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Forum: Analog Circuits LED-strip dimming without PWM


Author: F. L. (filmwerkstatt)
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Hello everyone!
I'm not an engineer but i want to do something special - or so it seems.

I want to power 2m of this LED strip (12V, 36W, 3A for 2m (120 LEDs)):
http://store.yujiintl.com/products/high-cri-led-ri...

with that Lithium-ion battery (Sony BP-U60, 14.4V, 3.9Ah):
http://www.sony.co.uk/pro/product/broadcast-produc...

I want to dim it from 0% to 100% (if possible) without PWM - i will use 
it for filming purposes and flickering is an absolute no-go. Sometimes i 
do slow-motion shoots with up to 2000fps - this might be extreme, but 
flicker-free till around 400fps would be nice. So PWM is out...

I tested it with my lab power supply: Around 7V its around 5% (and it 
doesn't flicker at normal framerates).

Additionally, i want to make sure i dont fully drain my batteries.
And, it's crucial that the brightness stays always the same - if the 
batteries can't deliver what i want it should be turned off or at least 
flash some red LED.

As i see it, i need:
1. Adjustable voltage regulator from ±14,4V to 7V - 12V, 3+A
2. Some relais that is triggered when voltage in the battery drops below 
the adjusted voltage
3. Heat absorber of some sort

Questions:
Where do i find a voltage regulator with my specifications? Do I have to 
make my own? If so, does someone know a schematic which withstands 3+ 
amps?
What is that 'relais' part called? Due it beeing dependent on the 
adjusted voltage, does a combo part exist somewhere?

Every though is much appreciated. Thanks!

: Moved by Moderator
Author: Joe F. (easylife)
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Depending on the LED driver you're using, an extremely high PWM 
frequency (10-20 KHz) might be an option.

Dimming white LEDs by reducing voltage/current in general does not work 
well.
Color temperature will change drastically or even shift to a color which 
could not in a bit be called "white" which makes it completely unusable 
for photographic lighting.

If reducing brightness in discrete steps is acceptable, I'd suggest to 
find a way to simply switch off every 3rd, 2nd, 2 of 3... LED and keep 
the remaining LEDs at full brightness.

Also applying neutral density foil is a good way to reduce brightness 
without changing color temperature.

Or setting up a mechanical "shutter" (e.g. black wrap) in front of the 
lamp might work as well.

Author: Georg M. (g_m)
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Fabian L. wrote:
> Where do i find a voltage regulator with my specifications?

http://www.conrad.ch/ce/de/content/filiale_dietlikon/

Author: russia (Guest)
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Try this:
Ebay-Artikel Nr. 361867758044

Author: F. L. (filmwerkstatt)
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thank you all for your thoughts!

Joe F. wrote:
> Depending on the LED driver you're using, an extremely high PWM
> frequency (10-20 KHz) might be an option.
>
> Dimming white LEDs by reducing voltage/current in general does not work
> well.
> Color temperature will change drastically or even shift to a color which
> could not in a bit be called "white" which makes it completely unusable
> for photographic lighting.
Sadly i don't have a color meter, and this would indeed destroy the 
purpose of LEDs with a CRI > 98. But from bare eye i don't see a color 
shift at all, and i'm kind of trained to detect color shifts in light 
sources. But i might be wrong of course.

> If reducing brightness in discrete steps is acceptable, I'd suggest to
> find a way to simply switch off every 3rd, 2nd, 2 of 3... LED and keep
> the remaining LEDs at full brightness.
this would be the best option, but i can't imagine the nigthmare of 
wiring 120 LEDs this way. And, if it should really work as a 
photographic light, it has to have the versitality of dimming in smaller 
steps.

> Also applying neutral density foil is a good way to reduce brightness
> without changing color temperature.
Of course, that would work, but I want to save energy, too. The battery 
lasts only around 1.3h @ 12V till completly empty and it would be nice 
if it would last longer if dimmed down.

> Or setting up a mechanical "shutter" (e.g. black wrap) in front of the
> lamp might work as well.
Well, i build a 'practical' - light source that is in frame. Even though 
i love black wrap, this isn't the place for it.

russia wrote:
> Try this:
> 
Ebay-Artikel Nr. 361867758044
this look very promising! If there's no other option than go with just a 
lot of hertz, 150KHz should be enough.

what about cutting power off if the voltage drops?

: Edited by User
Author: russia (Guest)
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Author: F. L. (filmwerkstatt)
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russia wrote:
> Youtube-Video "How to make a super bright LED light panel (for video work etc)"

oh my god, the internet is fantastic.
this is perfect and exactly what i need. thanks a bunch!!

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