How to Choose Power Rating for 680K Resistor: 0.5W 60s, 1.5W 5s Test Pulses?

Dear forum

In normal condition a resistor need to dissipate 50mW ..It have 680Kohm, and max voltage 100V

But I must dimension the resistor (in watt) when 2 test signal are applied to this resistor.

For one test signal, the resistor need to dissipate 0.5W for 60second
For the other test signal, resistor need to dissipate 1.5W for 5second

So question is:
-When i choose this resistor, what power I consider ?

I will buy a resistor of 1.5W (or moore), oe 0.5W ?

Thanks for your help

Roberto

The maximum power dissipation for a 680Kohm resistor at 100V is well below 50mW. Even allowing for elevated temperatures you are not going to need to de-rate the part. To get higher power for the specified resistor, you are going to need a very high voltage power supply, around 1Kv for the 1.5W. High voltage resistors are therefore needed, and they usually come in higher power ratings since they are physically larger to avoid voltage breakdown. I am not sure what you are trying to achieve, have I missed something, is the resistor 680 ohms? If so, then a 100V supply will be fine. How is the resistor to be tested, any test fixture may help the resistor dissipate heat, which will help it survive at elevated temperatures.
The data sheets provide temperature performance, which helps in dimensioning the power rating necessary for a resistor. Excess transient power may not destroy it, but it will likely effect the value and MTBF, therefore if you choose a resistor for the maximum peak power it has to handle you can't go wrong, since it is not always possible to design things like a PCB with larger pads to help dissipate higher transient power that may be encountered.
Cheers,
Richard