# LED comparision with same current and different voltages.

Discussion in 'General Electronics Chat' started by AcousticBruce, Mar 19, 2014.

1. ### AcousticBruce Thread Starter Active Member

Nov 17, 2008
58
0
A LED might need 30mA and lets try a couple voltages.

With a 9V battery (9v-2.1v) / .030A = 230 Ohms
With 220V supply (220V - 2.1) / .030A = ~7.3M Ohms

The 7.3 Ohm resister is @ 6.5 Watts
The 230 Ohm resister is @ 207 mW

So what about the LED? What is the different wattage? What if I did 1 million volts? What governs the max voltage?

2. ### ronv AAC Fanatic!

Nov 12, 2008
3,392
1,449
1-The resistor limits the current so the LED wattage is the same (2.1 X .030).

2- very big resistor

3- nothing except it wastes power.

3. ### spinnaker AAC Fanatic!

Oct 29, 2009
5,065
1,160
You can have any voltage you want as long as it is above the forward voltage of the LED. But the current must be limited to under the max current rating of the LED.

4. ### AcousticBruce Thread Starter Active Member

Nov 17, 2008
58
0
Thank you RonV.

I think I am getting this a little more. Tell me if I am right with this thought. The power from the 220V is mostly getting squelched by the resister and converting it to Watts. The LED has no idea and doesnt care what the original voltage was, it is only getting 30mA and not feeling the pressure of the 220V at all. a 9V @ 30mA and a 220V @ 30mA feels exactly the same to the LED.

Am I right about that last sentence?

5. ### ronv AAC Fanatic!

Nov 12, 2008
3,392
1,449
Right you are.

Whenever I see 220 volts I think AC volts. You have to do things a little different to run LEDs on AC.

6. ### AcousticBruce Thread Starter Active Member

Nov 17, 2008
58
0
yes 220 was meant to be DC. I just grabbed a common amount. AC I could just use 2 LEDs in parallel in opposite directions. This is what I want to learn about next. Maybe use a capacitor as the reactance resistance and a 60hz 120V lines and see what happens.

7. ### Bernard AAC Fanatic!

Aug 7, 2008
4,232
414
AAC does not support line operated LED's; but mine has been running for over 8 years 24/7 abet a bit dimmer. It does require more LED protection than just cap; line transients can be LED killer.

Last edited: Mar 20, 2014
8. ### BobTPH Active Member

Jun 5, 2013
806
121
In both cases, the voltage across the LED is the same (2.1V) The remaining voltage appears across the resistor.

Bob