#### d123

##### Advanced Member level 5

I just want to press a pushbutton and see an LED light up, but the innocent-looking 27R resistor is making me do hours of maths...

An LED is 30mA maximum current from 0°C to +25°C, fron there to +82°C it falls in a linear fashion to about 7mA, at +85°C the LED forward current drops immediately to 0mA, according to the datasheet graph of If vs. Tambient. Just trying to avoid unpleasant surprises of frying the LED due to resistance change across temperature.

27R, +-1%, +-100 ppm/°C, voltage across resistor = 800mV, temperature range of operation = 0°C to +85°C.

1) How many calculations do I need to do for this resistor? Is my list complete?

2) Would it really change from 27R to ~44R at +80°C Tamb so the current at 800mV wouldn't be ~27mA but would fall to ~18mA?

3) This takes ages just for one resistor. It's not viable to spend two hours calculating per resistor if a circuit has 20 or 50 resistors - is there a shorter method I can do without using online calculators or some free software that calculates all this for me? I have a very basic understanding of using Excel type spreadsheet programs to add or multiply rows only, that's it for my ability to create formulas in such programs.

A:

1) +-1% @ +25°C = 3 calculations

2) Iout @ +25°C then = 3 calculations

3) PD @ +25°C then = 3 calculations

B:

4) Self-heating @ +25°C for above three +-1% resistance values for +100ppm/°C

5) Change of resistance due to self-heating and Tambient rise above +25°C for the three +-1% resistance values for +100ppm/°C.

6) Change of Iout due to result of calculation 5

7) Change of PD due to result of calculation 5

C:

8) Self-heating @ +25°C for - 100ppm/°C

9) Change of resistance due to self-heating and Tambient rise above +25°C for these three resistance values for - 100ppm°/C

10) Change of I out due to self-heating and Tambient rise above +25°C for those three resistance values and - 100ppm/°C

11) Change of PD due to self-heating and Tambient rise above +25°C for those three resistance values and - 100ppm/°C

D (same steps as B but for Tambient fall below +25°C):

12), 13), 14), 15) Same four calculations/steps, three times each for + 100ppm/°C and Tambient fall below +25°C

E (same steps as C but for Tambient fall below +25°C):

16), 17), 18, 19) Same four calculations/steps, three times each for - 100ppm/°C and Tambient fall below +25°C

II have read rather a lot about ppm resistance change over the past years but never see clarification and examples I fully can be certain of about if +-100ppm/°C means + OR - with temperature rise or it means + with temperature rise and - with temperature fall from nominal room temperature.

Not interested in factoring in Johnson noise or any other causes of resistance shift just yet, only how many calculations I should be doing to get ideal, lowest and highest values for the resistor so as to see how much the 27mA may go up or down across temperature range of operation.

Thanks.