I thought that LCD was subject to 50 Hz mains flicker.
Scott wrote:
I thought that LCD was subject to 50 Hz mains flicker.
Not if you run them on DC
A friend and I were talking about the heat generated by TV studio
lighting in the old days. I assume these were tungsten lamps and are
now replaced by LCD or similar. I thought that LCD was subject to 50
Hz mains flicker. How is this synchronised accurately with the camera?
Andy Burns wrote:Probably as the frequency would be high enough to not matter to the
Scott wrote:
I thought that LCD was subject to 50 Hz mains flicker.
Not if you run them on DC
Via a SMPS?
On 2026-05-23 10:25, Scott wrote:
A friend and I were talking about the heat generated by TV studio
lighting in the old days. I assume these were tungsten lamps and are
now replaced by LCD or similar. I thought that LCD was subject to 50
Hz mains flicker. How is this synchronised accurately with the camera?
I know nothing beyond general physics about studio lighting, but my
general assumption would have been that any LED lighting would have to
use DC.-a However, it seems that we are both wrong in part and both right
in part ...
https://www.google.com/search?client=firefox-b- e&q=is+led+lighting+subject+to+mains+flicker
"""
Yes, LED lighting is inherently subject to mains flicker. Because LEDs
turn on and off instantly, they directly mirror the alternating current
(AC) of your power grid. However, whether you actually see it depends on
the quality of the internal components and external controls.
"""
[Follow the link above for much more that I could not easily
copy'n'paste-a --a in FF115.36.0esr regardless of how much is selected, I can only copy the first sentence or two at a time]
Jeff Layman wrote:
Andy Burns wrote:Probably as the frequency would be high enough to not matter to the
Scott wrote:
I thought that LCD was subject to 50 Hz mains flicker.
Not if you run them on DC
Via a SMPS?
cameras, but a linear PSU with sufficient smoothing would do it, run
them off a battery bank ... actually just realised I'd read the original
as LED rather than LCD (why would they use LCD?)
On 23/05/2026 11:51, Java Jive wrote:
On 2026-05-23 10:25, Scott wrote:
A friend and I were talking about the heat generated by TV studio
lighting in the old days. I assume these were tungsten lamps and are
now replaced by LCD or similar. I thought that LCD was subject to 50
Hz mains flicker. How is this synchronised accurately with the camera?
https://www.google.com/search?client=firefox-b-
e&q=is+led+lighting+subject+to+mains+flicker
[Follow the link above for much more that I could not easily
copy'n'paste-a --a in FF115.36.0esr regardless of how much is selected,
I can only copy the first sentence or two at a time]
Why try to copy an (imperfect) AI summary when the same search points to proper articles? https://www.nvcuk.com/technical-support/view/what-is-flicker-in-relation-to-lighting-2
A friend and I were talking about the heat generated by TV studio
lighting in the old days. I assume these were tungsten lamps and are
now replaced by LCD or similar. I thought that LCD was subject to 50
Hz mains flicker. How is this synchronised accurately with the camera?
On Sat 23/05/2026 10:25, Scott wrote:
A friend and I were talking about the heat generated by TV studio
lighting in the old days. I assume these were tungsten lamps and are
now replaced by LCD or similar. I thought that LCD was subject to 50
Hz mains flicker. How is this synchronised accurately with the camera?
Er, don't you mean LED?
There is a glaring error in that AI response - sorry if this is long.
It stated that older dimmers rapidly chopped the supply to effect an apparent light reduction. Wrong. They did not chop the AC (sinusoidal) waveform, they just started it late originally using a device know as a Silicon Controlled Rectifier or SCR but better known as a Thyristor.
This is a diode (only allows current to flow through it in one
direction) that blocks current once said current falls below a nominal
hold level usually of a few milliamps. As the mains waveform passes
through zero 100 times a second so the SCR blocks current every 20mS (correct - read on.). A third connection to the SCR will trigger the SCR
to conduct when about 3V is applied to it.
