Can i safely add PWM + Duty cycle logic to a constant current driver?

Thread Starter

markg85

Joined Sep 12, 2020
13
Hi,

Sorry for the long thread name! I'm not quite sure how to name this thread.

The situation is as follows. I currently have a lets string of about 100 LED's (white) that are being driven by a LDD-600H. Thus that's effectively a 600ma constant current driver. That driver has a PWM pin which i control using an ESP8266. As a side note, the ESP8266 is powered by a micro 48 to 3.3v circuit board, it's fed from the same source as the driver. With this logic i'm able to dim lights just fine.

That's the current working situation!

I have a couple of problems with this setup.
  1. Due to the ESP8266 i only have a 10 bit PWM duty cycle. For linear dimming that would already be low, but i also applied gamma correction, which gives me very visible PWM steps at the low dimming end.
  2. The PWM frequency of the LDD-600H is decent (1khz, max) but i like to up this to 5khz or so.

In terms of hardware, the ESP32 can do exactly this. It has a dedicated LED PWM which allows me to go to 12 bit duty cycle at a 5khz PWM frequency. If i drop the frequency a bit more i can increase the resolution.

So that's the route i want to try.

The trouble i'm having now is finding out if this logic would even work with constant current drivers. Say for example, would it work to connect the same LDD-600H but just not using that PWM pin. An external mosfet circuit with the gate controlled by a PWM pin on the ESP32 would then control the flow of energy from the output of the LDD-600H into the LED strip.

I vaguely remember a circuit like this being problematic.. But i don't know exactly why? I could just be seeing ghosts here and it might work. But i feel like i should ask first before trying this.

And if that doesn't work, what kind of driver do i need to make a logic like the above work?

Best regards,
Mark
 

Thread Starter

markg85

Joined Sep 12, 2020
13
... can i seriously not edit my own post?... That sucks.
I wanted to fix some typo's (like "lets" to "LED's")...
 

wayneh

Joined Sep 9, 2010
17,152
... can i seriously not edit my own post?... That sucks.
I wanted to fix some typo's (like "lets" to "LED's")...
It's possible that the privilege to edit your own post is attained after some minimum number of posts?

I don't think you can externally PWM a constant-current supply like that. It'd lose its mind trying to adjust to the varying load between full on and full off.
 

Thread Starter

markg85

Joined Sep 12, 2020
13
Do you have an Edit button at the bottom of your post?
Nope, not anymore. I do see it right after i post a comment but the edit button above my previous comment is also already gone. I'm guessing you're only allowed to edit a post within some minutes of posting?
 

wayneh

Joined Sep 9, 2010
17,152
I'm guessing you're only allowed to edit a post within some minutes of posting?
Not normally. It should "stick" and always be available. Do you see your username up in the right of the menubar? If you're not logged in, you won't get the edit button.
 

Thread Starter

markg85

Joined Sep 12, 2020
13
Not normally. It should "stick" and always be available. Do you see your username up in the right of the menubar? If you're not logged in, you won't get the edit button.
Yes i'm logged in.
I did, just to be sure, log out and back in. Didn't change anything.

At this very moment i can't edit my first and second post. I can edit my other posts in this topic (still).
 
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