# Effect of temperature

#### irsarasheed

Joined Dec 24, 2019
24
Can you please help me in understanding effect of temperature on embedded resistors and capacitors.

#### Papabravo

Joined Feb 24, 2006
12,937
I'm not familiar with the term "embedded resistors". Can you explain this term in more detail?

#### Janicer

Joined Jan 9, 2020
3
I know that, of course, use of temperature sensors does not vary with the temperature of the circuit but it is almost unavoidable and it is likely that the error between the measured value and actual value is equal to that of the temperature sensor. Therefore, and possibly for simplicity, I assume that the observed temperature can vary from one time to another, but I try to check as carefully as possible (these people must keep their temperature sensors as calibrated as possible or else they suffer them forever).

I am a working professional. Do I have to pay more for the same devices? What do I need to know?

#### dl324

Joined Mar 30, 2015
10,072
Can you please help me in understanding effect of temperature on embedded resistors and capacitors.
I don't know what embedded means with respect to resistors and capacitors.

At elevated temperatures, power dissipation needs to be derated for resistors and electrolytic caps will fail sooner when operating at elevated temperatures.

EDIT: I forgot to mention the most important factor. Resistors and capacitors vary with temperature. The nominal values are for 25C.

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#### crutschow

Joined Mar 14, 2008
24,380
Another effect is that the resistor and capacitor values will change with temperature.
How much change is usually given in the devices' data sheets.

#### MrChips

Joined Oct 2, 2009
20,277
If you wish you can get into the physics of the effect of temperature on conductors and this can get very complicated.

The resistance of a length of metallic wire is given by the equation:

where,
R = resistance
ρ = resistivity
l = length of wire
A = cross-sectional area of wire

As temperature increases, the physical dimension of metals will generally increase. Hence the length increases, resulting in higher resistance, maybe. Counter to that, the cross-sectional area will increase faster since the area varies as the square of the radius. Thus in theory, the resistance should decrease with increasing temperature.

More significantly, we have to look at the dependence of resistivity on temperature. This gets very complicated because now we have to look at the mobility of free electrons in metals. Suffice to say that manufacturers can balance the effect of temperature on resistivity and thermal expansion to create resistors with positive, negative, or zero temperature coefficient.

Now all of the above discussion is for DC operating conditions. At very high frequencies the above analysis breaks down completely.

This discussion is only about metallic conductors. Resistors are manufactured using different processes and materials, some of which can be composites with the desired characteristics.

If by "embedded" you mean resistors fabricated in integrated circuits, you are into a different game altogether. Resistors are fabricated using the same doping and masking process in the creation of FET transistors.

You can apply a similar analysis in the construction of capacitors. Besides the area and separation of the capacitor plates, the change in dielectric permittivity with temperature plays the dominating role.

Joined Jul 29, 2019
47
Heat destroys electronics. If the heat is high enough you will liquefy the solder and components can easily short circuit. That is why many industrial control panels have air conditioners to cool systems where high current is being used.

#### Rich2

Joined Mar 3, 2014
150
I've made a 555 circuit to sense the shed temperature and cut the heater off if it gets to 20c. The usual shed temperature is set to 10c controlled by a digital controller and fan heater. With it being such a smaI shed i didn't want to cook the bike if the relay contacts got stuck on the controller. Hence the safety circuit I made.

During testing I didn't want the chip or other resistors heating up and affecting the thermistor and I did notice even the small resistors warmed up very slightly when powered up. Then it dawned on me, the thermistor IS a resistor, and even a small current through it should raise the temperature? So why don't we see thermal runaway with thermistors?

#### Janis59

Joined Aug 21, 2017
1,057
Simply every RLC have specific TKR or TKC or TKL. Some have negative, some have positive. Then combinating, say, larger resistor of positive TKR with smaller resistor with negative TKR or vice versa, may get a "roughly temperature independent" resistor.

#### DickCappels

Joined Aug 21, 2008
6,187
Other things in the circuit often limit the current in practical circuits.

A negative temperature coefficient thermistor could go into runaway if you go to the trouble of making a suitable circuit.