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Taupe
Guest

Wed Feb 06, 2019 4:45 am   



Considering repurposing backlight LED's from discarded broken
TV's into a large LED lamp built into a picture frame & run
directly from 240 v AC.

Roughly if each LED need 3v to power on , I'd need 80 LED's linked in series
to total up to 240 v.

This will eliminate the need for separate power circuitry ( which would bulk
up the frame), thus have single power chord out of frame into wall
socket.

Will this kill all the LED's with voltage spikes?

Will the final lamp flicker at supply rate of 50hz?

Will it potentially be fire hazard from overheating & shorting?

--
'Journalism is printing what someone else does not want printed: everything
else is public relations.' - George Orwell

*Beat the system, don't let the corporate fascist take away your
privacy. https://www.privacytools.io/#

Jasen Betts
Guest

Wed Feb 06, 2019 10:45 pm   



On 2019-02-06, Taupe <jonahreal_at_yopmail.com> wrote:
Quote:
Considering repurposing backlight LED's from discarded broken
TV's into a large LED lamp built into a picture frame & run
directly from 240 v AC.

Roughly if each LED need 3v to power on , I'd need 80 LED's linked in series
to total up to 240 v.

This will eliminate the need for separate power circuitry ( which would bulk
up the frame), thus have single power chord out of frame into wall
socket.


no.

> Will this kill all the LED's with voltage spikes?

that depends on the power supply circuitry you use

> Will the final lamp flicker at supply rate of 50hz?

Unless you use a full wave rectifier it will. (else 100Hz)

> Will it potentially be fire hazard from overheating & shorting?

yes.

--
When I tried casting out nines I made a hash of it.

Computer Nerd Kev
Guest

Wed Feb 06, 2019 10:45 pm   



Taupe <jonahreal_at_yopmail.com> wrote:
Quote:
Considering repurposing backlight LED's from discarded broken
TV's into a large LED lamp built into a picture frame & run
directly from 240 v AC.

Roughly if each LED need 3v to power on , I'd need 80 LED's linked in series
to total up to 240 v.


You're not going to have control of how many LEDs are in the
backlight assembly in the first place anyway. LED forward voltages
also vary quite a bit, 3V is at the high end.

Quote:
This will eliminate the need for separate power circuitry ( which would bulk
up the frame), thus have single power chord out of frame into wall
socket.


You could quite easily have the power circuitry in a separate box and
connect that to the panel via another cord.

> Will this kill all the LED's with voltage spikes?

Without any current limiting, it will kill the LEDs even without
a voltage spike.

> Will the final lamp flicker at supply rate of 50hz?

Yes, though personally I wouldn't be concerned about that.

> Will it potentially be fire hazard from overheating & shorting?

Yes, but the LEDs will blow up and break the circuit before
you get the chance to leave it unattended anyway.


If you're looking for the simple way to do this, what you want
is a series resistance which will drop the extra voltage according
to Ohms law. R = 80V / I, with I being the current, and best
determined by how bright you want the light to be (the maximum
rating won't be known, but presumably the backlight will be too
bright for its purpose before you get to that). For safety, and
efficiency, I'd recommend that you do use a transformer or
switch-mode power supply as well.

Note that this won't be as efficient as a constant current LED
driver, and the efficency will be worse the greater the gap
between the LED backlight's voltage drop and 240VAC (unless a
lower voltage power supply is used). You'll also need to use
high wattage wire-wound resistor/s in order to handle the
wasted power, with their own cooling considerations.

--
__ __
#_ < |\| |< _#

keithr0
Guest

Thu Feb 07, 2019 1:45 am   



On 2/7/2019 7:36 AM, Computer Nerd Kev wrote:
Quote:
Taupe <jonahreal_at_yopmail.com> wrote:
Considering repurposing backlight LED's from discarded broken
TV's into a large LED lamp built into a picture frame & run
directly from 240 v AC.

Roughly if each LED need 3v to power on , I'd need 80 LED's linked in series
to total up to 240 v.

