Effects of current/voltage limiting of LEDs

Question

First of all, I have read many articles about the non-linear effects of voltage vs current of LEDs. I also know that current will rise as temperature rises, batteries have resistance, LEDs brightness is not linear. All in all, LEDs are finicky devices.

But lets say we have a 100% perfect scenario:

• The LED is rated for 9v at 700mA. The temperature somehow stays a constant 25C
• The power source, never mind what, is DC 1000000 volts and can output 1000000 amperes. The voltage remains constant no matter what the load is, i.e. 0 resistance.
• We have a switching voltage regulator that somehow handles all the power and is 100% efficient with 0 ripple, etc.

If I have a CC/CV setup with the output at 9v and current limited to 700mAh. The LED should now be at full brightness.

Let's say that I want to dim the LED by keeping the voltage output at 9v and reducing the current to 350mA.

• This would dim the LED right? Theoretically, 1/2x the CURRENT = 1/2x the brightness (Ignoring the fact that our eyes are non-linear and that the voltage stays at 9v)
• Is there anything wrong with doing this? Will I damage the LED by forcing 9v through it and allowing it only 350mAh?
• Is it really better to do keep current limiting at 700mA but reduce the voltage? This would result in the current used by the LED plummeting if I reduce the voltage a teensy bit

Now let's say I want to put the brightness back up to 100%. Assuming I reduced the current limiting to 350mA before and kept the voltage at 9v, I would now have to increase the current limit back up to 700mA

• Alas! I accidentally increased the current limit to 200 Amperes! (but kept the voltage at 9v)
• This shouldn't do anything, should it? The perfect LED simply draws 700mA at 9v (because current is simply drawn, it generally cannot be forced?)

If I reduced voltage before, yet kept the current limiter at 700mA, I would then have to increase the voltage. (keeping the current limiter at 700mA) - Oopsie! I accidentally increased the voltage to 15V. Now this is far beyond the overvolt acceptance for LEDs, The current would try to skyrocket, but it is being limited to 700mA. - Is there any detrimental impact on having the LED run at 15v 700mA? Any normal unregulated LED would have surely burnt itself up by now.

So here are the scenarios for the LEDs in a nutshell:

350mA @ 9v

500mA @ 7v

700mA @ 9V

700mA @ 15V

Would doing any of the above destroy the LED necessarily? Except for that 4th one.

It is not possible for me to use PWM to control the brightness of the LEDs

Sorry for exaggerating everything, I just wanted to make things crystal clear. Is anybody able to brighten me up on this topic? Thank you

Answer 1

You can't control both voltage and current into a load. you control one and the load will dictate the other. you can set limits on both in which case which ever limit is reached first will cause the load to dictate the other.

when you reduce the current to 350mA the voltage on real LEDs would probably decrease a little.

trying to control LED brightness by varying voltage has not proven to be a good strategy. LED voltage varies with temperature etc.

the best strategy is to vary the current limit and leave the voltage setting high enough to not interfere with current control dimming.

10000A and 9V is a tricky one, but if the leds are magically perfect they will take 700mA when fed 9V, (real LEDs are less predictable, and will probably take several amps, make noise and smoke, and then stop working)

Setting 700mA and 15V should be fine the LEDS will get 700mA and the voltage will stay wherever it needs to be for that to happen (with your perfect LEDs it will stay at 9V, but with real LEDs it's less predictable)

500mA and 7V might not have enough voltage to light the LED. you might reach the 7V limit but 0mA flows.

Answer 2

This can be made clearer with a graph (stolen from http://www.amperor.com/products/led/constant_voltage_constant_current_led_driver.html )

For your hypothetical LED, at most one of your scenarios can be possible. Note that this doesn't go away in a "perfect" LED, you can't make it into a "cliff"; it's determined by the Shockley diode equation which in turn depends on the underlying quantum mechanics.

So if it's at a point of 9V/700ma, then in order to dim to 350ma the forward voltage has to reduce by a small amount. Similarly 700ma at 15V isn't possible - either you can't get the voltage that high at that current, or the current must increase to give you that voltage.