LED Dirver Add-on

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csaba911

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Hi everyone....

Can this simple circuit be used to add extra protection to a otherwise not fully managed LED driver.
Originally the PWM was driving the FET direct, the 0.1 ohm also added..
Want to prevent my existing "expensive, aka bad choice" driver's to make a wrong decision again.
I thrown together a test on my desk and looks like works, I can set the max current with R1 even if the PWM stops on the full on state.

I need to make a driver for 32ch LED lighting system controlled by 32 PWM channel

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1. I don't how how it works. If the LED current is too high, the (-) input of IC1 go too high, IC1 output = low, IC2A output = high, turning on the MOSFET and keeping the current flowing in the LEDs.

2. You are using LEDs in constant voltage mode. They should be used in constant current mode. They are current devices not voltage devices.

3. I don't the LED current in your application so I can't give much detailed help. But I have attached a LED driver that is constant current. You set it for the current you want then drive the on/off pin with a PWM signal. (maybe 1khz or so) The LT1937 is for small LEDs. There are many parts for the big LEDs.
 
May be you can put the LED in parallel instead of series connection.that can solve your problem. but im not sure.
add a resistor to the LED about 470 ohm.
 
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Apart from the points noted above by Ron, the 4093 won't be able to provide enough current for charging/discharging the FET gate capacitance to switch the FET on/off cleanly enough to prevent significant heat generation, except at low frequencies. What is your PWM frequency?
 
sorry, forgot to include..
LEDs use 1100 ma

FET gate treshold 3v
pwm runs 106 khz
 
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The pwm comes from a 16ch pwm chip, I have not designed the software for the micro controlling the 2 pwm ic and it's not likely can be changed now.
 
At 106kHz PWM you'll need a 'proper' MOSFET driver; the 4093 just won't cope.
 
What 16 channel. PWM IC?
You should not put+12V across a string of leds! It is not god engineering.
 
2xPCA9685 driving the fet.
As I stated in the beginning the design is done, this circuit would be add on.
The LEDs grouped in 8x3's each driven by a ~9 amp constant current pwsupply.
The unit was working for over 2 year just fine until one of the PCA9685 stuck on full on and burning out half of the LEDs. The LEDs usually newer driven to 100% brightness, average 5-75%
The main driver/software wasn't designed by me nor have to any of the code. Can't even tell what micro was used since the markings are sanded off.

So I taught instead of starting everything from scratch I just add a limiter to each channel instead of buying 32 new constant current supply.
If you guys have better idea how to drive 96 3 watt led so they can be controlled individually in group of 3 than please let me know.
Btw this is a $2600 aquarium light. Out of warranty.
 
I think we are starting to understand what you have.
You have 16 PWM signals running at 106khz.
In your first schematic, where it says +12V. it is really a constant current supply.
There are really two 9A constant current supplies. Each supply is for 8 strings of 3 leds controlled by 8 PWMs.
LED current is about 1A.
It would have been nice to know this at the start.

I don't understand how a 9A constant current supply can drive 8 strings of LEDs that could be at 0% to 100% drive. There must be things that I do not understand.

Maybe you should tell us more about the 9A power supplies.
 
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Sorry I see where was the confusing coming from, the circuit had error, this is the correct version.

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Fixing logic is part of the problem.
Your PWM signal is built to drive MOSFET gates. The 4093 is not built to drive gates directly at 106khz. post #4.
I believe back in post #10 you stated the "12V" is really a 9amp constant current supply. (not the same thing)
In post #11 I am trying to find out what you have with little success.

Either, (I do not understand) or (you built a machine the will burn out LEDs) or (both).
 
The power-supply enclosed in epoxy, all I know from testing that all 4 output are individually controlled, so each out can be load up to 9A before start to limiting..

I taught about to scrap all the circuit and just use the fixture as is than start to build my own controllers.
I like they way its running, from web or usb via pc, color temperature can be adjusted and have many nice preset option like sunrise in Hawaii cloud cover effect, night storm, full moon phase simulator actually accurate to whatever location I select..

anyway the led's grouped like this.

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I'm not convinced that driving parallel chains of LEDs from one constant current source is good practice. If one chain fails, or is switched off, the other chains will pass extra current and probably fail in turn as a result. Is that what you're trying to avoid?
Relying on the FET to limit current could result in a lot of power being dissipated by the FET (we don't know how much because we don't know the voltage across the FET).
 
Here's what I'd try. As we don't know the full spec of your supply I simulated this with a fixed 12V supply.
 
The LEDs grouped in 8x3's each driven by a ~9 amp constant current pwsupply.
Click to expand...
#10
[quoteThe power-supply enclosed in epoxy, all I know from testing that all 4 output are individually controlled, so each out can be load up to 9A before start to limiting..[/quote] #14

OK so you power supply is 12V from a current limited supply not a constant current supply!!!!!!!!!
Why not add a resistor to each set of 3 LEDs. Size the resistor so there is the correct amount of current flows at 100%. Then use the PWM to reduce the current.

Never put voltage across a LED. Use a resistor or constant current source.
Every week we hear about some one driving a "3V" LED with 3 volts and it is too bright or too dim or some work and some don't. You have a 1A LED that needs some where between 2.5 to 4 volts to get 1A. While most of the LEDs do need 2.8 to 3.5V not all will.
 
I put everything back together in the way was originally, technically it was working fine for 2 years, installed 48 new led in the demaged section.
Looks like working.. Maybe the psup more advanced the we think, can really explained way cause the failure anyway, hope for the best. My corals isn't doing very well under T5
 
Just to trow my 2cent here, I build it, using Si4336 instead of IRL, using exact same as described in me schematic, the PWM just a 555 for testing.
After reading a few commercially available LED driver IC there is not much different other than max current rating, analog dimming, status output, open LED protection, etc... but all use the same basic concept, now before anyone start the FET gate driving issue, this fet is 3.3v direct MCU controllable...
No more comment required, start to remember why I haven't been here for 2 years..


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