You're not going to have control of how many LEDs are in the
backlight assembly in the first place anyway. LED forward voltages
also vary quite a bit, 3V is at the high end.

This will eliminate the need for separate power circuitry ( which would bulk
up the frame), thus have single power chord out of frame into wall
socket.

You could quite easily have the power circuitry in a separate box and
connect that to the panel via another cord.

Will this kill all the LED's with voltage spikes?

Without any current limiting, it will kill the LEDs even without
a voltage spike.

Will the final lamp flicker at supply rate of 50hz?

Yes, though personally I wouldn't be concerned about that.

Will it potentially be fire hazard from overheating & shorting?

Yes, but the LEDs will blow up and break the circuit before
you get the chance to leave it unattended anyway.


If you're looking for the simple way to do this, what you want
is a series resistance which will drop the extra voltage according
to Ohms law. R = 80V / I, with I being the current, and best
determined by how bright you want the light to be (the maximum
rating won't be known, but presumably the backlight will be too
bright for its purpose before you get to that). For safety, and
efficiency, I'd recommend that you do use a transformer or
switch-mode power supply as well.

Note that this won't be as efficient as a constant current LED
driver, and the efficency will be worse the greater the gap
between the LED backlight's voltage drop and 240VAC (unless a
lower voltage power supply is used). You'll also need to use
high wattage wire-wound resistor/s in order to handle the
wasted power, with their own cooling considerations.

R = 80V / I? That's not the Ohms law that I was taught at school, where
does the 80 come from?

Simple answer to all this do not stick it across the main with just a
dropper resistor, it's a really bad idea.

Taupe
Guest

Thu Feb 07, 2019 4:45 am   



Jasen Betts wrote:
Quote:
On 2019-02-06, Taupe <jonahreal_at_yopmail.com> wrote:
Considering repurposing backlight LED's from discarded broken
TV's into a large LED lamp built into a picture frame & run
directly from 240 v AC.

Roughly if each LED need 3v to power on , I'd need 80 LED's linked
in series to total up to 240 v.

This will eliminate the need for separate power circuitry ( which
would bulk up the frame), thus have single power chord out of frame
into wall
socket.

no.

can't be done?
what if 240v rectified to DC with a current limiting resistor?

Quote:

Will this kill all the LED's with voltage spikes?

that depends on the power supply circuitry you use


thats the point, not to use seperate box power supply

Quote:

Will the final lamp flicker at supply rate of 50hz?

Unless you use a full wave rectifier it will. (else 100Hz)

Will it potentially be fire hazard from overheating & shorting?

yes.


I'll put a fuse in circuit?

Taupe
Guest

Thu Feb 07, 2019 4:45 am   



Computer Nerd Kev wrote:
Quote:
Taupe <jonahreal_at_yopmail.com> wrote:
Considering repurposing backlight LED's from discarded broken
TV's into a large LED lamp built into a picture frame & run
directly from 240 v AC.

Roughly if each LED need 3v to power on , I'd need 80 LED's linked
in series to total up to 240 v.

You're not going to have control of how many LEDs are in the
backlight assembly in the first place anyway. LED forward voltages
also vary quite a bit, 3V is at the high end.


I'll alow myself some buffer & drop voltage to say 2v & increase series to
120 LED's

Quote:

This will eliminate the need for separate power circuitry ( which
would bulk up the frame), thus have single power chord out of frame
into wall socket.

You could quite easily have the power circuitry in a separate box and
connect that to the panel via another cord.


this is what i'm trying to avoid.

Quote:

Will this kill all the LED's with voltage spikes?

Without any current limiting, it will kill the LEDs even without
a voltage spike.


I'll include 3kohm series resitor.

Quote:

Will the final lamp flicker at supply rate of 50hz?

Yes, though personally I wouldn't be concerned about that.

Will it potentially be fire hazard from overheating & shorting?

Yes, but the LEDs will blow up and break the circuit before
you get the chance to leave it unattended anyway.


