Why I can`t drive 8 led diodes from 12v power supply if every diode need 1.5v?
Also, like you can see in my first post, specification for this diodes is not good, cheap chinese diodes and I can`t know real spec, they said:
Voltage: 1.5V~1.6V
Current: 20-25 mA
Peak(pulse): 700mA
Power: 200mW
But how can be 200mw with voltage 1.5V~1.6V and current 20-25 mA?!
Also, on ebay don`t have many sellers for 10mm IR diodes, I don`t know why...
Hi there,
I thought i might help clear up the voltage paradox problem a little here.
As others have already mentioned, the voltage of the 'diode' can change without you knowing it, and while they are running. You set them up to run and measure their voltage and it might be 1.500 volts, but once they run for a while and after a few months the voltage measured might be 1.400v or something like that.
This is a common wonder when people first encounter LEDs. They read that the LED voltage is like 1.5v, and it seems that they will always run at 1.5v. I think this is because we are previously so conditioned to think in terms of light bulbs which have filaments, and those devices usually require a fixed voltage. LEDs are not light bulbs though, and they need attention to current more than voltage. So you see we can not really say truthfully that an LED 'needs' 1.5v, we have to instead say that the LED 'needs' 20ma. Even though the data sheet will mention voltage as well as current, it's the current that we have to pay most attention to very unlike filament bulbs. Yes, the LED needs *at least* 1.5 volts to operate, but that's just a very rough estimate of what the voltage will be when it is first run up and after several hours of running and after several weeks or months of running when it may change to something else. Thus, our circuit must make sure that the current level, not the voltage level, stays constant for the life of the LED, even with the voltage changes i talked about above.
The basic rule for a filament light bulb is: "Let the current change to whatever it wants to, but keep the voltage as constant as possible".
The basic rule for an LED is: "Let the voltage change to whatever it wants to change to, but keep the current as constant as possible".
The reason we usually use a resistor in series with the LED is because that is the simplest way to limit current to a safe level during the entire life of the LED. There are better ways, but a resistor is the simplest and cheapest. The idea is to make sure you have enough voltage for the LEDs plus a little more, and use a series resistor to drop that little more. The result is an LED that can run for years.