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need of constant current drivers to drive High Bright LEDS

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prakashvenugopal

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Hi all,

I am using High bright (cool white) LED with forward voltage -= 3.2V and forward current= 350 mA.
I am placing 8 LEDS in series. I am feeding 24V DC to the LEDs using the SMPS. It is working fine.
My question is, if i am using the SMPS, there will not be any fluctuation in 24V DC. so there is no need to
use constant current LED drivers? Please suggest.

Thanks,
V. Prakash
 

Oh yes there is. LEDs have a poor temperature coefficient, so the current will vary from switch on (devices cool) to when they are hot. If you tolerance the voltage correctly, you can make sure that the devices do not over dissipate and burn out, but at the expense of them running a lit dim at other times. This is why the manufacturers say current not voltage!!!!
Frank
 
HI,

Even though when i use the constant voltage using SMPS, there is a need for constant current LED drivers. Please confirm.

Thanks,
V. Prakash
 

prakashvenugopal said:
Even though when i use the constant voltage using SMPS, there is a need for constant current LED drivers. Please confirm.
Click to expand...

Yes. Generally, for a given applied voltage, LED current has a strong positive temperature coefficient. That means that if the temperature rises, current increases. But if current increases, more power is dissipated and therefore the temperature rises. It's a positive feedback cycle: if you don't take special care to ensure the LEDs are properly ballasted with enough resistance, their current will run away (either until the LEDs break, or the power supply breaks).

Hope this helps.
 

It is your luck that the string is current regulating in itself .However , be vary since unbalance in currents /voltage drops across each will tend to fuse one and have the compensation provided by the other .
In any case is is advisable to have a mosfet regulator for each LED with 350mA provided to drain..
You can use transistor regulators too with current load at the collector .
 

Just finished design of one unit with 300mAmps load current.. I had raised the temp. from 25*C to 70*C in the soak testing for 8hours continuous and stable function.
 

Hi,

Please recommend the Led driver IC to drive 8 HB Leds with 24V Input Supply Voltage.

Thanks,
V. Prakash
 

prakashvenugopal said:
Hi, Please recommend the Led driver IC to drive 8 HB Leds with 24V Input Supply Voltage.Thanks, V. Prakash
Click to expand...
There is no need for a complicated IC based current regulator deisgn for such small applications. Just google to find some simple transistor based designs, and too one can be used based on the LM317 voltage regulator used as current regulator. The said circuit with LM317 is also found with discussions on Eda board.
Cheers
 

If you are expesting high temperature fluctuation in the environment where LEDs will operate, the best way is to make your own CC sink using one NPN tranbsistor and one N-MOSFET. The NPN transistor has negative temperature cioefficient and, therefore will compensate for positive coefficient of the LEDs.

 
I had tried the simple LM317 version which works OK for A single LED ...However , when the temperature increased from 20-75*C there were deviations in intensity and I was unhappy at the result .

- - - Updated - - -

I had to work on 18watt LEDs and had to parallel 15 nos spread to a distance of 35meters .Further ,I had to control the dimming and intensity variation from starlight to full sun as this was on the theater roof lighting .I used a CV type 300wtts supply and a 3842 so that the buss bar of 30VDC line was running across the roof .
The single panel control at the base station was enough to cater for the same .The AC roof temp was 20*C and the LED heatsink had risen to 75*C (AC not working with LEDs ON for 16Hrs ) during summers as the theater was in desert area...
 

HI,
I need to adjust the brightness of the LEDs. How to adjust the brightness in the mentioned circuit?

Thanks,
V. Prakash
 

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