Led strip DC driver pwm problem - Page 1
Forum section: electronics / beginners

author: cosminnci
author_rank: Regular Contributor
timestamp: October 16, 2014, 12:25:27 pm
content: 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

author: Simon
author_rank: Unknown
timestamp: October 16, 2014, 12:31:50 pm
content: 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.

author: cowana
author_rank: Frequent Contributor
timestamp: October 16, 2014, 12:57:50 pm
content: 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

author: cosminnci
author_rank: Regular Contributor
timestamp: October 16, 2014, 01:01:48 pm
content: Hello,we use a 3014 led strip 120led /m it can be cut each 1m after each 5 led there is 1 100R resistor.

author: Simon
author_rank: Unknown
timestamp: October 16, 2014, 01:17:16 pm
content: 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

author: cowana
author_rank: Frequent Contributor
timestamp: October 16, 2014, 01:17:40 pm
content: Are you sure the strip needs 220v?Assuming a white 3014 LED with the following specs:3.2v Vf (typical)50mA If maximumFor 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!

author: cosminnci
author_rank: Regular Contributor
timestamp: October 17, 2014, 05:23:24 am
content: they are 220v led`s and you can`t cut them only every 1m120leds/m 1 rez each 5 leds (120/5 =24) groups of 5 leds220v/24 groups /5 leds =1.83the resistor is 100Rso I suppose the voltage is 1.83v

author: Simon
author_rank: Unknown
timestamp: October 17, 2014, 06:03:48 am
content: 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,

author: cosminnci
author_rank: Regular Contributor
timestamp: October 17, 2014, 06:35:12 am
content: at 0 pwm they are offat 3 turn on very lowand at 10 it is max brightness

author: Simon
author_rank: Unknown
timestamp: October 17, 2014, 06:38:29 am
content: 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 ?

author: cosminnci
author_rank: Regular Contributor
timestamp: October 17, 2014, 07:51:17 am
content: I cant test that I don`t have a 1k4w resistorin 1 m of led strip I have 22 resistors of 100R and 120 ledsthere are grouped 1 resistor followed by 6 leds see the attached imgthe ledstrip is sold as 220v strip with a simple rectifier and connector.they worked for hours alreadyattached is a img of a segment of the led strip.

author: Simon
author_rank: Unknown
timestamp: October 17, 2014, 07:59:23 am
content: Well then run it at 10 (out of 255 - 4%) and measure the voltage and tell us what that is.

author: cosminnci
author_rank: Regular Contributor
timestamp: October 17, 2014, 08:05:03 am
content: 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 brightnessand 140V at lowest brightness

author: Simon
author_rank: Unknown
timestamp: October 17, 2014, 08:08:17 am
content: 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 ?

author: cosminnci
author_rank: Regular Contributor
timestamp: October 17, 2014, 08:19:44 am
content: 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.

author: Simon
author_rank: Unknown
timestamp: October 17, 2014, 08:21:24 am
content: Connect them back up to the supplied adapter and measure the voltage on the LED strip

author: cowana
author_rank: Frequent Contributor
timestamp: October 17, 2014, 08:48:27 am
content: 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

author: Simon
author_rank: Unknown
timestamp: October 17, 2014, 08:51:28 am
content: 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.

author: bobcat
author_rank: Regular Contributor
timestamp: October 17, 2014, 10:01:55 am
content: 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.

author: Simon
author_rank: Unknown
timestamp: October 17, 2014, 10:04:28 am
content: 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

author: cosminnci
author_rank: Regular Contributor
timestamp: October 17, 2014, 10:44:05 am
content: 1m led strip connected to the original power supply says 217v dcI modified the pwm frequency to 3.9khz to avoid flicker.

author: Simon
author_rank: Unknown
timestamp: October 17, 2014, 10:49:20 am
content: 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.

author: cosminnci
author_rank: Regular Contributor
timestamp: October 17, 2014, 11:49:13 am
content: 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.

author: Simon
author_rank: Unknown
timestamp: October 17, 2014, 11:50:40 am
content: 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

author: cosminnci
author_rank: Regular Contributor
timestamp: October 17, 2014, 12:01:36 pm
content: could I remap the arduino pwm 0-5v to 2-4v by hardware?then I would have available all the 255 steps of pwm

author: Simon
author_rank: Unknown
timestamp: October 17, 2014, 12:04:20 pm
content: I assume your actually refering to the voltage equivalent to the duty cycle. You want the mosfet switched from 10V to ensure it's resistance is low and minimize losses and overheating. You need to only use the range of PWM that gets you the brightness you need, what are you using as an input ? you need to map input to output.

author: cosminnci
author_rank: Regular Contributor
timestamp: October 17, 2014, 12:07:46 pm
content: arduino pin goes tough an 4n25 optocoupler that drives the mosfet.the output side of the optocoupler is powered from a 10v source.the schematic is in the initial post.

author: Simon
author_rank: Unknown
timestamp: October 17, 2014, 12:35:52 pm
content: I know, it's just odd that you refer to the PWM output in terms of a "voltage" you need to map the output to your LED characteristic and of course to any input you are using to control it.