life testing resistors in high temp

[engineer1000]

I want a method of high temp testing resistors. If I know the temp coefficient I can calculate the theoretical value change with temp. I also understand when loading the resistance with a current to match the power it will generate its own heat which needs to be taken into account.
What I don't know is how do I test a batch of resistors say 20 at the same time.
If I put them in parellel some resistors will take more current than others due to the tolerance, so not sure how to do it

 

[ravindragudi]

Use a constant current source to input the required current into the resistors. Have series configuration for this. you can have 3 or 4 such series strings, independent of each other so you need as many current sources.

 

[srizbf]

I want a method of high temp testing resistors. .....

high temp testing ... can you give more info on what is exactly required.

 

[engineer1000]

high temp testing ... can you give more info on what is exactly required.

Right I am new to high temp design work. I need to test a number of different passive components at high temp 150C to confirm that they perform as per the spec and see what happens if I go slightly over the spec'ed temp.
I'm starting with resistors as these appear to be the simplest. Now I have looked at automotive specs and mil specs they give info on what parameters to test but none seem to give a test method of batch testing a number of resistors.
say I want to do a batch from 3 different manufactures and say 20 resistors per batch.
I could power up one resistor near its max power put it in the oven for a predetermined no. of hrs and measure the resistance at the end and once its cooled down.
What I want is a method of powering say 20 at the same time

 

[schmitt trigger]

It is very simple actually.

1-Measure, with an accurate and CALIBRATED ohmeter (at least 4 1/2 digits) the value of each resistor. Write it down. Make sure you null out the probe's resistance, and for values lower than 100 ohms use Kelving sensing. Also record the ambient temperature.
2-Consult the manufacturer's datasheets to see the power derating at 150C. You may find, for instance, that a 1 watt resistor can only dissipate 0.1 watt at that temperature.
3-Use the method suggested by ravindragudi
4-Every week or so, unpower the test jig, allow the resistors to FULLY cool down to the temperature in step 1, and measure them again with the same instrument of step 1.
5-plot the results.
6-Repeat. Usually 1000 hours testing is enough, but more stringent applications may require longer periods.

 

[engineer1000]

It is very simple actually.

1-Measure, with an accurate and CALIBRATED ohmeter (at least 4 1/2 digits) the value of each resistor. Write it down. Make sure you null out the probe's resistance, and for values lower than 100 ohms use Kelving sensing. Also record the ambient temperature.
2-Consult the manufacturer's datasheets to see the power derating at 150C. You may find, for instance, that a 1 watt resistor can only dissipate 0.1 watt at that temperature.
3-Use the method suggested by ravindragudi
4-Every week or so, unpower the test jig, allow the resistors to FULLY cool down to the temperature in step 1, and measure them again with the same instrument of step 1.
5-plot the results.
6-Repeat. Usually 1000 hours testing is enough, but more stringent applications may require longer periods.

Thanks scmitt trigger that answer is excatly what I needed