If you're looking for the simple way to do this, what you want
is a series resistance which will drop the extra voltage according
to Ohms law. R = 80V / I, with I being the current, and best
determined by how bright you want the light to be (the maximum
rating won't be known, but presumably the backlight will be too
bright for its purpose before you get to that). For safety, and
efficiency, I'd recommend that you do use a transformer or
switch-mode power supply as well.


already have existing power supply, thought I'd get rid of it by running
directly of 240v AC

Quote:

Note that this won't be as efficient as a constant current LED
driver, and the efficency will be worse the greater the gap
between the LED backlight's voltage drop and 240VAC (unless a
lower voltage power supply is used). You'll also need to use
high wattage wire-wound resistor/s in order to handle the
wasted power, with their own cooling considerations.


LED's (in strips of 8-10 ) were stuck to metal body of TV to cool
will do same with metal back for frame.

Taupe
Guest

Thu Feb 07, 2019 4:45 am   



keithr0 wrote:
Quote:
On 2/7/2019 7:36 AM, Computer Nerd Kev wrote:
Taupe <jonahreal_at_yopmail.com> wrote:
Considering repurposing backlight LED's from discarded broken
TV's into a large LED lamp built into a picture frame & run
directly from 240 v AC.

Roughly if each LED need 3v to power on , I'd need 80 LED's linked
in series to total up to 240 v.

You're not going to have control of how many LEDs are in the
backlight assembly in the first place anyway. LED forward voltages
also vary quite a bit, 3V is at the high end.

This will eliminate the need for separate power circuitry ( which
would bulk up the frame), thus have single power chord out of frame
into wall socket.

You could quite easily have the power circuitry in a separate box and
connect that to the panel via another cord.

Will this kill all the LED's with voltage spikes?

Without any current limiting, it will kill the LEDs even without
a voltage spike.

Will the final lamp flicker at supply rate of 50hz?

Yes, though personally I wouldn't be concerned about that.

Will it potentially be fire hazard from overheating & shorting?

Yes, but the LEDs will blow up and break the circuit before
you get the chance to leave it unattended anyway.


If you're looking for the simple way to do this, what you want
is a series resistance which will drop the extra voltage according
to Ohms law. R = 80V / I, with I being the current, and best
determined by how bright you want the light to be (the maximum
rating won't be known, but presumably the backlight will be too
bright for its purpose before you get to that). For safety, and
efficiency, I'd recommend that you do use a transformer or
switch-mode power supply as well.

Note that this won't be as efficient as a constant current LED
driver, and the efficency will be worse the greater the gap
between the LED backlight's voltage drop and 240VAC (unless a
lower voltage power supply is used). You'll also need to use
high wattage wire-wound resistor/s in order to handle the
wasted power, with their own cooling considerations.

R = 80V / I? That's not the Ohms law that I was taught at school,
where does the 80 come from?


assume LED require 2v & 15mA to operate.

current Resistor = v240- 100 x 2v/ 15ma

40 /.015 a =2666 ohm resistor say 3kohm

i'll probably put in 120 LED's to start off with, then work back taking out
LED's until we get best brightness.

Quote:

Simple answer to all this do not stick it across the main with just a
dropper resistor, it's a really bad idea.


Phil Allison
Guest

Thu Feb 07, 2019 6:45 am   



Taupe wrote:
Quote:


Considering repurposing backlight LED's from discarded broken
TV's into a large LED lamp built into a picture frame & run
directly from 240 v AC.

Roughly if each LED need 3v to power on , I'd need 80 LED's linked in series
to total up to 240 v.



** The AC supply peaks at 340V and swings positive and negative compared to earth. Typical white LEDs are bright at 3.3 to 3.5 V.

So you need about 100 LEDs in series.

Some flicker may be noticeable under certain conditions, but 50Hz operation is common for dial lights and works OK.

If you add a bridge rectifier, then the flicker frequency moves to 100Hz and should be invisible.



..... Phil




Quote:
This will eliminate the need for separate power circuitry ( which would bulk
up the frame), thus have single power chord out of frame into wall
socket.

Will this kill all the LED's with voltage spikes?

