Voltage regulator for LED panel

[th.dub]

Hi,

I am currently planning an LED panel for video/photography work. The plan is to connect 100 LEDs in parallel on a PCB. For the power source, I would ideally like to use a commercial powerbank so that I can easily replace the battery without needing a special type of battery. Also, a powerbank is easily recharged via USB, making it more portable.

The LEDs I will use have a max. forward voltage of 3.6V and a max. forward current of 20mA.
The plan is to connect the LEDs to the output of an LM2596 buck-converter (cheap adjustable chinese version) and to connect a a female micro-USB socket to the input of the buck-converter.

Since the buck-converter outputs between 1.3 and 35V, I would like to know how I could connect a potentiometer to it in order to limit the output voltage to the range 1.3V - 3.6V for dimming (without blowing the LEDs).

Also, can anybody point out something I may have overlooked when choosing a commercial "powerbank" as a power source? I would of course make sure to chose one with at least 2A output current and an over-discharge protection.

Thanks in advance.

 

[BobK]

Driving 100 LEDs in parallel with a voltage source is a real no-no. Please read this resource on driving LEDs.

https://www.electronicspoint.com/resources/got-a-question-about-driving-leds.5/

Bob

 

[cjdelphi]

The lm2596 already has adjustable voltage, no need for a pot

You simply need 100 resistors or less (using higher voltage led's in series)

 

[th.dub]

@BobK , the post suggests to connect them in parallel though? Just with a resistor in front of each. So connecting in parallel is fine, as long as they each have a resistor? I

@cjdelphi , what I initially meant was how to limit the potentiometer to a maximum of 3.6V so I don't blow the LEDs by turning it up too high.

 

[BobK]

It is possible but wasteful. The most efficient way to do it is run as many as possible in series, then drive each series string from a switch mode constant current supply.

Did you look at the resource I linked?

Bob

 

[th.dub]

I did look at the post, but it I was struggling to see the difference between methods 1 and 2 for driving arrays of low power LEDs until now.The ease of recharging and availability of power supply is incredibly important for me though. And since all commercial powerbanks deliver 5V, using LEDs in series isn't really an option.

So a setup with an initial buck-converter for dimming, and 100LEDs with a resistor each would be appropriate considering I would like a simple voltage power supply?

 

[AnalogKid]

And since all commercial powerbanks deliver 5V, using LEDs in series isn't really an option.

Sure it is. Once you commit to building something with a pre-fab switching power supply inside, the only issue is which power supply. If you use an adjustable boost converter to drive 10 parallel strings of 10 LEDs in series, the resistor power loss is greatly reduced. Plus, you are reducing greatly one cause of uneven LED brightness.

http://www.ebay.com/itm/DC-3-32V-St...285699?hash=item4643af7fc3:g:Y0MAAOSwwbdWM1RT

NOTE: A standard USB output can source 5 W, while your minimum load is 7.2 W.

ak

 

[BobK]

It's not really too bad. You waste about 1/3 of the power driving them that way.

Dimming, however, should be done by PWM, not by varying the voltage. Much more efficient since it keeps the LEDs off a percentage of the time.

Bob

 

[th.dub]

@AnalogKid , that sounds like a good option! A have a 2A (unlike the normal 1A), 5V power source, so the load should not be a problem. Would you be able to tell me how could I go ahead to modify the voltage booster to use a classical potentiometer that is easily accessible, rather than the tiny screw one? Also, how could i get achieve linear dimming? I believe that if i have 10 parallel strings of LEDs, adjusting the voltage will make very little difference between 0V and around 25V, but a massive difference between 25V and 35V?
@BobK , I read about the efficiency of PWM, but that makes it redundant for video use since the off-time would lead to flickering in the video.

 

[BobK]

, I read about the efficiency of PWM, but that makes it redundant for video use since the off-time would lead to flickering in the video.

Not if the frequency is high enough. It will flash many times during a single frame, which will not be visible at all in the video.

And it is the easiest way to get something like linear dimming. You would have to change the voltage in a non-linear way to achieve the same.

Bob

 

[th.dub]

Not if the frequency is high enough. It will flash many times during a single frame, which will not be visible at all in the video.

And it is the easiest way to get something like linear dimming. You would have to change the voltage in a non-linear way to achieve the same.

Bob

Would you know what minimum frequency is required for this? The fastest I could find was around 10-15kHz (here or here).
And is it correct to assume that the voltage booster would be less efficient when boosting 5V to 35V, rather than only 25V (in a dimmed state)? That was my logic to use voltage dimming, since that would cut the losses made in the voltage booster? It's using an LTC1871 chip (datasheet)

Sorry if that's nonsense, but it kinda made sense in my head.

