Author Topic: Driving this dual coloured COB LED?  (Read 3349 times)

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Online Zero999

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Re: Driving this dual coloured COB LED?
« Reply #25 on: February 06, 2023, 08:34:28 am »
MEANWELL have some mains input CC led drivers that will do 36V leds.
And some have 3 types of brightness control input,  0-10V, PWM or external potentiometer.

Check out something like this one for $28
https://www.digikey.com/en/products/detail/mean-well-usa-inc/XLG-50-AB/9858488

Another great idea but sadly for me (I didn't mention this earlier; my apologizes) I plan to make it a portable light likely running off of high power lithium cells (hence the boost converter).

MEANWELL won't let you off that easy, they have a step-up DC input option for that too.   :-DD

Input 12V  (9-18V range)
Output up to 43V LED
CC 1.05A (a little under your 1.2A LED spec, but that's good, the LED will have a good lifespan not being driven right on the edge of failure/running hot)
Power 45W
Analog input for brightness control (I think on these ones you need couple resistors and regulated source to get the pot to produce a 0.25V to 1.3V signal. (unlike that previous one that can take pot directly on pins)
91% efficient.
Even cheaper, $21

Check out this one
https://www.digikey.co.nz/en/products/detail/mean-well-usa-inc/LDH-45A-1050W/7704776
Datasheet
https://www.meanwellusa.com/upload/pdf/LDH-45(DA)/LDH-45-spec.pdf

MEANWELL have a huge range of modules, i only looked for a minute so there might be ones that can do 1.2A if you spend some time looking.
The LDH-65-1400 maybe, it does 1.4A, but you could hardware limit it to 1.2A avg by limiting the max scale of the brightness control voltage.
$28
https://www.digikey.co.nz/en/products/detail/mean-well-usa-inc/LDH-65-1050W/12759739
The original poster's LED is common anode.

There's no mention on that datasheet of isolation, or whether the + outputs of two modules can be commoned. I strongly recommend talking to the manufacture before ordering.
 

Offline Psi

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Re: Driving this dual coloured COB LED?
« Reply #26 on: February 06, 2023, 09:08:48 am »
ah true, i missed that.
Greek letter 'Psi' (not Pounds per Square Inch)
 

Online Zero999

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Re: Driving this dual coloured COB LED?
« Reply #27 on: February 06, 2023, 07:28:14 pm »
Here's an idea. Pot1 controls the brightness of both the warm and cold white. Pot2 alters the colour temperature, but changing the currents through D1 and D2. It's not perfect, because if a Pot2's wiper goes open, it'll blow up the LEDs. U1 and U2 can be the cheap old LM358.
« Last Edit: February 06, 2023, 07:30:50 pm by Zero999 »
 

Offline spostma

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Re: Driving this dual coloured COB LED?
« Reply #28 on: February 07, 2023, 10:11:13 pm »
Zero999's solution
https://www.eevblog.com/forum/beginners/driving-this-dual-coloured-cob-led/msg4682669/#msg4682669

could be simplified a little by omitting the MOSFET on the higher-voltage LED.

I sometimes do something similar to make a red/green LED toggle by having a single pull-up resistor on both LEDs,
the Cathode of the Green (2V4) LED fixed to ground, and the Cathode of the Red (1V8) LED via a BSS123 controlled MOSFET to ground.
 

Offline LooseJunkHaterTopic starter

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Re: Driving this dual coloured COB LED?
« Reply #29 on: February 07, 2023, 10:13:28 pm »
Here's an idea. Pot1 controls the brightness of both the warm and cold white. Pot2 alters the colour temperature, but changing the currents through D1 and D2. It's not perfect, because if a Pot2's wiper goes open, it'll blow up the LEDs. U1 and U2 can be the cheap old LM358.


Ohhhh I really like this design! However question, why a NPN transistor (BD131) and not instead a MOSFET? Would a MOSFET not be more efficient and easier to drive?
 

Online Zero999

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Re: Driving this dual coloured COB LED?
« Reply #30 on: February 07, 2023, 10:52:07 pm »
Here's an idea. Pot1 controls the brightness of both the warm and cold white. Pot2 alters the colour temperature, but changing the currents through D1 and D2. It's not perfect, because if a Pot2's wiper goes open, it'll blow up the LEDs. U1 and U2 can be the cheap old LM358.


Ohhhh I really like this design! However question, why a NPN transistor (BD131) and not instead a MOSFET? Would a MOSFET not be more efficient
What makes you think a MOSFET would be more efficient?

Quote
and easier to drive?
Yes and no. A MOSFET has a virtually infinite current gain, so don't take any steady state current from the op-amp's output. The disadvantage is it has a high gate capacitance, which makes oscillation very likely. I chose the BD130 because it should have just enough gain to give a collector current of 600mA, when driven by the LM358.
 

Offline LooseJunkHaterTopic starter

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Re: Driving this dual coloured COB LED?
« Reply #31 on: February 07, 2023, 11:13:10 pm »
which makes oscillation very likely.
Isn't this really simple to overcome with a 10k resistor?

What makes you think a MOSFET would be more efficient?

Actually I think you're right. Since both the transistor and fet would both be doing constant current by creating wasted heat, they'd both be equally inefficient, right? Or would the NPN be even less efficient because of lost current between base/emitter?
 

Online Zero999

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Re: Driving this dual coloured COB LED?
« Reply #32 on: February 07, 2023, 11:34:20 pm »
which makes oscillation very likely.
Isn't this really simple to overcome with a 10k resistor?
That would make it worse because it would increase the RC time constant.

