Hey

can someone explain me what is temperature coefficient of current through resistor if voltage on resistor has temperature coefficint TCv[ppm/K] and temperature coefficient of resistor is TCr[ppm/K]
I understand that I[T]= (U * ( 1 + TCu*ΔT )) / (R * (1 + TCr*ΔT )) but i have no idea how to get the temperature coefficient of current from that equation.

Thank you

 

Hey

can someone explain me what is temperature coefficient of current through resistor if voltage on resistor has temperature coefficint TCv[ppm/K] and temperature coefficient of resistor is TCr[ppm/K]
I understand that I[T]= (U * ( 1 + TCu*ΔT )) / (R * (1 + TCr*ΔT )) but i have no idea how to get the temperature coefficient of current from that equation.

Thank you

Expand the equation:

I*R * (1 + TCr*ΔT ) = U * ( 1 + TCu*ΔT )

and solve for ΔI/ΔT

 

Yes i did it. But the final temperature coefficient of current is a real value or a function of ΔT?

 

Parts that are bothering me include the ideas that voltage or current have a temperature coefficient. They don't. A resistor has a temperature coefficient because the resistance changes as temperature changes. However that effects the current or the voltage are not a property of the current or the voltage, they are a property of the resistor.

 

Yes i did it. But the final temperature coefficient of current is a real value or a function of ΔT?

Look at the dimensions or units of

(1 + TCr*ΔT)

What are the dimensions?

Hence TCr*ΔT must have the same dimensions.

What are the dimensions of TCr?

 

Parts that are bothering me include the ideas that voltage or current have a temperature coefficient. They don't. A resistor has a temperature coefficient because the resistance changes as temperature changes. However that effects the current or the voltage are not a property of the current or the voltage, they are a property of the resistor.

Voltage definitely can have a temperature coefficient. For example you can force some current through diode and use voltage over diode as reference voltage. The temperature coefficient is -2mV/K.

Look at the dimensions or units of

(1 + TCr*ΔT)

What are the dimensions?

Hence TCr*ΔT must have the same dimensions.

What are the dimensions of TCr?

I found out that if we have some voltage and resistance with temperature coefficient the final temperature coefficient of current is

TCi = 1/ΔT * ( (1 + TCu*ΔT) / (1 + TCr*ΔT) - 1), what is preatty useless because because this is not linear relation anymore.

Thank you for your help.

 

Voltage definitely can have a temperature coefficient. For example you can force some current through diode and use voltage over diode as reference voltage. The temperature coefficient is -2mV/K.

Again, that's a property of the diode and not the voltage or current flowing through it.

 

Again, that's a property of the diode and not the voltage or current flowing through it.

We did not understand each other.
https://www.google.si/search?q=PTAT...AUICigB&biw=1242&bih=602#imgrc=V5z9GT2Vv9kpyM:

On this picture is circuit which produce proportional to absolute temperature - PTAT voltage over resistor. My question is: what is temperature coefficient of current if the voltage has some TC and also the resistor has TC.

 

Voltage can not have TC. Current can not have TC. Physical parts can have TC.
If resistor has TC1 and Vbe has TC2 then two parts in series have TC1 + TC2

 

Can you please describe me why voltage cannot have some temperature dependence?

 

Can you please describe me why voltage cannot have some temperature dependence?

A volt is defined by a standard in a laboratory. That volt is the same voltage at every time and every place. If a volt could have a temperature coefficient, a volt would be a different voltage at different temperatures. Notice that the definition of a volt says nothing about temperature:

Definition of volt
: the practical meter-kilogram-second unit of electrical potential difference and electromotive force equal to the difference of potential between two points in a conducting wire carrying a constant current of one ampere when the power dissipated between these two points is equal to one watt and equivalent to the potential difference across a resistance of one ohm when one ampere is flowing through it.

https://www.merriam-webster.com/dictionary/volt

 

I clearly understand that volt as unit is constant. But also ohm is defined by standard?

 

Can you please describe me why voltage cannot have some temperature dependence?

For a constant current amount, the voltage measured across 1 part, or more parts, can change with the temperature of the parts. The current did not cause the change because the current is defined as constant. The voltage did not cause the change because a volt is the same at all temperatures. The TC must be the TC of the part or parts. No person says, "My volt is feeling high today because the weather is cold." What people might say is, "The voltage across the diode is 0.56 volts instead of 0.6 volts because the diode is very hot today."

There is always a physical part involved in a TC because a volt is always the same size volt and an amp is always the same size amp.

 

I clearly understand that volt as unit is constant. But also ohm is defined by standard?

Yes, an ohm is defined as a standard and an ohm is always the same size of an ohm. Physical parts have resistance and that resistance often changes when the temperature changes. The ohms are still the same size ohms, there are simply more of them or less of them.

 

This is a very important point. It is small but important because everything in electronics is built of voltage, current, frequency, and how they change. It is important to know which part is changing and how you call it so other people know what you mean.

 

Hey

can someone explain me what is temperature coefficient of current through resistor if voltage on resistor has temperature coefficint TCv[ppm/K] and temperature coefficient of resistor is TCr[ppm/K]
I understand that I[T]= (U * ( 1 + TCu*ΔT )) / (R * (1 + TCr*ΔT )) but i have no idea how to get the temperature coefficient of current from that equation.

Thank you

Read my first post.

Voltage over resistor is temperature dependent, resistance of resistor is temperature dependent. My question was how the current through resistor is temperature dependent.

 

Voltage over resistor is temperature dependent, resistance of resistor is temperature dependent.

Voltage over a resistor is temperature dependent and that is declared at a constant current, whatever you want that current to be.
If the current is dependent on temperature, then you should declare a constant voltage.
If, for 1 ma, the voltage across a resistor is 1 volt + 1 millivolt per degree Kelvin then you can use Ohms Law to find the change in current if the voltage is held to a constant voltage.

 

The voltage across the resistor is not temperature dependent. It depends on the resistance (which is) and the applied current, as defined by ohms law. If you define a specific current, then the voltage will change with the temperature but only because the resistance is changing. Ohms law has no temperature term, but the resistance of a real resistor does.

 

Ok, i see where is the problem.

We have temperature dependent voltage source. Connect resistor which is temperature dependent on this voltage source. How the resistor current change with temperature. That was my question.

 

Now you should be able to do the math.
From I = E/R
dI = dE/dR
dI = (E + TCe)/(R+TCr)
Keep expanding until you have all the TC described.