you will get a graph that shows sort of a “box”, inside of which the device is going to work, and outside it’s going to burn up.

Here’s a typical SOA (safe operating area) graph:

Hi, yes the LM317 will get hotter than the LM338, even though they are dissipating the same POWER.

Of the POWER that has to be dissipated from the devices, the 317 has lower surface area compared to the 338 so to transfer the same amount of POWER the 317 has a higher surface temperature.

Thermodynamics.

Tom....... :)

TomGeorge: Thermodynamics.

Tom....... :)

Actually no, just heatflow, thermodynamics is something else, to be pedantic.

I am a pedantic

Thermodynamics.

"Thermo" from greek means heat (not temperature)

"dynamics" from the greek too; means movement.

So talking about heat moving from inside the component to the outside is basic Thermodynamics.

Kind regards.

Hi,

I am a pedantic

There is always one or two in the bunch..lol

The subject that I learnt it in was called, Thermodynamics ME162.

Tom.... :) Heat: amount of vibration at the atomic level. http://topdocumentaryfilms.com/what-is-one-degree/

TomGeorge: Hi, yes the LM317 will get hotter than the LM338, even though they are dissipating the same POWER.

Of the POWER that has to be dissipated from the devices, the 317 has lower surface area compared to the 338 so to transfer the same amount of POWER the 317 has a higher surface temperature.

Thermodynamics.

Tom....... :)

Your reply misses the point of the OP's question and, if he didn't notice the smiley at the end of it, he would also be misled.

A device with a higher power RATING will absolutely NOT run any cooler than a device of lesser rating when dissipating the SAME amount of power.

TomGeorge: Hi, There is always one or two in the bunch..lol

The subject that I learnt it in was called, Thermodynamics ME162.

Tom.... :) Heat: amount of vibration at the atomic level. http://topdocumentaryfilms.com/what-is-one-degree/

"The core temperature of the sun is approximately 27 million degrees".

"Is that in C or K?"

"What does it matter?"