Originally the SCR only worked in one current direction, so when the
mains reversed polarity the SCR did not trigger and the dimmer thus had
a fixed minimum amount of dimming because only one half of the
sinusoidal waveform is being used.
Devices developed and along came the Triac which could be viewed as two
SCRs connected in reversed parallel so giving a much greater range of dimming as it used both halves of the sinusoidal waveform. This was eventually advanced into a Quadrac which had other internal switching
that allowed almost 0-100% perfect dimming.
The problem with SCR/Triac/Quadrac dimmers is that at the instant of the device switching into the conducting state the voltage applied to the
device being controlled went from zero to the actually voltage point of
the waveform* in typically 2-3uS and many recipients of such objected or failed - especially prevalent in the voltage conversion electronics
inside a LED lamp. Standard dimmers for incandescent bulbs had this
effect and because of the way the waveform was cut they were/are known
as leading edge dimmers.
[*240Vrms is our nominal mains voltage, but that equates to about 340V
peak, and that voltage controlled by a Thyristor could go from a few
volts to almost 340V in a few microseconds which is one huge jump.]
The solution for LED lights is to switch the power off early before the waveform reaches a natural zero. This is known as trailing edge dimming
and with modern microelectronics it is easy to manufacture a system that turns the power off. If you fit trailing edge dimmers to all the lights
that you want to vary - as we have done - it is extremely rare for a LED bulb of any type to fail and there will be no visible flicker. The human
eye has a persistence of around 1/12th of a second: as the lamp is
flashing at 50 or 100 times a second it appears to us to be constant.
This picture (https://tinyurl.com/5sh6y3np) shows the three mains
waveforms described above. The red on the left is a sinusoidal AC
waveform and one complete cycle lasts 20ms (for UK 50Hz mains).
The blue waveform is what we in the UK call a leading edge dimmer where
the supply is switched on delayed after the start of the waveform.
The green waveform is that of a trailing edge dimmer which switches off before the waveform has finished. (The terminology is American.)
On 24/05/2026 18:24, Woody wrote:
There is a glaring error in that AI response - sorry if this is long.
It stated that older dimmers rapidly chopped the supply to effect an
apparent light reduction. Wrong. They did not chop the AC (sinusoidal)
waveform, they just started it late originally using a device know as
a Silicon Controlled Rectifier or SCR but better known as a Thyristor.
This is a diode (only allows current to flow through it in one
direction) that blocks current once said current falls below a nominal
hold level usually of a few milliamps. As the mains waveform passes
through zero 100 times a second so the SCR blocks current every 20mS
(correct - read on.). A third connection to the SCR will trigger the
SCR to conduct when about 3V is applied to it.
Originally the SCR only worked in one current direction, so when the
mains reversed polarity the SCR did not trigger and the dimmer thus
had a fixed minimum amount of dimming because only one half of the
sinusoidal waveform is being used.
Devices developed and along came the Triac which could be viewed as
two SCRs connected in reversed parallel so giving a much greater range
of dimming as it used both halves of the sinusoidal waveform. This was
eventually advanced into a Quadrac which had other internal switching
that allowed almost 0-100% perfect dimming.
The problem with SCR/Triac/Quadrac dimmers is that at the instant of
the device switching into the conducting state the voltage applied to
the device being controlled went from zero to the actually voltage
point of the waveform* in typically 2-3uS and many recipients of such
objected or failed - especially prevalent in the voltage conversion
electronics inside a LED lamp. Standard dimmers for incandescent bulbs
had this effect and because of the way the waveform was cut they were/
are known as leading edge dimmers.
[*240Vrms is our nominal mains voltage, but that equates to about 340V
peak, and that voltage controlled by a Thyristor could go from a few
volts to almost 340V in a few microseconds which is one huge jump.]
