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LED car lights flicker - no need!

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Major Scott

Jan 1, 1970
0
Surely they can design LED lights on cars to have a higher frequency PWM? Even £100K cars flicker dramatically, especially when filmed. It makes them look really cheap. All it would take is a higher frequency PWM, or a smoothing capacitor?
 
Surely they can design LED lights on cars to have a higher frequency PWM=
? Even =A3100K cars flicker dramatically, especially when filmed. It m=
akes them look really cheap. All it would take is a higher frequency PW=
M, or a smoothing capacitor?

They flicker for a reason. If they smoothed the current they might just as
well use DC direct from the battery. I don't know the technical reasons why but
apparently using the equivalent DC voltage required to get the same brightness
as you can get by strobing them would burn them out. I'm sure some electronics
guru on here can explain more. But it does lead to interesting effects on
video as you say :eek:)

NJR
 
D

Dave Plowman

Jan 1, 1970
0
They flicker for a reason. If they smoothed the current they might just
as well use DC direct from the battery. I don't know the technical
reasons why but apparently using the equivalent DC voltage required to
get the same brightness as you can get by strobing them would burn them
out. I'm sure some electronics guru on here can explain more. But it
does lead to interesting effects on video as you say :eek:)

Pulsing an LED is a way of getting a higher light output from it without
overheating. Overheating an LED kills it in short order. Seeing a flicker
from them on a video is the same effect as wagon wheels appearing to turn
backwards on old cowboy and indian films - stroboscopic effect.
 
Pulsing an LED is a way of getting a higher light output from it without
overheating. Overheating an LED kills it in short order. Seeing a flicker

Its odd though isn't it. The way they're constructed must mean the amount of
heat generated for a given voltage or current must slowly tail off so although
they'd heat up too much at constant voltage X you can pulse them at for
arguments sake X*2 producing the same or even more total light but without a
doubling of the heat generated so allowing for cooling down to safe levels
during the OFF periods of the pulse. Or something like that.

NJR
 
D

DavidR

Jan 1, 1970
0
They flicker for a reason. If they smoothed the current they might just as
well use DC direct from the battery. I don't know the technical reasons
why but

The effect relies on the persistance of the eyes to make it appear that the
average brightness is higher. Smoothing at source would be less energy
efficient.

Agreed the effect is not pleasant. It would help if they could introduce
softer start for indicators and brake light dimming when conditions suggest
a slow moving queue.
 
M

Major Scott

Jan 1, 1970
0
Pulsing an LED is a way of getting a higher light output from it without
overheating. Overheating an LED kills it in short order.

Take for example the brake/tail lights. These are often pulsed for tail and on for brake. So what you said doesn't make sense. Anything less than full voltage on (as for brake) will be lower heat.
Seeing a flicker
from them on a video is the same effect as wagon wheels appearing to turn
backwards on old cowboy and indian films - stroboscopic effect.

It's way worse than that - the duty cycle is quite a lot less than 50%, so you see them off, with the occasional on.
 
M

Major Scott

Jan 1, 1970
0
The effect relies on the persistance of the eyes to make it appear that the
average brightness is higher.

Easy enough to double the frequency of the flicker, then you wouldn't notice it. Remember 50Hz CRT monitors?
Smoothing at source would be less energy efficient.

I don't believe you. Switched mode power supplies are very cheap nowadays, especially compared with the cost of a car, especially a £100K carwhich has the same problem.

You can get a very smooth DC voltage of any level out of one - just lookat your PC power supply then think of a smaller version of it. There are in fact smaller versions of it on your motherboard changing 12 volts to the CPU voltage (which is in fact variable).
Agreed the effect is not pleasant. It would help if they could introduce
softer start for indicators

I prefer them to go on and off suddenly. The only problem I have is flickery tail lights.
and brake light dimming when conditions suggest a slow moving queue.

I don't agree with different brightnesses of brakes. We already have two brightnesses of red - tail and brake. Adding more would just lead to confusion, you would wonder if it was a tail or a brake.
 
M

Major Scott

Jan 1, 1970
0
They flicker for a reason. If they smoothed the current they might just as
well use DC direct from the battery.

No, provide them with a lower DC voltage to make them dimmer. Say 12V for brake and 9V for tail.
I don't know the technical reasons why but
apparently using the equivalent DC voltage required to get the same brightness
as you can get by strobing them would burn them out. I'm sure some electronics
guru on here can explain more. But it does lead to interesting effects on
video as you say :eek:)

So you're saying that on brake they are also strobed? I have never noticed a brake strobing. It's the tails that do it.
 