Will the final lamp flicker at supply rate of 50hz?

Will it potentially be fire hazard from overheating & shorting?

--
'Journalism is printing what someone else does not want printed: everything
else is public relations.' - George Orwell

*Beat the system, don't let the corporate fascist take away your
privacy. https://www.privacytools.io/#


Jasen Betts
Guest

Thu Feb 07, 2019 6:45 am   



On 2019-02-07, Taupe <jonahreal_at_yopmail.com> wrote:
Quote:
Jasen Betts wrote:
On 2019-02-06, Taupe <jonahreal_at_yopmail.com> wrote:
Considering repurposing backlight LED's from discarded broken
TV's into a large LED lamp built into a picture frame & run
directly from 240 v AC.

Roughly if each LED need 3v to power on , I'd need 80 LED's linked
in series to total up to 240 v.

This will eliminate the need for separate power circuitry ( which
would bulk up the frame), thus have single power chord out of frame
into wall
socket.

no.
can't be done?
what if 240v rectified to DC with a current limiting resistor?


that would be poswersupply circuitry.... (perhaps I misunderstood the
original question)

Quote:
Will this kill all the LED's with voltage spikes?

that depends on the power supply circuitry you use

thats the point, not to use seperate box power supply


Quote:
Will the final lamp flicker at supply rate of 50hz?

Unless you use a full wave rectifier it will. (else 100Hz)

Will it potentially be fire hazard from overheating & shorting?

yes.

I'll put a fuse in circuit?


That should be enough, brightness may change significantly with AC
voltage variations.

You probably want to do 70 LEDs and 30V drop in the resistor depending
on how bright you want this that resistor could get hot.

--
When I tried casting out nines I made a hash of it.

Jasen Betts
Guest

Thu Feb 07, 2019 7:32 am   



On 2019-02-07, Taupe <jonahreal_at_yopmail.com> wrote:
Quote:
keithr0 wrote:
On 2/7/2019 7:36 AM, Computer Nerd Kev wrote:
Taupe <jonahreal_at_yopmail.com> wrote:
Considering repurposing backlight LED's from discarded broken
TV's into a large LED lamp built into a picture frame & run
directly from 240 v AC.

Roughly if each LED need 3v to power on , I'd need 80 LED's linked
in series to total up to 240 v.

You're not going to have control of how many LEDs are in the
backlight assembly in the first place anyway. LED forward voltages
also vary quite a bit, 3V is at the high end.

This will eliminate the need for separate power circuitry ( which
would bulk up the frame), thus have single power chord out of frame
into wall socket.

You could quite easily have the power circuitry in a separate box and
connect that to the panel via another cord.

Will this kill all the LED's with voltage spikes?

Without any current limiting, it will kill the LEDs even without
a voltage spike.

Will the final lamp flicker at supply rate of 50hz?

Yes, though personally I wouldn't be concerned about that.

Will it potentially be fire hazard from overheating & shorting?

Yes, but the LEDs will blow up and break the circuit before
you get the chance to leave it unattended anyway.


If you're looking for the simple way to do this, what you want
is a series resistance which will drop the extra voltage according
to Ohms law. R = 80V / I, with I being the current, and best
determined by how bright you want the light to be (the maximum
rating won't be known, but presumably the backlight will be too
bright for its purpose before you get to that). For safety, and
efficiency, I'd recommend that you do use a transformer or
switch-mode power supply as well.

Note that this won't be as efficient as a constant current LED
driver, and the efficency will be worse the greater the gap
between the LED backlight's voltage drop and 240VAC (unless a
lower voltage power supply is used). You'll also need to use
high wattage wire-wound resistor/s in order to handle the
wasted power, with their own cooling considerations.

R = 80V / I? That's not the Ohms law that I was taught at school,
where does the 80 come from?

assume LED require 2v & 15mA to operate.

current Resistor = v240- 100 x 2v/ 15ma

40 /.015 a =2666 ohm resistor say 3kohm

i'll probably put in 120 LED's to start off with, then work back taking out
LED's until we get best brightness.