The solution for LED lights is to switch the power off early before
the waveform reaches a natural zero. This is known as trailing edge
dimming and with modern microelectronics it is easy to manufacture a
system that turns the power off. If you fit trailing edge dimmers to
all the lights that you want to vary - as we have done - it is
extremely rare for a LED bulb of any type to fail and there will be no
visible flicker. The human eye has a persistence of around 1/12th of a
second: as the lamp is flashing at 50 or 100 times a second it appears
to us to be constant.
This picture (https://tinyurl.com/5sh6y3np) shows the three mains
waveforms described above. The red on the left is a sinusoidal AC
waveform and one complete cycle lasts 20ms (for UK 50Hz mains).
The blue waveform is what we in the UK call a leading edge dimmer
where the supply is switched on delayed after the start of the waveform.
The green waveform is that of a trailing edge dimmer which switches
off before the waveform has finished. (The terminology is American.)
I was glad that I had remembered most of what you said, from my
electronic engineering days at university.
One thing I've always wondered is how fluorescent tubes are dimmed. My school, in the mid 1970s, had a lecture theatre which was lit by loads
of fluorescent tubes. These could be dimmed gradually, or would go from fully off to fully on over about 2 seconds without any of the normal flash-flash-on of a bimetallic starter. Using technology that was around
in the 1970s, how were fluorescent tubes dimmed? I seem to remember that like so many dimmers, even modern LED lamps such as Philips Hue that
have built in dimming circuitry, they could not be dimmed completely:
below some minimum brightness they went between dim and off.
There is a glaring error in that AI response - sorry if this is long.
It stated that older dimmers rapidly chopped the supply to effect an apparent light reduction. Wrong. They did not chop the AC (sinusoidal) waveform, they just started it late originally using a device know as a Silicon Controlled Rectifier or SCR but better known as a Thyristor.
This is a diode (only allows current to flow through it in one
direction) that blocks current once said current falls below a nominal
hold level usually of a few milliamps. As the mains waveform passes
through zero 100 times a second so the SCR blocks current every 20mS (correct - read on.). A third connection to the SCR will trigger the SCR
to conduct when about 3V is applied to it.
Originally the SCR only worked in one current direction, so when the
mains reversed polarity the SCR did not trigger and the dimmer thus had
a fixed minimum amount of dimming because only one half of the
sinusoidal waveform is being used.
Devices developed and along came the Triac which could be viewed as two
SCRs connected in reversed parallel so giving a much greater range of dimming as it used both halves of the sinusoidal waveform. This was eventually advanced into a Quadrac which had other internal switching
that allowed almost 0-100% perfect dimming.
The problem with SCR/Triac/Quadrac dimmers is that at the instant of the device switching into the conducting state the voltage applied to the
device being controlled went from zero to the actually voltage point of
the waveform* in typically 2-3uS and many recipients of such objected or failed - especially prevalent in the voltage conversion electronics
inside a LED lamp. Standard dimmers for incandescent bulbs had this
effect and because of the way the waveform was cut they were/are known
as leading edge dimmers.
[*240Vrms is our nominal mains voltage, but that equates to about 340V
peak, and that voltage controlled by a Thyristor could go from a few
volts to almost 340V in a few microseconds which is one huge jump.]
The solution for LED lights is to switch the power off early before the waveform reaches a natural zero. This is known as trailing edge dimming
and with modern microelectronics it is easy to manufacture a system that turns the power off. If you fit trailing edge dimmers to all the lights
that you want to vary - as we have done - it is extremely rare for a LED bulb of any type to fail and there will be no visible flicker. The human
eye has a persistence of around 1/12th of a second: as the lamp is
flashing at 50 or 100 times a second it appears to us to be constant.
...or convert to (low voltage) DC and use PWM (pulse width modulation).
With a high frequency (kHz), there won't be any problems with flicker.
...or convert to (low voltage) DC and use PWM (pulse width modulation).
With a high frequency (kHz), there won't be any problems with flicker.
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