Question about LEDs

Thread Starter

lordeos

Joined Jun 23, 2015
33
Hi everyone,

Thx for the detailed answers but somehow i think my message didn't came across ... i'll try to explain.

First of all i ment indeed 1miliamp --> my mistake sorry

info :
- fixed resistor : 1megaohm
- potresistor : varies from 0k to 100k
- red led : VF --> starts at 1.4
- light bulb : 100 miliamp @ 6V
- supply voltage: 6V

if i put the 1 megaohm resistor in series with the trimmer (set at 100K) the led is barely shining but the voltage drop over the LED is still 1,4V and the display of my power supply shows a current of 1 miliamp or 0.001A. From other experiments i did ,it seems to be that a LED almost has a fixed voltage drop of at least 1.4V even with almost no current flowing trough it and you almost need infinite resistance to drop to 0V over the LED , is this correct what i'm thinking ??? 100 k + 1 megaohm seems not to be enough to get the LED below 1.4V

When i do the same experiment and replace the LED by a 6v lightbulb the 6v supply voltage is completely dropped over the two resistors (1 megaohm + trimpot @ 100k --> resulting in 0V reaching the lightbulb (so lightbulb not working).

I'm convinced that i'm missing something about the "working" of an LED and i miss some valuable info thats why i want your professional info.

Why is a LED so hard to shut down completely (0 V voltage drop) where as the lightbulb is 0V even with less resistance in my fixed resistors ?

Thx Mike
 

blocco a spirale

Joined Jun 18, 2008
1,546
Hi everyone,

Thx for the detailed answers but somehow i think my message didn't came across ... i'll try to explain.

First of all i ment indeed 1miliamp --> my mistake sorry

info :
- fixed resistor : 1megaohm
- potresistor : varies from 0k to 100k
- red led : VF --> starts at 1.4
- light bulb : 100 miliamp @ 6V
- supply voltage: 6V

if i put the 1 megaohm resistor in series with the trimmer (set at 100K) the led is barely shining but the voltage drop over the LED is still 1,4V and the display of my power supply shows a current of 1 miliamp or 0.001A. From other experiments i did ,it seems to be that a LED almost has a fixed voltage drop of at least 1.4V even with almost no current flowing trough it and you almost need infinite resistance to drop to 0V over the LED , is this correct what i'm thinking ??? 100 k + 1 megaohm seems not to be enough to get the LED below 1.4V

When i do the same experiment and replace the LED by a 6v lightbulb the 6v supply voltage is completely dropped over the two resistors (1 megaohm + trimpot @ 100k --> resulting in 0V reaching the lightbulb (so lightbulb not working).

I'm convinced that i'm missing something about the "working" of an LED and i miss some valuable info thats why i want your professional info.

Why is a LED so hard to shut down completely (0 V voltage drop) where as the lightbulb is 0V even with less resistance in my fixed resistors ?

Thx Mike
Maybe you should read all the posts again, especially those explaining the difference between a light bulb and an LED.

Since V=IR it is impossible to get 1mA through a 1M resistor with a 6V supply so something is wrong somewhere.

And obviously, If you have infinite resistance in series with the LED you will of course measure 0V across it. Replace your resistor with air and you'll see what I mean.
 
Last edited:

dannyf

Joined Sep 13, 2015
2,197
I think the messages people have been sending your way is that with a supply voltage of 6v, loop resistance of at least 1MOhm, there is no way you can drive 1ma through that thing.

Physically impossible.

Figure out what went wrong there and then we have a basis for a discussion.
 

Thread Starter

lordeos

Joined Jun 23, 2015
33
Hi guys,

well beginners mistake i guess, my display on my power supply seems not to go that low :)

so i did a measurement with my voltmeter and it turns out to be 000.5 microampere (is this better). as an extra i put in one LDR in series with the LED and put some black tape on the LDR making it 10 megaohm of resistance. Now my LED dropped down to 1.1 --> no more light.

Is amazing that a so low current (almost none) is still able to produce a 1.1V of in the LED
 

Thread Starter

lordeos

Joined Jun 23, 2015
33
so just to clarify ... the only resistor in series with the LED is the 10 megaohm LDR and it produces 000.5 ua or microampere
 

MikeML

Joined Oct 2, 2009
5,444
...You can view a diode (led or otherwise) as a resistor whose "resistance" is a function of the current going through it.
No you cant. Just because you can measure a voltage across a LED (Vf) and a current through it (If), just because you can numerically divide Vf by If does not make the result a "resistance"!
 

Wendy

Joined Mar 24, 2008
22,436
Like I said, you have to get the voltage to the minimum, Vf. There are a lot of people who try to use the Vf to bias the LED, big mistake, since it is a current device.
 

dannyf

Joined Sep 13, 2015
2,197
so i did a measurement with my voltmeter and it turns out to be 000.5 microampere (is this better).
The maximum current in that circuit, putting aside the led, is 6v/10Mohm = 0.6ua. So your new figure makes perfect sense.

Is amazing that a so low current (almost none) is still able to produce a 1.1V of in the LED
Yeah. The equivalent resistance of a led at very low current levels can be in the multi-MOhm range -> see the simulation I showed earlier.

If you could lower the current further - putting in a few of those LDRs or a GOhm resistor (or a TOhm resistor -> aka air / vaccum / a piece of plastic), you can drive that voltage drop to (near) zero.

That's simply physics. No amount of authority can change that, :)
 
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