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led after glow

klauslundsgaard

New Member
hi
this is my first post on the site and my knowledge in electronics is near not existing !

i am making a light panel for enlargers.
i use 25 pcs 3 w 3,4V leds in line and a constant current driver.
i have got a new model of drivers and see a after glow in around 1/3 of the array of leds for 1-3 seconds.
the old model did not have the problem.


i read on a site that a capacitor of 0,1 micro farad with resistance of 100 ohm will fix the issue.
is this capacitor ok for my set up ?
how / where do i mount it ?

kindly
klaus lundsgaard
denmark

att photo of the light panel
 

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Perhaps, output shutdown after input off is not intended to be "active" or controlled and just coasts to zero. A "passive" load might speed that up a bit... as previously suggested.

A shunt cap might only be to reduce stray AC induced on long strings acting as antenna to line electric fields on DC without protective earth ground on 0V. Usually very dim when off. The LEDs rectify the stray AC to DC. Where did you read that?

With very low current, the best LEDs with the lowest leakage may be brighter at low << 1mA currents from the slow decay time of seconds. It may also mean the newer drivers store more energy and do not switch off output with low input.
 
You must be seeing the after effects of the phosphor used in the LED.

Some possible fixes :



Regards, Dana.
It's called a "phosphor" but it's really fluorescent material - with half-life in the nano-second range. White LEDs can be turned completely off at more than 100kHz. In other words, white LED phosphors are NOT phosphorescent with long lifetimes. The web page is COMPLETELY WRONG about phosphors for LEDs.

Did you really read the causes of glow in this page? They are comical. "Old wiring" causes afterglow? Really. Does old wiring also make my music sound like Hank Williams tunes?

Afterglow is caused by constant current sources that don't shut off completely or people designing around power supplies that don't shut off immediately when unplugged from mains power - especially when a larger supply is powering a very small load. Or any other weird designs of power supplies or poor attempts at turning off the supply - which should be shut off hard, close to the LED, and not upstream, near mains power and allow energy stored in inductive or capacitive elements to drain to zero volts.
 
ZipZapOuch appears to agree with my conclusions or at least does not contradict any.

Your link recommends a plastic X-rated capacitor to suppress AC noise for every assumption of cause.
I agree phosphor glow is never a cause.
Excess stored energy decay time is the prime cause.
Since wht. LEDs rise in impedance rapidly below 2.5V from the 1 Ohm range for a 1~2W LED to > 10k ohms only a resistive load rated for say 1 to 5% of power rating will improve it. A similar problem on dimmable LEDs on Triac dimmers might fail to turn off unless at least one incandescent 10W bulb was wired in parallel or about 5% of the rated 500W load.
.
I personally found all the dry contact switches near my dimmable ceiling PAR LEDs get interference and blink every time a dry contact heater or fan switch goes OFF. Adding a suitable cap across the line like below or even less 10 nF across the switch, fixed the glitch.

1708634682756.png


If you are really concerned, it can be fixed with more details on the driver and LED specs links.
 

ZipZapOuch


This paper seems to indicate some phosphors used decay times are in mS, as well as
ones with nS.


Not sure if this is commercial but interesting :


And further papers seem to indicate number of layers compounding effect.

Does old wiring also make my music sound like Hank Williams tunes?

I would say yes, for you, that's a possibility.....:)


Regards, Dana.
 
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copy paste from Tony Steward:
I personally found all the dry contact switches near my dimmable ceiling PAR LEDs get interference and blink every time a dry contact heater or fan switch goes OFF. Adding a suitable cap across the line like below or even less 10 nF across the switch, fixed the glitch.

1708634682756.png

thank you for explaing in plain english !
i am greatful for the lectures i got from members, but to be honest i do not understand the slightest !
i have ordered the cap and then ill see.
thanks a lot
klaus
denmark
 
It won't help the slow decaying turn-off, but it may help if you happen to see a blink ok ceiling lights from the coffee heater or bathroom fan turn off transient glitch.

As I explained it needs a resistor. and we need the model numbers of everything
 
LED bulbs may glow faintly, especially when there's a large distance between the switch and bulb, caused by capacitive coupling from other parallel wiring.
 
Not all phosphors have the same decay rate yet all commercial white LEDs have a phosphor decay rate much faster than the eye can detect in the microsecond range as someone stated. But this is a conducted storage decay time issue with capacitors.
 
LED bulbs may glow faintly, especially when there's a large distance between the switch and bulb, caused by capacitive coupling from other parallel wiring.
I wonder where have you heard, read or seen this? That effect is on ungrounded DC supplies to high resistance LED e.g. >= 5m long Stripleds only when the power is off, as I indicated.
 
I wonder where have you heard, read or seen this? That effect is on ungrounded DC supplies to high resistance LED e.g. >= 5m long Stripleds only when the power is off, as I indicated.
On AC mains supplies, there can be significant capacitance between live and switched live. When the switch is open, some current will flow through that capacitance.

For some LED lamps, that current can be enough to make them glow.

In fact, in some LED lamps, the current limiting device is a capacitor. Any capacitance in series with the lamp will reduce the current they take, but the little remaining can cause glow.

The same thing happens with neon power indicators.
 
This does not show me the problem how it looks or how it is wired. Can you make a video?
1709241401380.png


I see two strings of disconnected LEDs.
Similar LED datasheets show each LED will be 3.15 V approx. at 250 mA nominal.
1709241428221.png

The Driver of constant current puts out 240 to 260 mA up to 65 V maximum.

That tells me you can string 65V/ 3.1V=20.96 LEDs or ~ 21 of these LEDs in series.
Meaning it will exceed the 65 V MAX rating and not perform as expected with 25 or 28 LEDs in series!

It may work with 25 but it may cause driver stress internally or may fail prematurely or any number of odd symptoms.

I see that your driver and LED strings are mismatched.
Each 1 watt LED has a suitable heatsink area but you don't have enough voltage at 250 mA.

It is always important to the LED string to the driver, or get another driver to match your string (s) and then match your strings.

If you have a meter to measure Vdc when connected, let me know voltage. (carefully)

You can reduce the strings to be equal and share the constant current such as 25 each in parallel or 25S-2P "25 LED Series strings with in 2 in Parallel" Thus each String will share the 250 mA and at 125 mA, The LED voltage drops to around 2.9V. So 25 x 2.9 = 72.5V +/-20%? which is still too high,

So it should be shortened 23S-2P or whatever those need to be so that the voltage is less than the rated max voltage of 65V.

FYI
To trigger an alert to someone type @ then the 1st few letters of their name, then select it, to automate a message.

klauslundsgaard
 
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