That won't give predictable results, mains voltage is not constant,
and varies slowly minute to minute and also has breif dips and peaks.

eg: you may find that the lamp flickers every time the vacuum-cleaner or
refrigerator starts up. This is why people use LED drivers.

Are you going to put the whole backlight lens assembly in the frame or
just the LED strip?

it seems that it would be difficult to change the number of LEDs in the
backlight assembly.

--
When I tried casting out nines I made a hash of it.

Computer Nerd Kev
Guest

Thu Feb 07, 2019 7:45 am   



keithr0 <user_at_account.invalid> wrote:
Quote:
On 2/7/2019 7:36 AM, Computer Nerd Kev wrote:
Taupe <jonahreal_at_yopmail.com> wrote:
Considering repurposing backlight LED's from discarded broken
TV's into a large LED lamp built into a picture frame & run
directly from 240 v AC.

Roughly if each LED need 3v to power on , I'd need 80 LED's linked in series
to total up to 240 v.

You're not going to have control of how many LEDs are in the
backlight assembly in the first place anyway. LED forward voltages
also vary quite a bit, 3V is at the high end.

This will eliminate the need for separate power circuitry ( which would bulk
up the frame), thus have single power chord out of frame into wall
socket.

You could quite easily have the power circuitry in a separate box and
connect that to the panel via another cord.

Will this kill all the LED's with voltage spikes?

Without any current limiting, it will kill the LEDs even without
a voltage spike.

Will the final lamp flicker at supply rate of 50hz?

Yes, though personally I wouldn't be concerned about that.

Will it potentially be fire hazard from overheating & shorting?

Yes, but the LEDs will blow up and break the circuit before
you get the chance to leave it unattended anyway.


If you're looking for the simple way to do this, what you want
is a series resistance which will drop the extra voltage according
to Ohms law. R = 80V / I, with I being the current, and best
determined by how bright you want the light to be (the maximum
rating won't be known, but presumably the backlight will be too
bright for its purpose before you get to that). For safety, and
efficiency, I'd recommend that you do use a transformer or
switch-mode power supply as well.

Note that this won't be as efficient as a constant current LED
driver, and the efficency will be worse the greater the gap
between the LED backlight's voltage drop and 240VAC (unless a
lower voltage power supply is used). You'll also need to use
high wattage wire-wound resistor/s in order to handle the
wasted power, with their own cooling considerations.

R = 80V / I? That's not the Ohms law that I was taught at school, where
does the 80 come from?


I got it into my head at one point that he'd said the voltage drop of
the backlight was 80V, so I started doing
((240VAC x 1.414) - 80V) / I
Then realised that he hadn't mentioned the Vf at all and edited
everything to remove all the specifics, but I accidentally left the
80V in there instead of just the V.

Quote:
Simple answer to all this do not stick it across the main with just a
dropper resistor, it's a really bad idea.


Not knowing too much about LCD backlights, if they designed them with
a voltage drop close enough to the peak mains voltage that the series
resistor could be a reasonable wattage, there could be a case for
running from mains. If, of course, the wiring and case are suitably
insulated or grounded.

I did a bit of research and this doesn't seem to be the case though,
this page says that they range from 24V - 66V:
https://sites.google.com/site/lcd4hobby/8-backlight/a-led-backlight

I also stumbled upon this, which appeals to me for some reason:
https://www.instructables.com/id/A-very-Simple-LCD-Backlight-Fix/

--
__ __
#_ < |\| |< _#

Computer Nerd Kev
Guest

Thu Feb 07, 2019 8:45 am   



Taupe <jonahreal_at_yopmail.com> wrote:
> assume LED require 2v & 15mA to operate.

If you're able to mess about with the individual LEDs, I suggest
measuring one to find its actual forward voltage rather than
guessing. This is easily done by connecting it via, say 1K, to a 5V
power supply and measuring over the LED leads with a multimeter.

Quote:
current Resistor = v240- 100 x 2v/ 15ma

40 /.015 a =2666 ohm resistor say 3kohm


I guess you meant (240V - 200V) / 0.015A. If the panel uses LEDs in
parallel, the current may need to be higher than that for just one.