D

Dave Plowman

Jan 1, 1970
0
Take for example the brake/tail lights. These are often pulsed for tail
and on for brake. So what you said doesn't make sense. Anything less
than full voltage on (as for brake) will be lower heat.

LEDs are current, not voltage, driven.
 
D

Dave Plowman

Jan 1, 1970
0
No, provide them with a lower DC voltage to make them dimmer. Say 12V
for brake and 9V for tail.

I'd suggest you look up the Ladybird book of electronics to get a clue
about how LEDs work.
 
D

DavidR

Jan 1, 1970
0
Easy enough to double the frequency of the flicker, then you wouldn't
notice it. Remember 50Hz CRT monitors?
I don't believe you. Switched mode power supplies are very cheap
nowadays, especially compared with the cost of a car, especially a £100K
car which has the same problem.

What is generated
------ ------
| | | |
| |______| |_____

What the eye perceives
-------- --------
| \ | \
| \___| \____

By averaging the power in the top waveform, the peak intensity is reduced
and would not use eye's ability to fill in the gaps. Therefore a
smooth waveform requires more power at source.

Picking a Cree led at random, the data sheet shows that the increase in
luminous output falls relative to the increase in current (ie, doubling the
current produces less than a doubling of output), so at first sight it would
seem that pulsing is counterproductive. Which means that there are other
factors that make it advantageous.
I don't agree with different brightnesses of brakes. We already have two
brightnesses of red - tail and brake. Adding more would just lead to
confusion, you would wonder if it was a tail or a brake.

If you can't maintain a gap in a slow moving queue without the help of brake
lights, are you sure you're competent?
 
D

Dave Plowman

Jan 1, 1970
0
When there's a series resistor, then you can think of them as voltage
driven.

Why would you have a 'series resistor' if they are pulse driven?

But in any case it is irrelevant. It's the current they are driven with
that matters - not the voltage.
 
Major said:
When there's a series resistor, then you can think of them as voltage
driven. Anyway electronics to lower the current can be made without
pulsing.

No, the series resistor merely determines the maximum current that could
flow through the LED.

Daniel
 
I

Ian Jackson

Jan 1, 1970
0
Dave Plowman said:
Why would you have a 'series resistor' if they are pulse driven?

But in any case it is irrelevant. It's the current they are driven with
that matters - not the voltage.
Nothing is really only 'current driven'. It's more correct to say that
LEDs need to be driven from a power source which provides a fairly
well-defined current. Even when you drive them with pulsed current, the
amplitude of the pulses will be determined by the voltage producing the
pulses. At any instant, the power dissipated in the LED is simply the
product of the voltage across it (typically 2V, depending on the colour)
and the current flowing through it. When pulsed, the average power is
also determined by the mark-space ratio of the pulses.
 
D

Dave Plowman

Jan 1, 1970
0
If this pulsing can make them appear brighter than they are, why don't
they use it in domestic LED bulbs?

No idea. They are crap and I wouldn't have one in the house.
 
D

Dave Plowman

Jan 1, 1970
0
Nothing is really only 'current driven'. It's more correct to say that
LEDs need to be driven from a power source which provides a fairly
well-defined current.

Very well defined if you're driving them hard. The source voltage is
irrelevant. Provided it is more than the forward voltage drop of the LED
or LED chain.

Even when you drive them with pulsed current, the
amplitude of the pulses will be determined by the voltage producing the
pulses. At any instant, the power dissipated in the LED is simply the
product of the voltage across it (typically 2V, depending on the colour)
and the current flowing through it. When pulsed, the average power is
also determined by the mark-space ratio of the pulses.

As I said, it's the current flow through the LED that you design for.
 
D

DavidR

Jan 1, 1970
0
Major Scott said:
If this pulsing can make them appear brighter than they are, why don't
they use it in domestic LED bulbs?

Because a large number of morons in and around the motor industry have
become attached to the idea that if some lighting is good, brighter lights
must be better.

Thy don't seem to recognise that the the current standard of lighting is
making the roads more dangerous.
 
S

Steve Firth

Jan 1, 1970
0
Dave Plowman said:
No idea. They are crap and I wouldn't have one in the house.

The LED bulbs I have used have been anything but "crap". They use 1/10th
the electricity of equivalent halogen bulbs and can be bought as flood or
spotlight versions.
 
I

Ian Jackson

Jan 1, 1970
0
In message
<2082196069388490010.841939%steve%[email protected]
g> said:
The LED bulbs I have used have been anything but "crap". They use 1/10th
the electricity of equivalent halogen bulbs and can be bought as flood or
spotlight versions.
I was under the impression that they did use a pulsed supply in domestic
lighting.
 
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