RGB LED, Why voltage regulator? - Page 1
Forum section: electronics / beginners

author: horle
author_rank: Newbie
timestamp: July 21, 2017, 12:26:32 pm
content: 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?

author: Zero999
author_rank: Super Contributor
timestamp: July 21, 2017, 12:47:34 pm
content: 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.

author: 3db
author_rank: Frequent Contributor
timestamp: July 21, 2017, 12:50:28 pm
content: 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 specsbecause the regulator is dissipating this energy.

author: Zero999
author_rank: Super Contributor
timestamp: July 21, 2017, 12:52:56 pm
content: A voltage regulator does seem like overkill though. A simple resistor probably would have done the same thing, at a lower cost.

author: StillTrying
author_rank: Super Contributor
timestamp: July 21, 2017, 01:13:41 pm
content: I'd have probably used abig enoughzener in series meself, to lower the CC source's dissipation.

author: Zero999
author_rank: Super Contributor
timestamp: July 21, 2017, 06:12:20 pm
content: Quote from: StillTrying on July 21, 2017, 01:13:41 pmThat 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.

author: StillTrying
author_rank: Super Contributor
timestamp: July 21, 2017, 10:39:05 pm
content: Quote from: Hero999 on July 21, 2017, 06:12:20 pmI 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.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 21, 2017, 11:48:40 pm
content: Quote from: StillTrying on July 21, 2017, 01:13:41 pmWith 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.

author: mariush
author_rank: Super Contributor
timestamp: July 22, 2017, 12:08:22 am
content: 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.

author: horle
author_rank: Newbie
timestamp: July 24, 2017, 02:21:41 pm
content: 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 expectedSo instead of dissipating the excess power in the CC sources and possibly putting them at risk, we use that beefy voltage regulator.

author: 3db
author_rank: Frequent Contributor
timestamp: July 24, 2017, 05:36:08 pm
content: Quote from: horle on July 24, 2017, 02:21:41 pmThat 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

author: Zero999
author_rank: Super Contributor
timestamp: July 24, 2017, 06:08:07 pm
content: Quote from: 3db on July 24, 2017, 05:36:08 pmFixed.

author: 3db
author_rank: Frequent Contributor
timestamp: July 24, 2017, 10:44:15 pm
content: @Hero999Sorry, Mike's post stuck in my mind and I'd forgotten about your earlier post.My bad as they say in modern speak.3DB