# Maintain constant Power in varying load

1. Sep 21, 2007

### weeb0

how to maintain constant Power in varying load

Hi,

I'm having some questions about the way I could achieve to maintain the power in a resistor constant. My problem, is that the resistor is quite small (less than 100ohm at room temperature). When running, the resistor, a heating resistor, heats up to 300celcius.

So, the resistor change in function of the temperature. So, to maintain that temperature, I want to keep the power of the resistor constant.

I have some ideas to achieve this, but I have some misunderdstanding so I miss something.

Here is my mind :

RL = Resistor Load (heating resistor)

1. find IRL : adding an Rsense of about 1ohm serially with the RL. 1A = 1V on the ouput of the opamp
2. multiply the IRL with the VRL. Obtain Power 1W = 1V.
3. find delta Power (substract Power_Setpoint from Power (step2))
4. ?????? -> using (step 3) change the current or the voltage applied to the Rsense to obtain the perfect power.

I though to use a microcontroller to control this, but I was wondering some precision issues...

So, do somebody have some suggestions ?
Do some circuit do this ?
Am I doing this right ?

Last edited: Sep 22, 2007
2. Sep 22, 2007

### alvaros

The only way is to measure both the voltage and the current across the resistor.

The only way is to measure the temperature of the resistor. If the resistance of the resistor changes with temperature ( like all resistors ) one way is to measure, again, the voltage and the current across the resistor.