Also, 240VAC has a peak voltage of 240 x 1.414 = 339V. So for
calculating for maximum current you should use the peak voltage, eg.
(339V - 200V) / 0.015A = 9267R = ~10Kohm. Or you could add LEDs in
series so that they drop 300V instead of 200V, which also prevents
wasting up to (339V - 200V) x 0.015A = 2.1W of power (actually that's
not too bad, but more likely figures would be 0.15A and 21W or
greater) at the very peak of the mains voltage cycle.

However that way the LEDs are only lit for a breif part of the
waveform and the appearance would be more sensitive to variations
in the mains than if you had the wider 140V gap above the LED's
turn-on voltage. The LEDs would also appear brighter at lower current
because they are on for more time, which might allow a greater
resistance to be used in series and thereby offset some of the
efficiency loss mentioned in the last paragraph.


I have a side-lit LED backlight pulled from a digital photo frame
LCD, which I use as a slide viewer. To work out how to power that
I simply connected it via a 1K resistor and increased the voltage
from my bench supply until I noticed a light. I then tried a lower
value series resistor and raised the voltage above the turn-on
point to see the stage at which increasing the voltage stopped
producing a significant increase in brightness, the current thereby
being approximately the maximum. Then I found a brightness that
suited the purpose. Finally I ended up using a 100R wire-wound
resistor in series with a 18VDC plugpack, and it works well without
the resistor getting burning hot, which is all that I was after.

--
__ __
#_ < |\| |< _#

keithr0
Guest

Thu Feb 07, 2019 11:45 am   



On 2/7/2019 4:41 PM, Computer Nerd Kev wrote:
Quote:
keithr0 <user_at_account.invalid> wrote:
On 2/7/2019 7:36 AM, Computer Nerd Kev wrote:
Taupe <jonahreal_at_yopmail.com> wrote:
Considering repurposing backlight LED's from discarded broken
TV's into a large LED lamp built into a picture frame & run
directly from 240 v AC.

Roughly if each LED need 3v to power on , I'd need 80 LED's linked in series
to total up to 240 v.

You're not going to have control of how many LEDs are in the
backlight assembly in the first place anyway. LED forward voltages
also vary quite a bit, 3V is at the high end.

This will eliminate the need for separate power circuitry ( which would bulk
up the frame), thus have single power chord out of frame into wall
socket.

You could quite easily have the power circuitry in a separate box and
connect that to the panel via another cord.

Will this kill all the LED's with voltage spikes?

Without any current limiting, it will kill the LEDs even without
a voltage spike.

Will the final lamp flicker at supply rate of 50hz?

Yes, though personally I wouldn't be concerned about that.

Will it potentially be fire hazard from overheating & shorting?

Yes, but the LEDs will blow up and break the circuit before
you get the chance to leave it unattended anyway.


If you're looking for the simple way to do this, what you want
is a series resistance which will drop the extra voltage according
to Ohms law. R = 80V / I, with I being the current, and best
determined by how bright you want the light to be (the maximum
rating won't be known, but presumably the backlight will be too
bright for its purpose before you get to that). For safety, and
efficiency, I'd recommend that you do use a transformer or
switch-mode power supply as well.

Note that this won't be as efficient as a constant current LED
driver, and the efficency will be worse the greater the gap
between the LED backlight's voltage drop and 240VAC (unless a
lower voltage power supply is used). You'll also need to use
high wattage wire-wound resistor/s in order to handle the
wasted power, with their own cooling considerations.

R = 80V / I? That's not the Ohms law that I was taught at school, where
does the 80 come from?

I got it into my head at one point that he'd said the voltage drop of
the backlight was 80V, so I started doing
((240VAC x 1.414) - 80V) / I
Then realised that he hadn't mentioned the Vf at all and edited
everything to remove all the specifics, but I accidentally left the
80V in there instead of just the V.

Simple answer to all this do not stick it across the main with just a
dropper resistor, it's a really bad idea.

