RGB LED, Why voltage regulator?

[horle]

Hey guys!

I just got a new pcb design from my colleague which I don't understand and he is on vacation now.

So it's a RGB led illumination. We have 6 RGB leds (Osram LRTB GFUG) in series.
Each color channel is powered by a constant current source Diodes Inc. BCR420U.

The led is designed to be used with 24VDC. All channels will be connected to the same power supply through mosfets. So far I understand the design.

But what I cannot understand is why he added an 18V voltage regulator MC78M18CDTG to the red channel.

I know the red leds have less forward voltage but I don't understand why he had to limit the voltage here at all.
I mean they get a constant current so the voltage shouldn't matter. At least that's what I thought.

Is there any possible reason I miss here? Or did he just have a blackout during the design?

[Zero999]

Perhaps the lower forward voltage meant that the power dissipation of the constant current source's transistor would have been exceeded with a power supply voltage of 24V?

Please post a schematic and picture of the circuit.

[3db]

Because the voltage of the red leds is lower the constant current device will have more voltage across it than the others.
This means it will dissipate more power. Perhaps this power would have exceeded the specs of the device.
By using the separate regulator to reduce the voltage the constant current devices can now operate within their specs
because the regulator is dissipating this energy.

[Zero999]

A voltage regulator does seem like overkill though. A simple resistor probably would have done the same thing, at a lower cost.

[StillTrying]

I'd have probably used a big enough zener in series meself, to lower the CC source's dissipation.

[Zero999]

I'd have probably used a big enough zener in series meself, to lower the CC source's dissipation.

That would work but is a zener really necessary? The constant current source keeps the current the same, so a resistor is all that's needed, as the voltage drop across it will be constant.

[StillTrying]

a resistor is all that's needed, as the voltage drop across it will be constant.

I agree, a mosfet, a 78M18 and a CC source, what could possibly go wrong!
I hope we eventually get to know the answer to the mystery.

[mikeselectricstuff]

I'd have probably used a big enough zener in series meself, to lower the CC source's dissipation.

With a constant-current source, a resistor is all you need. The  only benefit of the zener is if you wanted all colours to behave similarly if the supply voltage dropped below the dropout threshold of the current regulator.

[mariush]

It looks pretty clear to me... those led drivers are linear, they work like voltage regulators only adjust voltage based on current.
So you have red leds with forward voltage of let's say 1.6 ..1.8v .. times 6 that's around 10-12v , and you have the green and blue colors with up to 3v forward voltage so around 18v input.

So 18v in for red channel means around 6v drop on the driver, 24v in for the green and blue means 24-18 = 6v drop on those ... so you have a sort of even heat across all three led drivers.
The 18v linear regulator may be cheap and convenient and easy to solder on circuit board and with nice and big surface to dissipate heat easily.

[horle]

Thanks for your replys.

Distribution of power dissipation would have been my guess. I couldn't make sense of anything else. Seems my understanding of electronics is better than expected

So instead of dissipating the excess power in the CC sources and possibly putting them at risk, we use that beefy voltage regulator.

[3db]

Thanks for your replys.

Distribution of power dissipation would have been my guess. I couldn't make sense of anything else. Seems my understanding of electronics is better than expected

So instead of dissipating the excess power in the CC sources and possibly putting them at risk, we use that beefy voltage regulator.

That would seem to be the most logical conclusion from the information provided.
This doesn't mean that a resistor as suggested by Mike wouldn't have been adequate.

3DB



 

[Zero999]

Thanks for your replys.

Distribution of power dissipation would have been my guess. I couldn't make sense of anything else. Seems my understanding of electronics is better than expected

So instead of dissipating the excess power in the CC sources and possibly putting them at risk, we use that beefy voltage regulator.

That would seem to be the most logical conclusion from the information provided.
This doesn't mean that a resistor as suggested by Mike and Hero999 wouldn't have been adequate.

3DB

Fixed.

[3db]

@Hero999
Sorry, Mike's post stuck in my mind and I'd forgotten about your earlier post. 
My bad as they say in modern speak. 

3DB