Led strip DC driver pwm problem

[cosminnci]

Hello,

I have the following schematic for a 220v led strip dimmer,
it is driven with 3.9KHz pwm from an arduino.
there would be 2 problems:
the mosfet heats up (the led strip is ~ 140W / 220v)
the led strip brightness is responsive only for a small pwm range (10 steps of 255)

what gives the led strip sensitivity and how could I adjust it for using the full range of 255 steps form pwm ?

thank you,
regards,
Cosmin

[Simon]

What is the total forward voltage of the LED strip ? You will need your average voltage to overcome this first..

Yes your mosfet could get warm, try using a smaller gate resistor and put it on a heatsink.

[cowana]

Do you have a datasheet for the LED strip? It is very common for 220v rated LED stips/bulbs to have a built in SMPS (switch mode power supply) - this will try to give a constant output across a wide range of input conditions. This type of setup would not be dimmable using your setup.

Andy

[cosminnci]

Hello,

we use a 3014 led strip 120led /m it can be cut each 1m  after each 5 led there is 1 100R resistor.

[Simon]

hang on, if you cut it every 5 LED's that means you have banks of 5 LED's in series and then those all in parallel, that is 5x 3.8V = 19V!!!!!!! your trying to put 220V in ? no wonder only 10 in 255 steps work that's 8.6V on average and much too much overvoltage, that is why your mosfet gets hot.

your lucky you didn't blow them up

[cowana]

Are you sure the strip needs 220v?

Assuming a white 3014 LED with the following specs:
3.2v Vf (typical)
50mA If maximum

For 5 LEDs in series with a 100R resistor, this would require 19v. I'd imagine the LED strip has a built in power supply to generate this - it does not sound like it is designed to run on 220v directly?

Edit: Simon beat me to it!

[cosminnci]

they are 220v led`s and you can`t cut them only every 1m
120leds/m 1 rez each 5 leds  (120/5 =24) groups of 5 leds
220v/24 groups /5 leds =1.83
the resistor is 100R
so I suppose the voltage is 1.83v

[Simon]

White LED's are 3.5-3.8V ish, 1.8V sounds like red. Are you sure they are all in series. It does not add up for me.

Can you confirm exactly what the LED's do when you start your PWM at 0 and work up,

[cosminnci]

at 0 pwm they are off
at 3  turn on very low
and at 10 it is max brightness

[Simon]

Then they are certainly not 220V because with 4% duty they are full on, they can't possibly be 220V, do this: connect them straight to 220VDC with a 1K 40W resistor (based on 200mA going into LED's) and tell us the voltage drop on the LED strip.......

what amperage are the LED's rated for ?

[cosminnci]

I cant test that I don`t have a 1k4w resistor

in 1 m of led strip I have 22 resistors of 100R and 120 leds
there are grouped 1 resistor followed by 6 leds  see the attached img

the ledstrip is sold as 220v strip with a simple rectifier and connector.
they worked for hours already

attached is a img of a segment of the led strip.

[Simon]

Well then run it at 10 (out of 255 - 4%) and measure the voltage and tell us what that is.

[cosminnci]

so to measure voltage drop on the led strip I basically measure the voltage on the 2 wires I power it with ?
that reads 220v at full brightness
and 140V at lowest brightness

[Simon]

Very strange, how can they be full brightness at 4% if they are made for 220V anyway ? something does not add up, have a good look at what they were supplied with, are you sure it does not do any voltage reduction ?

[cosminnci]

no, they only come with a rectifier and a wall plug.
I ran them with a bridge rectifier with my config and worked for a few hours already.
they are not sold as dimmable in this configuration, but they do dimm, I got them working but the range is too small and the steps are very visible.

[Simon]

Connect them back up to the supplied adapter and measure the voltage on the LED strip

[cowana]

I suspect the issue could just be the non-linear current/light output of LEDs. Coupled with the logarithmic response of the human eye, this means you'll see far more dimming levels at very low currents. Once the current has got to 50% of it's maximum, your eye won't notice much increase in light - certainly going from 60-70% is a lot less noticeable than 10-20%.

The solution may just be to use a higher resolution time, allowing for more PWM levels (say 16 bit) - then ramp through them at an increasing rate as the desired brightness increases.

Andy

[Simon]

yes but your saying 4% = full brightness and now you think that the other 96% is just wasted...... measure the voltage with the original power pack, this is the only way you will know where you are and what you need to do.

[bobcat]

If there is no filter capacitor on the power supply, then you are trying to do PWM on pulsating DC. Random turn on points on the power waveform.

[Simon]

That won't be much of a problem as it's 498Hz mdulating 50Hz, nothing bad there. The human eye will average it all out but there is definitely a problem with voltages

[cosminnci]

1m led strip connected to the original power supply says 217v dc
I modified the pwm frequency to 3.9khz to avoid flicker.

[Simon]

217V sounds like mains, what can i say, wind it up past 10 and see if it goes bang. you will need your software to "remap" to the correct duty range, yes you might need 16bit but I'm not sure the arduino will let you use the 6bit counter.

[cosminnci]

I was thinking I might have not chosen the right valued for driving the mosfet or the mosfet type.
the mosfet works from 2-10v range and the led strip reacts in the 2-4 vrange so remaping the signal would be the solution.

[Simon]

with the mosfet fully on you won't get any more power than connecting it to the original power supply if it is indeed made to work on 220V

[cosminnci]

could I remap the arduino pwm 0-5v to 2-4v  by hardware?
then I would have available all the 255 steps of pwm