LED's pulsing when on low value?

Hi, I've been adding code to my sketch for the adafruit #805 12-bit PWM board and have been testing it on 1 channel with one LED. Tonight I hooked up my main aquarium light which is using 8 of the channels, I'm using 9 LDD-700's to drive them. Each driver is driving 4 - 9w LED's for the except of 2 which are driving 1w moon lights.

When the lights are dimmed low, starting at around 150 out of 4095, they start to pulse? (I'm guessing its always happening but can't see it when they are bright) When the PWM value gets down to 15 and lower you can really see it. All of the channels are doing this. When the value set to 0 they do shut off.

I have the PWM wire from the drivers going directly to the 12-bit PCB, should I have a resistor inline or something? I never had this problem with my other controller so I know its not the light itself. I'm using cat5 wire, about 6ft for the 8 channels and a 16awg wire for ground. Ground is going to the pin next to the PWM outputs on the mega2560.

should I have a resistor inline or something?

No.

What is your power supply? Can it cope with the current? Have you got some large supply smoothing capacitors across the supply?

What happens when you add the LEDs one at a time? How many do you need to add before this effect becomes apparent?

I have a meanwell NES-350-48, its 350w at 48v which should be more than enough. I was running my light off my storm LED controller with no issues. The power supply has a fan that turns on and off and it hardly runs so the power supply isn't being worked. It also seems to make no difference if I only have one hooked up. The drivers are hooked directly to the power supply, no capacitors. It also makes no difference how I power my arduino, nothing on it is getting warm.

Product ID 805 from Adafruit is this but 815 is this

So let's assume you are using #815. I am very familiar with the chip on that board as I use it for my own LED shields
While the PCA9685 generates a PWM signal with 12 bits of resolution to my knowledge there are no low cost commercial general purpose LED constant currrent drivers such as the LDD-700 available that accept a 12 bit dimming signal.
Most of these only accept 8 bit.

That's likely the reason why it is pulsing at low PWM rates.

Oops, its the 815.

That is not what I wanted to hear, very disappointing.

That's a nice looking shield you made, how many 9w LED's could it drive?

I just discovered something interesting. I disconnected everything hooked it back up one PWM pin at a time testing between and there was no pulsing. Then I ran my ramping schedule and it started pulsing. Now no matter what I do. reset, repower it will always pulse. Maybe its my sketch causing it.

Does anyone have a simple sketch that will ramp down to zero?

robsworld78:
The drivers are hooked directly to the power supply, no capacitors.

In that case that is what your problem is. You need to add some capacitors to the output of your power supply. As you say the output voltage is 48V then these need to be rated at a voltage of at least 60V. You should try with something like 470uF.

robsworld78:
I just discovered something interesting. I disconnected everything hooked it back up one PWM pin at a time testing between and there was no pulsing. Then I ran my ramping schedule and it started pulsing. Now no matter what I do. reset, repower it will always pulse. Maybe its my sketch causing it.

No it is not the sketch, this confirms that you have power supply instability a capacitor will help with this.

I think you are right Grumpy, explains why I never noticed this during testing. I've been playing and it only does it certain times. I'm definitely pushing the limits on voltage, I guess I didn't think that mattered as much as pushing the limits on amperage.

How many capacitors should I be adding? You mention 470uF, is that the only number I need to watch for or is there voltage ratings as well? Are the caps physically large or small? Sorry the electronic stuff isn't really my thing, I can put together but don't know what it does. :slight_smile:

As I said you need a voltage rating of at least 20% more than the voltage you are actually using. The value was just a guess to what I would start with. You can not have too much capacitance but you can have too little.

At this sort of voltage yes they are physically big.

Oh sorry I didn't realize the cap was 60v. I will look around tomorrow for them. How many should I get?

I would get three, use one at first and if it doesn't work put the other two in parallel.

Right on, thanks, hopefully I can find them locally.

I was at Active today but they didn't have any 470uf so I got some 63v 1000uf, will these be ok?

You mention I need to wire them to the output of the power supply, does it go inline or across + and -? I'm thinking + of cap goes to + of output and - of cap goes to power supply ground?

That value will be fine.

Wire them + to the +ve of the supply and - to the -ve of the supply. Wire them as close to the LED strip as you can.

Thanks I'll give it try next time it happens, they were working great last night. :slight_smile: