Query on constant current drivers - how do they work?

[gregmcc]

I have a query on constant constant current drivers, specifically the Meanwell LDD-700h driver powering LED's

Lets say I have the circuit attached - a 48V PSU supplying power to the LDD (which is a 700mA constant current driver controlled by PWM) which puts out a voltage from 2-56V (from the datasheet)

Now I hook up 3 power LED's with a forward voltage of 3V at 700mA.

I get the LDD can drive the LED's at 700mA but each LED has a voltage of 3V - 9V in total.

If I connect the LDD wont it supply a voltage of 48V to the LED's - how does the LDD know that I only have 3 LEDs and that it must only supply a max of 9V. Is that controlled by the PWM? If so I could potentially put 48V across the LED's by messing around with the PWM?

https://www.electro-tech-online.com/custompdfs/2013/06/LDD-H-spec.pdf

BTW I"m going to control the PWM using an Arduino.

 

[dougy83]

The PSU regulates its output based on the measured output current. In your case, it will increase the output voltage if the current is <700mA and increase the output voltage if the current is >700mA (the output voltage will remain constant when it has 700mA flowing through the output).

That said, it is the LEDs on the output that set the voltage; if one LED is connected and its forward voltage drop is 3.4V at 700mA, then the output voltage will be 3.4V (until the LED heats up and the voltage changes for the same current). If you put a 10 ohm resistor on the output, then the output voltage will be 700mA*10 = 7V

The PWM signal sets the output current; if you feed 100% D PWM in, the output current should be 100% of 700mA = 700mA. If you feed in 50% D PWM, then the output current will be regulated to 350mA (and the output voltage will decrease accordingly).

 

[gregmcc]

Thanks - great explanation. You learn something new everyday!

I had no idea that the PSU worked that way. I thought it gave out a constant 48V.