Why bulbs and LED diodes keep glowing after being switched off?

[pepediaz]

Homework Statement

Just curiosity

Relevant Equations

Maybe magnetic induction ones and electromagnetic energy/momentum formulae (just supposing)

The bulb/LED is connected to a long cable consisting of two insulated wires.

I want to know how is this phenomenon happening in a physical way. I mean, not referring to the quality of used materials.

I suppose that for the bulb, it happens kinda Joule effect, and when the circuit becomes open, the tungsten has to cool down. Even though, that seems more like a thermodynamical question. I wanted to know how can we explain it with electromagnetism (maybe currents and power consumption are interesting to know?)

For the LED, I also think that there is some electromagnetic explanation before tackling the question with solid state arguments.

Thank you beforehand!

 

[berkeman]

They don't. Incandescent bulbs and LEDs go out right away when you disconnect the driving current. It sounds like whatever you are seeing is in devices that have current supply circuits that do not cut off the driving current right away when you change their switches to "OFF".

 

[pepediaz]

But it's a circuit I have made, with a long cable and a battery. The effect of dimly lighting after being switched OFF happens. Both for bulb and LED. I don't know why.

 

[berkeman]

But it's a circuit I have made, with a long cable and a battery. The effect of dimly lighting after being switched OFF happens. Both for bulb and LED. I don't know why.

How long of a cable? What type of cable? How much capacitance does that cable have? Is the switch at the battery end or at the LED end? If it is at the battery end, what happens if you move it to the LED end?

 

[Steve4Physics]

I guess you are referring to the LED lights used for domestic and commercial lighting.

The main reason they continue to glow is this ...

In order to produce white(-ish) light, blue LEDs are often used but with a layer of phosphors to get the colour-temperature about right. The phosphors are chemicals which can store energy for many seconds and re-emit it as light of the required wavelengths (colours).

The LED lights continue to glow after being switched off (or even completely removed from the supply circuit) because the phosphors (not the actual LEDs) are emitting their remaining stored energy as light.

 

[PAllen]

Also, I think the thermal explanation for incandescent bulbs must be correct. How can the filament cool from the emitting temperature in exactly 0 seconds?

 

[berkeman]

How can the filament cool from the emitting temperature in exactly 0 seconds?

It's not instantaneous like that, but I did a test with some incandescent lights in my home before posting what I did above. They go out "in the blink of an eye", and do not appear to go out with any perceptible glow period (at least for me).

 

[pepediaz]

I have plot the circuit the best I could. The cable is 10 m long, and the two insulated wires have each one a section of 1.5 mm•mm.
Let's suppose there is no capacitance.
Changing the position of the bulbs is indifferent.

 

[Omega0]

I would like to mention this link: https://ledlightinginfo.com/why-do-light-bulbs-glow-when-switched-off

It should clearly be said that white hot light bulbs definitely take a short time to cool down when you still can see the light dimming from orange to red.
Naturally, for LEDs it is a completely different story. They are super fast and you perfectly use them for communication protocols. As mentioned above, if there is some slowly dimming light this is not the way the LED behaves but a component more. This so-called phosporescence comes from specific materials (which not necessarily have to do with phospor).

PS: For laser LEDs you should avoid to check directly if they go out instantly... believe me, they do.

 

[PAllen]

It's not instantaneous like that, but I did a test with some incandescent lights in my home before posting what I did above. They go out "in the blink of an eye", and do not appear to go out with any perceptible glow period (at least for me).

I see about .25 seconds on any of my clear incandescents. A trick to avoid retinal overstimulation is to turn toward the clear bulb only at the moment of turning it off. Then I clearly see the filament fade and get redder.

 

[berkeman]

A trick to avoid retinal overstimulation is to turn toward the clear bulb only at the moment of turning it off. Then I clearly see the filament fade and get redder.

Ah, good point. That probably was contributing to my sense of the turn-off time being so short.

 

[PAllen]

As for LEDs, I have two different types in my house. For one, no matter how I try to catch an afterglow, I see none. For the other type, I see nearly a full of second afterglow. Per earlier post, it appears one uses phosphors with long phosphorescence, while other either uses no phosphors, or uses ones with decay too rapid to be seen.

 

[Steve4Physics]

It's not instantaneous like that, but I did a test with some incandescent lights in my home before posting what I did above. They go out "in the blink of an eye", and do not appear to go out with any perceptible glow period (at least for me).

