Too much available current?

[PetrosA]

I'm pretty comfortable in the world of AC volts, but feel pretty lost in the DC world. Because LEDs and their drivers are becoming more common in my world, one of the things that I'd like to understand better is the relation between performance and available current in DC circuits. I've read in a number of places that when LEDs (and other DC loads) are given too much available current (and I'm assuming the voltage is remaining constant in these cases) that it can lead to shortened lifespan or malfunction. (There is no such thing as "too much available current" in the AC world and a 100W light bulb will work just as well on a 10A breaker as it would on a 100A breaker as long as the voltage is correct.) So how is current limited in a DC circuit without regulating voltage? What is the difference between a wall wart that provides 500 mA and one that provides 1.2A and why can it be bad to use the 1.2A one with a device that only requires 500mA if both are rated for the same output voltage?

[SnakeBite]

i'm pretty newbie in this forum but i just finish my first year of EE in the university here in Israel. and the far as i know AC and DC are not so different in the terms of the implementing . i don't know if what you said is true because if a light bulb is rated to deliver 75w of power and your main AC is for example 220v then you current will have to be around 2.93A if you put more than that even if you keep your voltage constant you will overload you light bulb and it will blow. there is no way that you can "feed" a regular light bulb with 100A and it will now blow (that what i think and learned).
and DC Devices is more sensitive for current changes then AC devices so if you feed 1A instead of 500mA or even 800mA it will fry out.

Ido

[alm]

Nothing to do with AC vs. DC, but with voltage sources vs. current sources. A LED is a current sink, so you have to drive it with a constant current source. If this constant current is too high, you're overloading the LED. The total available current doesn't matter for a voltage source (the available current for the ideal voltage source is unlimited), just like the ideal current source can supply an infinite voltage (unlimited compliance).

[ChrisGammell]

alm is on the right track, but it's a bit beyond that.

The real difference is that an LED is a diode whereas a lightbulb is effectively a resistor.

So if you have a lightbulb on 120V AC power, it's the internal resistance of the lightbulb that is controlling how much current is drawn from the source. In the case of a 100W lightbulb, it's likely about 144 Ohms (P=V^2/R so 100W = 120V^2 / X...X = 144). The manufacturer of the lightbulbs design them to draw the correct current from the sockets and to optimize light output for the current drawn (and light temperature and longevity and a bunch of other things that aren't relevant here). The circuit breakers you mention are only in place to LIMIT the total amount of current allowed through to a certain device, they have no impact on the device itself. So in the above example, if you had 500 100W light bulbs all hooked up in parallel, each asking for .8333 A (the amount the bulb wants based on 120V supplies and 144 ohm internal resistance), the circuit breaker would see too much current flowing through it and shut down to protect your circuit (and probably your house wiring from burning down). 

The differences with a Light Emitting Diode (LED) is beyond a certain voltage drop across the device, it will sink as much current as it can. See http://en.wikipedia.org/wiki/Light-emitting_diode#Technology for a few more details about it. So say you hook up a bench voltage source across an LED that is set to 1V drop. If the voltage source can supply 10A, it will supply AS MUCH as it can past the biasing point. So what to do? Well, if you have a 1V supply and the LED is going to "eat" .7V of that in order to be biased on, you have .3 V remaining to play with. If you put a resistor in series with the LED, the .3V across that resistor will determine the current for the entire circuit! Boom, easy current limit. So if your LED can only hand 30 mA (max), you pop a 10 ohm resistor in there and your LED won't go "pop". Of course you can also do what alm said and instead supply your LED with a current source, but a voltage source + a resistor in series effectively IS a current source.

[Zero999]

Nothing to do with AC vs. DC, but with voltage sources vs. current sources. A LED is a current sink, so you have to drive it with a constant current source. If this constant current is too high, you're overloading the LED. The total available current doesn't matter for a voltage source (the available current for the ideal voltage source is unlimited), just like the ideal current source can supply an infinite voltage (unlimited compliance).

Yes, an AC example of a device requiring a constant current, would be a fluorescent tube which requires a ballast to limit the current.

Most DC loads require a constant voltage but as with AC a fuse may be required for safety reasons.

[PetrosA]

Thanks for the answers - they help a lot. I see that at 40 yrs. old, I'm not done learning

[Simon]

Thanks for the answers - they help a lot. I see that at 40 yrs. old, I'm not done learning

That's the spirit ! with that attitude you can go a long way

[IRLOONY]

 ;)None of us ever stop learning ,more so in Electronics being there is always something new to play with,Im 44 and still play as if im 12 but now i have my own Shed its been 32 years since i first used LEDs on my pushbike in the 70s.
 oh the cheap option is in a DC system Vs-Vf/If=R