Electronic Circuits and Projects Forum

Originally Posted by Sceadwian

http://www.electro-tech-online.com/c...s_select-1.pdf for one.
Resistor wattage ratings are free air with abundant ambient flow.

My apologizes on the value given though being at full rating, I forgot to put the diode drop in my calculation, so the calculator provided naturally derates a few percent by whatever the voltage drop of the diode is, good for most everyday situations.

The other point which I cannot cover is the 'E' range from which the User is choosing his 'nearest' resistor value.

I have to leave the User some leeway, like all calculators its just a Tool and has to be used with some common sense.

Resistors are made with materials that withstand very high temperatures.
Maximum power ratings for parts do not acknowlege injury to people nearby or damage to nearby plastic. Phenolic circuit boards can be charred.
Maximum power ratings should be de-rated to reduce the maximum temperature.

Originally Posted by audioguru

Resistors are made with materials that withstand very high temperatures.
Maximum power ratings for parts do not acknowlege injury to people nearby or damage to nearby plastic. Phenolic circuit boards can be charred.
Maximum power ratings should be de-rated to reduce the maximum temperature.

Nobody said anything about injury to people.

I still say its acceptable to run resistors at their rated wattage. In fact you confirm that by your above statement Resistors are made with materials that withstand very high temperatures.


If you think that a 0.16Watt dissipation from a 0.25W resistor is going to char Phenolic circuit boards, I would suggest you get out and dust off your text books and give them a good read.


Note the word RATED... this means Tamb= 25C free air cooling, if you you take the trouble to read the datasheet sheets on resistor heating generation at increased ambient temperatures you will see that the RATING is reduced.

As always in your weak counter arguments you try to take points out of context in order to try and prove your point.

Do as you tell other NOOBS, read the datasheet.!

Originally Posted by ericgibbs

I still say its acceptable to run resistors at their rated wattage.

Then it overheats when enclosed. It is ALWAYS enclosed.
Then the case is at 250 degrees C or more, it will severely burn a person, it will melt a nearby plastic capacitor or wire insulation and it will char a phenolic circuit board. It might peel the printed wiring off a pcb.
I do not operate a resistor at its maximum power rating. I don't operate ANYTHING at its maximum rating.

Originally Posted by audioguru

Then it overheats when enclosed. It is ALWAYS enclosed.
Then the case is at 250 degrees C or more, it will severely burn a person, it will melt a nearby plastic capacitor or wire insulation and it will char a phenolic circuit board. It might peel the printed wiring off a pcb.
I do not operate a resistor at its maximum power rating. I don't operate ANYTHING at its maximum rating.

Why do you insist in including extremes as part of your explanation.???

If I put my my fingers in wall mains socket I will get a shock and possibly burnt fingers, thats why I DON'T do it.!

Any 1st year student knows that an enclosed resistor will over heat.

We also know that a component running at 250C could damage nearby components, BUT what has that to do with the LED resistance calculator.???

If you choose not to run resistors at their rated wattage, thats your choice, its NOT technically required.

You say you DON'T operate anything at its maximum rating, does this apply to your domestic appliances...

To make it easier for you to understand, if I was specifying a resistor that I was going to operate in an ambient temperature of say 100C, I would look at the datasheet for the resistors and make a Wattage selection based on that Tambient.

If the required wattage was say 1Watt at 25C in free air and I had a Tamb of say 100C it could mean I would have to choose a 2 or 3Watt resistor.... do you follow that OK.??

Originally Posted by Sceadwian

Both I and the PDF I linked state that quiet clearly, you're aware of this inherently... New user will not be, new users will not even have the possibility of looking at the datasheet for their resistors because the bulk majority of these concerns are inferred and never explicitly stated resistors almost never actually have data sheets. Which is why the app note that I posted exists!

Your datasheet supports my point, 25C in free air.

How am I supposed to know the environment the user is operating his resistor and LED in.???

Are you suggesting that I should over specify the wattage, to suit the extremes of 250C that 'agu' keeps dreaming up.

ie; 3W or 5W resistor for a actual dissipation of 0.16Watt.???

You will find in engineering that you design to meet the required specification, not the worst case conditions for every component.

The upgraded calculators look great!

I'm digging this thermistor one. Sometimes I have to linearize the output voltage of an NTC thermistor in a resistor-divider network for a microcontroller. So I cheat and use a lookup table with three or four values and then linear-interpolate a piecewise reconstruction of the exponential curve. Using this calculator might prompt me to put in a few more values and get that curve closer.

Originally Posted by duffy

The upgraded calculators look great!

I'm digging this thermistor one. Sometimes I have to linearize the output voltage of an NTC thermistor in a resistor-divider network for a microcontroller. So I cheat and use a lookup table with three or four values and then linear-interpolate a piecewise reconstruction of the exponential curve. Using this calculator might prompt me to put in a few more values and get that curve closer.

hi,
Pleased to hear that.

Thanks for the wattage suggestion.

The advantage in having Online calculators at ETO is that we can enhance them with members idea's.

Always open to new calculator suggestions.