Quote
What makes you think a MOSFET would be more efficient?

Actually I think you're right. Since both the transistor and fet would both be doing constant current by creating wasted heat, they'd both be equally inefficient, right? Or would the NPN be even less efficient because of lost current between base/emitter?
Yes the MOSFET would be a tiny bit more efficient, as it doesn't require any base current, but the difference is small enough not to matter.
 

Offline langwadt

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Re: Driving this dual coloured COB LED?
« Reply #33 on: February 08, 2023, 12:24:56 am »
What's the supply voltage vs LED forward voltage?

The current needs to be controlled, not the voltage.

The LEDs being common anode doesn't help matters.

would work with an "upside-down" buck, e.g. https://www.mouser.dk/new/texas-instruments/ti-tps92511-driver/
 

Offline LooseJunkHaterTopic starter

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Re: Driving this dual coloured COB LED?
« Reply #34 on: February 08, 2023, 01:28:56 am »
What's the supply voltage vs LED forward voltage?

The current needs to be controlled, not the voltage.

The LEDs being common anode doesn't help matters.

would work with an "upside-down" buck, e.g. https://www.mouser.dk/new/texas-instruments/ti-tps92511-driver/

You know what's funny, almost as soon as you posted this comment, I came across a similar I.C called the MP4700. https://www.monolithicpower.com/en/mp4700.html

I think these I.C's appear to more commonly be referred to as "low side buck converters". Definitely more complex than driving a transistor/fet in a linear manor, however, potentially a lot more efficient since they'd automagically PWM + duty cycle to limit current and not be visibly noticeable? I think I'm going try to find more alternatives (preferably some cheaper IC's; the MP4700 is about $1 and the tps92511 is about $2) but I'll explore this route as well. Further suggestions, tips, and ideas welcomed!
 

Online Zero999

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Re: Driving this dual coloured COB LED?
« Reply #35 on: February 08, 2023, 01:48:37 pm »
The TPS92511 is limited to 500mA, but that's probably enough, as it's never a good idea to drive LEDs at the maximum current.

The MP4700 will need a lower voltage supply, just like my LM358 op-amp suggestion, but that's not a problem, as you can just use whatever is powering the boost converter.

A buck converter will be theoretically more efficient, than a linear regulator, but bear in mind, if the output voltage of the boost converter is only a few volts above the forward voltage of the LEDs, it might not gain you much. Suppose VF = 37.5V and VIN = 40V then the efficiency of a linear regulator is 37.5/40 = nearly 94%.
 

Offline LooseJunkHaterTopic starter

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Re: Driving this dual coloured COB LED?
« Reply #36 on: February 08, 2023, 04:17:41 pm »
The TPS92511 is limited to 500mA, but that's probably enough, as it's never a good idea to drive LEDs at the maximum current.

The MP4700 will need a lower voltage supply, just like my LM358 op-amp suggestion, but that's not a problem, as you can just use whatever is powering the boost converter.

A buck converter will be theoretically more efficient, than a linear regulator, but bear in mind, if the output voltage of the boost converter is only a few volts above the forward voltage of the LEDs, it might not gain you much. Suppose VF = 37.5V and VIN = 40V then the efficiency of a linear regulator is 37.5/40 = nearly 94%.

You have again solidified my decision into using a transistor/fet for constant current. The efficiency gains are not worth the extra hassle for my use case. Thank you! :p
 

Online Zero999

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Re: Driving this dual coloured COB LED?
« Reply #37 on: February 08, 2023, 06:31:36 pm »
Ideally you need a single buck-boost converter, with a negative output voltage and current, that way you can dispense with the boost converter and linear regulator.

I had a quick search and found the MAX25610. It's not a beginner friendly IC as it's surface mount and has a thermal pad, which will require hot air and a proper PCB. An evaluation kit is available, but it's not cheap.

https://www.analog.com/media/en/technical-documentation/data-sheets/MAX25610A-MAX25610B.pdf
https://www.mouser.co.uk/ProductDetail/Maxim-Integrated/MAX25610EVKIT?qs=T3oQrply3y%252BPYTt501wHzw%3D%3D
 

Offline LooseJunkHaterTopic starter

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Re: Driving this dual coloured COB LED?
« Reply #38 on: March 02, 2023, 06:54:43 pm »
Got around to building the circuit on a breadboard, except with a N-channel mosfet (currently waiting for some N-BJT's to come in the mail) but it doesn't work. It simply turns on-or-off the mosfet, depending on how much I turn the potentiometer (circuit attached).

When I measure the gate of the mosfet, I can see the voltage varies between 3.680-3.660v, depending on how much I turn the potentiometer.

What's wrong with the circuit? I tried to remove the 10k resistor and remove ground from the pot but neither worked. Help?


 

Online Zero999

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Re: Driving this dual coloured COB LED?
« Reply #39 on: March 02, 2023, 07:33:59 pm »
The circuit you've posted is doing what is expected. The op-amp is simply acting as a buffer between the potentiometer and MOSFET. There's no current sense resistor and no feedback from the output current.

Refer to the schematic I posted above. The potentiometer sets the reference voltage, which the op-amp compares to the voltage across a sense resistor.
https://www.eevblog.com/forum/beginners/driving-this-dual-coloured-cob-led/msg4685614/#msg4685614

I wouldn't recommend using a MOSFET. It's too prone to oscillation. Use a power BJT.
 


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