Hi @berkeman, here’s an experiment that’s worth trying...

1. Ideally use a dark room with an incandescent bulb the only light source.
2. Close your eyes and avoid looking in the direction of the bulb (so your eyes are partly dark-adapted).
3. Using your finest coordination skills, simultaneously switch off the bulb, open your eyes and observe the bulb.
4. With a bit of luck, you should see the filament change from yellow to orange to dull red; it’s pretty quick (less than 0.5s maybe) so you might have to try it a couple of times.

 

[berkeman]

Hi @berkeman, here’s an experiment that’s worth trying...

My wife is already worried about me and my time spent on PF. I'll just take your word for it on this one...

 

[Keith_McClary]

The phosphors are chemicals which can store energy for many seconds and re-emit it as light of the required wavelengths (colours).

Also fluorescent lights. The phosphors on CRT TV tubes are chosen to emit over the time of one frame to minimize flickering.

 

[Gordianus]

I've seen many large incandescent bulbs that "die" slowly. They have a "large" mass and cool down in some tenths of a second.
As for LEDs, in addition to the phosphor, some LED-driving circuits may have an output smoothing capacitor that keeps providing current.

 

[pepediaz]

Thank you all, now I can imagine better how this phenomenon works.

Is there a way to estimate the currents and power consumption when the circuit is open?

 

[Merlin3189]

Also fluorescent lights. The phosphors on CRT TV tubes are chosen to emit over the time of one frame to minimize flickering.

If you go back to CRTs, then oscilloscopes could have long persistence phosphors lasting seconds (they fade gradually, so depends where you call it cut off.) Similarly radar displays needed the image to remain reasonably visible for one rotation of the antenna, which would have been seconds.

My first thought when you mentioned CRTs persistence, was watching the screen collapse to a dot and last for 10 seconds or more. I think that might have been the slow discharge of capacitors keeping electrons bombarding the screen until the hot cathode had ceased to release any electrons (all landing on the central dot because the magnetic scanning had ceased.)

 

[Merlin3189]

View attachment 293948

I have plot the circuit the best I could. The cable is 10 m long, and the two insulated wires have each one a section of 1.5 mm•mm.
Let's suppose there is no capacitance.
Changing the position of the bulbs is indifferent.

Coming right back to this, you seem to have drawn a bulb in series with a capacitor connected to the AC power. The switch is in parallel with the capacitor and supplies extra current when closed. But even without the switch a small current can flow via the capacitor, which might be enough to light the LEDs in the bulb all the time.
Even if the LEDs eventually go out, they may have been sustained by the charge on an internal capacitor. The external, wiring, capacitance would simply extend this period by allowing partial recharging of the internal capacitor.

 

[Steve4Physics]

Is there a way to estimate the currents and power consumption when the circuit is open?

'Open' would mean the circuit is off. In this case, the current and power are both zero.

If the circuit is operating, you would need to know the power consumption for the type of lamp being used. E.g. suppose you have a lamp rated by the manufacturer as 10W when used with a 230V AC. supply.

Assuming losses in the connecting wires are negligible, the power is simply 10W.

The current is the power divided by the supply voltage. so the current is 10/230 = 0.0435A approx.

(The voltage and current above are intended to be rms (root mea square) values - a special type of average used when dealing with alternating supplies.)

 

[Merlin3189]

Is there a way to estimate the currents and power consumption when the circuit is open?

I looked at 10m cable, 1mm diam wires, 2mm apart, PVC insulation and got about 1nF capacitance, for about 3MΩ impedance at 50 Hz and about 80uA current, neglecting the bulb impedance. A lot of 'about's in there and perhaps I shouldn't neglect the bulb's (complicated) impedance. Maybe I'll try putting 80uA through one and see if it glows dimly.
Power seems messy and I haven't worked out even a rough estimate. And I doubt a power meter would work at such low current.

 

[pepediaz]

First of all, thank you all for your responses, they are giving me a lot of insight in this issue.

So, as I can observe, the phenomenon is mainly due to the materials and the little capacitances the current finds on its way.

I have thought about hypotetical physical phenomena: magnetic induction, residual potential, cables covering remaining polarized, the charge in the flow of current in one part of the wire induces flow in the other one...

Maybe I am mixing things that are not important here. But just for getting the comprehensive intuition over the bulbs and their dim glowing after switching off.