You really understand resistance usage?

Resistance as a basic electronic components, widely used in a variety of circuits, generally we also think resistance is the simplest electronic components.In addition to power, there is no excessive stress.If today I say this small resistance, many senior electrical engineers are not really know how to use,do you believe it?
In the selection of resistance,if avoided high-frequency circuit special application, generally we only consider the power of resistance.for the average engineer,As long as select the appropriate power by the current that flow through the resistor,then can be counted as qualified.
If take into account the instantaneous power, it can be counted as a good engineer. But be able to properly use the resistance, only two things were not enough,pleaes read the following text then you will understand.
Look at a simple circuit diagram,by the current flowing through the sample LED lights from sampling resistor R1 and R2,then adjust PWM output,to ensure the LED current constant that in different voltage.




 There is a place make confuse, why sample resistor use two 3.9 ohm resistor?If with a 2 ohm resistor also seems ok.
In my personal understanding, this circuit form has the following advantages:
1. Multiple resistance is more conducive to heat, two 1W 2ohm resistor in parallel compared with a 2W 1ohm resistor used alone, the current through the same.Most time two resistors in parallel the temperature rise a little,resistance withstand power will be significantly decreased at high temperature (for example, higher than 70 degrees Celsius), so that the effect of in parallel circuit is better.
2. Multiple resistance can get a more ideal resistance value by combine.

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