Led driver

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I need to drive 36 red and green led (150 mA each), power supply is 10 Ah 12V, or 24 V (=2*12V), 24 V is twice as expensive as 12V. So, what is suggested cost effective way to do that, one fet or some real led driver with buck or boost functionality ?

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6 strings of 6 leds? That would be about 12V and 120ma.

Imagecraft compiler user

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Leds are Green ASMT-QGBE-NFH0E, V(f) = 3,6...4,1 V and Red ASMT-QRB2-FCD0E V(f) = 3,1...3.55 V, max current is 150 mA. 36 pcs is not exact it can be 30...40 pcs, I did same calculation as bob and got 6*6=36. Driver ????.

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Quote:
24 V is twice as expensive as 12V

Quote:
cost effective way to do

With these given LM317 (not 7805) based constant current source for each chain of 3 (roughly) LED's.
Assuming LED's in series are more handy to manage, 24V source will ask just for six 317's imho.

Last Edited: Wed. Jan 12, 2011 - 04:00 PM
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With green leds V(f) can be 4,1 V so with 7805 only 2 (12V) or 5 (24V) can be chained so maybe some boost driver (TS19371 or similar), dimming is also nice feature. How much is voltage loss using 7805? I haven't any experience about driver circuits and needed coil sizes (physical)

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Sorry for misleading with 7805 - one should use LM317 in a current source configuration. This circuit works from 3V drop over regulator. Proven.

Quote:
dimming is also nice feature

Well this was not pointed initially; with such a requirement it's definitely easier manage boost regulator. For instance, Zetex (Diodes Inc.) has a number of boost regulators capable of delivering around 1A for LED's. Output controlled by low frequency PWM or DC. Imho meets the specs. Inductors parameters are in data sheets.

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So, what do you want these LEDs do?

Free the toes!

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Torby wrote:
So, what do you want these LEDs do?

Battery operated starting lights (red and green) and false start control to motorbike races on ice track. Bikes used are like this:
http://fi.wikipedia.org/wiki/J%C...
In this link only pictures are in english.

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Series / Parallel with resistors.

Switch them using a low side switch. Pwm with the switch for dimming..

Use the 12V source.

oddbudman

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oddbudman wrote:
Series / Parallel with resistors.
Switch them using a low side switch. Pwm with the switch for dimming..
Use the 12V source.

oddbudman


That was my first plan but after some calculations solution is far from perfect without battery voltage controlled PWM.
V(f) green leed is about 3,8 V, so if we think battery voltage is 10...14 V, only 2 leds in chain. I(led) is 150 mA, 12V-2*3,8V = 4,4V -> R(led) = 30 ohm, Wasted power is 1,1 W. If V(bat) = 14V I(led) = 210 mA, wasted power is 1,4W. Using 24V battery (20...28V) do not help. So it looks that I have to use boost driver.

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I've used the LM3404 before, it should handle this with ease.

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Check NCV3065. Step up DC-DC current supply from 12V, used in automotive environments. External switch (N-Channel Mosfet). It controls the current that flows trhoug a sense resistor in series with the LED string, and you can use this resistor to equalize the current flowing through different branches. It can go up to 40V with ease, and deliver the current you want (since the key components are the external MOSFET and coil).

Guillem.
"Common sense is the least common of the senses" Anonymous.