In an online circuit simulator, I was observing the power of a resistor along with the heat produced in the resistor.

When a voltage of +9v is applied across a resistor of 10 ohm connected in series with a led, power or P= 4.79w.

When a voltage of +64v is applied across a resistor of 3k ohm connected in series with a led, power or p= 1.29w

Also I understand (correct me if I am wrong) to maintain the same current through a circuit when the voltage is increased, the resistance has to be incresed which causes power of the resistor to increase along with the heat quite so much as to melt the resistor (assuming resistors used are 1/4w power).

Now my question is : 1. Neglecting current needed by led, why doesn't the 10 ohm resistor melt or heat up (I tried it practically with a 9V battery) ? 2. Why doesn't the led burn practically when I connect it to the 9v battery as the simulator shows 692.16mA?

Thanks! enter image description here


He difference between theory and practice is greater in practice than in theory :-)

The main issue here is that the real life battery is unlikely to be capable of supplying the current seen in the simulator. If it was then a 20 mA LED would die and a low wattage resistor would be "noticeably warm" (or soon go open circuit).

The way to check this in practice is to use a voltmeter to measure the voltage across the 10 Ohm resistor when a 9V battery is used (left hand circuit).
I = V / R
So I i amps = Vresistor / 10 in this case. Also, measuring the LED Vf in this situation would be informative.

Report back.

  • \$\begingroup\$ Thanks. I got it. The battery in the simulation can provide infinite current as compared to a practical one which gives about 500mA, hence the resistor doesn't really get hot enough to melt. Right? \$\endgroup\$ – putu06 Dec 19 '15 at 17:49

9V battery can definitely kill LEDs so yours must have been crap or discharged. And that's in line with the datasheet for such a battery; it can easily dish out 500mA.

  • \$\begingroup\$ Ibid: youtu.be/KidBQaoNjPY?t=98 \$\endgroup\$ – Fizz Dec 19 '15 at 0:26
  • \$\begingroup\$ I did connect a 10 ohm resistor in the circuit as said in the question. \$\endgroup\$ – putu06 Dec 19 '15 at 20:01
  • \$\begingroup\$ @putu06: That's not enough to make a difference to most LEDs. But you're not saying what LED you have or what wattage the resistor is. 500mA through 10ohms will give you 2.5W, which will smoke most resistors. Nobody found your precise experiment funny enough to do on youtube, but 12V DC source will do this to a 47 ohm resistor: youtube.com/watch?v=iii_2WYMXh4 The power there is about 3W. \$\endgroup\$ – Fizz Dec 19 '15 at 20:11

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