Not knowing too much about LCD backlights, if they designed them with
a voltage drop close enough to the peak mains voltage that the series
resistor could be a reasonable wattage, there could be a case for
running from mains. If, of course, the wiring and case are suitably
insulated or grounded.


The older ones used fluorescent tubes and a high voltage power supply to
drive them. The more modern LED ones are more likely to use a constant
current supply. The voltage across the panel and the current going into
it would depend on how the LEDs are connected, series, parallel, or
series parallel, and how many LEDs are involved.

Quote:
I did a bit of research and this doesn't seem to be the case though,
this page says that they range from 24V - 66V:
https://sites.google.com/site/lcd4hobby/8-backlight/a-led-backlight

I also stumbled upon this, which appeals to me for some reason:
https://www.instructables.com/id/A-very-Simple-LCD-Backlight-Fix/

Gawd, how crude! Obviously this was a fluorescent lit display.


I've got a project to take the panel out of an old display that I got
from a local op-shop, put it in an old radar set in lieu of the CRT and
use a Raspberry pi to simulate a radar picture.

Computer Nerd Kev
Guest

Thu Feb 07, 2019 10:45 pm   



keithr0 <user_at_account.invalid> wrote:
Quote:
On 2/7/2019 4:41 PM, Computer Nerd Kev wrote:
keithr0 <user_at_account.invalid> wrote:
On 2/7/2019 7:36 AM, Computer Nerd Kev wrote:

Not knowing too much about LCD backlights, if they designed them with
a voltage drop close enough to the peak mains voltage that the series
resistor could be a reasonable wattage, there could be a case for
running from mains. If, of course, the wiring and case are suitably
insulated or grounded.

The older ones used fluorescent tubes and a high voltage power supply to
drive them. The more modern LED ones are more likely to use a constant
current supply. The voltage across the panel and the current going into
it would depend on how the LEDs are connected, series, parallel, or
series parallel, and how many LEDs are involved.


Yes, the constant current supplies in LED light bulbs are sometimes
linear, with the series LED forward voltage high enough that they
don't waste too much power. The same could presumably be applied to
LCD backlights, but with a SMPS in there for the logic voltages
anyway, I doubt that there'd be much gained by doing so.

Quote:
I did a bit of research and this doesn't seem to be the case though,
this page says that they range from 24V - 66V:
https://sites.google.com/site/lcd4hobby/8-backlight/a-led-backlight

I also stumbled upon this, which appeals to me for some reason:
https://www.instructables.com/id/A-very-Simple-LCD-Backlight-Fix/

Gawd, how crude! Obviously this was a fluorescent lit display.


Easy to spot that one.

Quote:
I've got a project to take the panel out of an old display that I got
from a local op-shop, put it in an old radar set in lieu of the CRT and
use a Raspberry pi to simulate a radar picture.


Neat. Bonus points if it displays data from flightradar24. :)

--
__ __
#_ < |\| |< _#

Taupe
Guest

Thu Feb 07, 2019 11:45 pm   



Phil Allison wrote:
Quote:
Taupe wrote:


Considering repurposing backlight LED's from discarded broken
TV's into a large LED lamp built into a picture frame & run
directly from 240 v AC.

Roughly if each LED need 3v to power on , I'd need 80 LED's linked
in series to total up to 240 v.



** The AC supply peaks at 340V and swings positive and negative
compared to earth. Typical white LEDs are bright at 3.3 to 3.5 V.


good point, should allow for 340v peak

Quote:

So you need about 100 LEDs in series.

Some flicker may be noticeable under certain conditions, but 50Hz
operation is common for dial lights and works OK.

If you add a bridge rectifier, then the flicker frequency moves to
100Hz and should be invisible.



.... Phil




This will eliminate the need for separate power circuitry ( which
would bulk up the frame), thus have single power chord out of frame
into wall
socket.

Will this kill all the LED's with voltage spikes?

Will the final lamp flicker at supply rate of 50hz?

Will it potentially be fire hazard from overheating & shorting?

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