Is resistor really necessary for this LED configuration?

Question

LED 4v, 40mA

Battery 6v

I'm wondering if it's really necessary to have a resistor (56 ohms)

Because the above configuration is typical for most LED key chain sold on the market. Powered by 2 CR2016 creating 6v, and the LED forward voltage is 4v max. However, LED key chain on the market does not use any resistor.

So.... is it really necessary with the configuration above?

If so, why doesn't LED key chain have one?

What's the risk?

Answer 1

The need for additional components relates to the ENTIRE circuit. The existing circuit may have a resistive drop somewhere, but who knows and the need is based on many things. You don't indicate the additional application, so it is hard to know what you need for current limiting. Provide exact details (preferably a schematic of your intended circuit) to get a good solid answer.

Answer 2

Zero, you asked basically the same question two days ago.  Did you read the response?  If you bought an already assembled LED key chain light, that probably functions normally, why are you concerned with adding a current limiting resistor?  The only risk, if you must know, that you run would be to shorten the life of the LED.  Who or what is telling you to add a 56 ohm resistor to a purchased item?

Added:  They make some LEDS with built-in blinker circuits in that all you do is power them up with a battery.  This could be the same thing in your case.  The current limiting resistor could be built-in to the LED case itself.