 

[AnalogKid]

LED brightness changes with the total energy through the diode. Voltage-mode, current-mode, PWM, whatever, for a particular brightness you need a certain amount of energy from the source. The energy at 50% brightness will be less than at peak brightness, but the conversion circuit efficiency might actually be worse. This gets into the down-and-dirty details of how a boost converter works and how the individual components function, things like FET gate charge, edge rate, inductor resistance, etc.

ak

 

[KJ6EAD]

…And since all commercial powerbanks deliver 5V, using LEDs in series isn't really an option…

There is at least one powerbank that delivers up to 19V.

http://www.tekkeon.com/products-mypowerall.html

 

[BobK]

If the frame rate is 120 fps, a 15000Hz PWM will flash 125 times during each frame. You will not see any flicker affect.

Bob

 

[Herschel Peeler]

Hi,

I am currently planning an LED panel for video/photography work. The plan is to connect 100 LEDs in parallel on a PCB. For the power source, I would ideally like to use a commercial powerbank so that I can easily replace the battery without needing a special type of battery. Also, a powerbank is easily recharged via USB, making it more portable.

The LEDs I will use have a max. forward voltage of 3.6V and a max. forward current of 20mA.
The plan is to connect the LEDs to the output of an LM2596 buck-converter (cheap adjustable chinese version) and to connect a a female micro-USB socket to the input of the buck-converter.

Since the buck-converter outputs between 1.3 and 35V, I would like to know how I could connect a potentiometer to it in order to limit the output voltage to the range 1.3V - 3.6V for dimming (without blowing the LEDs).

Also, can anybody point out something I may have overlooked when choosing a commercial "powerbank" as a power source? I would of course make sure to chose one with at least 2A output current and an over-discharge protection.

Thanks in advance.

I think there is nothing bad about your original idea. You only need about 2 Amps. TheLM2596 can supply 3 Amps. Put a small resistor in series with each LED, maybe 10 to 27 Ohms or so. You can't run it from USB. How long will the light be on at any given time? Just a few minutes? If hours the battery may be too much to charge from USB. This would be running from a 12 V battery.
An option, forget the LM2596 and use an 18650 lithium battery. It takes about 10 hours to charge from USB at 100 mA. That will give you your 2 Amps for about an hour. Any bigger and you will have to go with a bigger battery and/or a regular power supply.
Not that the other suggestions are bad. Just not as simple as using a single 18650 battery.

 

[Herschel Peeler]

Hi,

I am currently planning an LED panel for video/photography work. The plan is to connect 100 LEDs in parallel on a PCB. For the power source, I would ideally like to use a commercial powerbank so that I can easily replace the battery without needing a special type of battery. Also, a powerbank is easily recharged via USB, making it more portable.

The LEDs I will use have a max. forward voltage of 3.6V and a max. forward current of 20mA.
The plan is to connect the LEDs to the output of an LM2596 buck-converter (cheap adjustable chinese version) and to connect a a female micro-USB socket to the input of the buck-converter.

Since the buck-converter outputs between 1.3 and 35V, I would like to know how I could connect a potentiometer to it in order to limit the output voltage to the range 1.3V - 3.6V for dimming (without blowing the LEDs).

Also, can anybody point out something I may have overlooked when choosing a commercial "powerbank" as a power source? I would of course make sure to chose one with at least 2A output current and an over-discharge protection.

Thanks in advance.

A problem with using the LM2596. LEDs are 3.5 V. The battery is3.7 V. The LM2596 needs a few volts in higher than the out. Why use the LM2596 if the battery is at the right voltage in the first place?
No the LEDs won't blow out at a volt or so over the 3.5 V. Most LED flash lights run the 3.5 V LED right off of 4.5 V battery, some with even no resistor between, but that is not suggested if you want reliability.

 

[Colin Mitchell]

If you use 20 strings of 5 LEDs, you will be able to turn down the voltage by removing the 10-turn pot and connecting the three leads of a normal pot to the 3 holes on the PC board. 5 x 3.7 = 18.5v You will be able to adjust the voltage from 17v to 23v with a 220R resistor in each string. Don't go over 24v.
The current will be 20mA x 20 strings = 400mA @ 23v = 10 watts.

 

[Herschel Peeler]

If you use 20 strings of 5 LEDs, you will be able to turn down the voltage by removing the 10-turn pot and connecting the three leads of a normal pot to the 3 holes on the PC board. 5 x 3.7 = 18.5v You will be able to adjust the voltage from 17v to 23v with a 220R resistor in each string. Don't go over 24v.
The current will be 20mA x 20 strings = 400mA @ 23v = 10 watts.

Battery powered?

 

[th.dub]

If you use 20 strings of 5 LEDs, you will be able to turn down the voltage by removing the 10-turn pot and connecting the three leads of a normal pot to the 3 holes on the PC board. 5 x 3.7 = 18.5v You will be able to adjust the voltage from 17v to 23v with a 220R resistor in each string. Don't go over 24v.
The current will be 20mA x 20 strings = 400mA @ 23v = 10 watts.

Would this be more efficient than using 10 strings at 34V? And is driving them at a higher voltage than 3.6 each okay?