Do LED's "draw" current?

[PizzaCombo]

Hi,

So it is my understanding that things like DC motors "draw" the current it needs (i.e if your supply can give out 5 amps, the motor will only draw what it needs, not necessarily 5 amps)

Do LED's work in the same way? It seems that by placing an LED in the circuit, it just happens to be intercepting the already flowing current, and not drawing only what's needed (thus the need for a resistor so it doesn't overheat and pop).

My question is, is there a difference between things like motors and LED's when it comes to current "draw"?

 

[Harald Kapp]

Yes, there is a difference, but it is not as you describe it.

A motor, a tungsten lamp, a resistor etc. "draw" a current when connected to a voltage source.
The current through a resistor follows Ohm's law: I=V/R.
The current through a lamp is similarly controlled by the lamp's resistance. But a lamp's resistance increases with the temperature of the filament which in turn rises when current flows. Therefore a cold lamp has a high inrush current that diminishes when the lamp is lit.
A motor's current is controlled in part by the resistance of the motor's windings, in part by the inductive effect of the motor's coil and by the counter emf generated by the motor's rotation.

An LED's characteristic has a very steep current-vs-voltage curve. Therefore a small change in voltage will generate a big change in current. This effect easily leads to destruction of the LED. Therefore you do not connect an LED directly to a voltage source. You need to control the current through the LED. The voltage across the LED is a consequence of the current and the LED's characteristic.

Read more in the tutorial section "Got a question about driving LEDs?"

 

[PizzaCombo]

Yes, there is a difference, but it is not as you describe it.

A motor, a tungsten lamp, a resistor etc. "draw" a current when connected to a voltage source.
The current through a resistor follows Ohm's law: I=V/R.
The current through a lamp is similarly controlled by the lamp's resistance. But a lamp's resistance increases with the temperature of the filament which in turn rises when current flows. Therefore a cold lamp has a high inrush current that diminishes when the lamp is lit.
A motor's current is controlled in part by the resistance of the motor's windings, in part by the inductive effect of the motor's coil and by the counter emf generated by the motor's rotation.

An LED's characteristic has a very steep current-vs-voltage curve. Therefore a small change in voltage will generate a big change in current. This effect easily leads to destruction of the LED. Therefore you do not connect an LED directly to a voltage source. You need to control the current through the LED. The voltage across the LED is a consequence of the current and the LED's characteristic.

Read more in the tutorial section "Got a question about driving LEDs?"

Great explanation, I will read the tutorial.

Thanks Harald

 

[HellasTechn]

In other words the LED just like a DC motor or a simple lamp will only draw the current required to poerate BUT this has to do with whe source voltage and its resistance.

Like the friend above stated.
Current (I) = Voltage (V) / Resistance (R).

if you raise voltage keeping steady resistance current will rise.

Given that the LED has a certain resistance that practically does not change because of temprature and it can handle a certain ammount of miliamps (2-3 if i remember correctly) depending on the LED then when you raise voltage currnet will also raise and you will burn it.
So you put a resistor in series with the LED to increase its total resistance so it will handle the extra currnet.

 

[(*steve*)]

Given that the LED has a certain resistance that practically does not change because of temprature

Everything was fine up to there.

Look at section 0 of https://www.electronicspoint.com/got-question-driving-leds-t256849.html

LEDs (just like incandescent lamps) have a non-linear voltage vs current relationship. Neither act according to ohms law. However, incandescent lamps are non-linear in a way that tends to result in stability whereas LEDs are non-linear in a way that tends to instability.

The purpose of the series resistor is to swamp out that non-linearity over an expected range of voltages so the current becomes predictable and stable.

 

[HellasTechn]

Didn't knew that.

I thought that for example 3.5mm LED's had stable current draw because they dont ge warm after use.

I knew that for example the XML-T6 LED's produce massive ammount of heat so that it is only
logical hat their resistance will shift.