# Thermal Lag

#### Mazaag

Joined Oct 23, 2004
255
Hi guys,

I had a question about heating up resistors and thermal lag... I have a control system which uses thermocouples as the feedback sensors, and I alter the voltage across the resistor (AC) according to error...

Now, I understand that there is thermal lag in terms of the resistor heating up.. my question is as follows : will it matter whether the voltage applied is AC or DC ? in other words, is it possible to decrease the lag time in heating up the resistor if DC is used instead of AC ?

Thanks

#### thingmaker3

Joined May 16, 2005
5,084
AC or DC will give the same result.

#### Mazaag

Joined Oct 23, 2004
255
Can you explain to me why .. ?

Like... how does the AC heat up the resistor , and is it different from the way DC does ?

#### beenthere

Joined Apr 20, 2004
15,819
The current through the resistor is what does the heating. The machanism is exactly alike for AC and DC.

As long as the AC current is measured as RMS, then the power dissipated in the resistor is exactly the same for AC as DC.

#### Mazaag

Joined Oct 23, 2004
255
How does the current produce the heat ? is it the affect of friction between the electrons and the atoms on the material?

#### Distort10n

Joined Dec 25, 2006
429
You have to love physics!

The way I look at it, and if I remember correctly, it is the vibration and/or collision of molecules that help to cause heat. If I had my physics' notes I could tell you more. Here is a link that asks the same question: