DIY pwm led driver Q's

F

funkyfish77

New member
OK my goal with this is a simple affective pwm DC/DC driver for one 20 watt multichip LED . I started here http://pcbheaven.com/circuitpages/LED_PWM_Dimmer/
Well this is what I have now it works fine.
picture.php

picture.php

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I don't have a extra led yet so I have not tested it qon a 20 watt led yet . Changes I have made to the circuit are r1 got changed to 1kohm the pot is 20k and I changed the transistor to a tip42. Here is the schamatic
picture.php

Question 1 from what you can see is there anything I can do to my curcuit to make it better or more dependable without adding much complexity?
Question 2 when I check my voltage on the output it vary's in voltage depending on the brightness of the leds . I thought a pwm signal would still read 12v no matter the brightness
Of the leds?
Well I'm open to all opinions so lets have them.
Thanks in advance

Funky
 
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funkyfish77 said:
Question 1 from what you can see is there anything I can do to my curcuit to make it better or more dependable without adding much complexity?
Click to expand...

How are you controlling current? Remember LEDs are diodes, and without a current comparator to control your PWM I don't expect them to last long... If you're using a resistor to control current, note that you're sacrificing a ton of efficiency and stability (the diode Vf changes with respect to current).

I would not use this for high current applications.

funkyfish77 said:
Question 2 when I check my voltage on the output it vary's in voltage depending on the brightness of the leds . I thought a pwm signal would still read 12v no matter the brightness
Of the leds?
Click to expand...

A multimeter effectively averages over a time window. Since the PWM is a square wave (that is, cycling between 0V and 12V), your multimeter is seeing what the average voltage is in an update cycle. In order to see the PWM waveform, you really need an oscilloscope.
 
That circuit uses resistors to limit current, and the 555 to chop the power on and off. If dedicated LED ic's didn't exist this might be a reasonable approach, but they do. ;) I would suggest something like the CAT4101 instead.
 
I kind of thought that was happening with the multimeter thanks . Im pretty new at these things so bear with me.
Yes I was going to use a resistor to limit voltage . The LED runs at 11v 2 amps my power supply is 12.36v with or without load so I only have to drop about 1v . I get a .6v drop with my circuit then a 1 ohm resistor that should get me under 11 v and 1800ma.

I looked at the cat4101 driver but they can't handle the amp load if there is a simaluar chip that can handle 3 or 4 amps that would be cool . Any use full links would be great

Funky
 
Personally, I wouldn't use:
- BJTs in a switch
- A linear regulator

You're likely going for efficiency (one of the top reasons for going LED), and to drop 1.2W in a transistor and 2W in a resistor is silly (thats 15% of your power right there), never mind the linear regions of the transistor as its transitioning. If you want to DIY something, I suggest a buck converter controller with external FET. Or at the very least, a FET to control your PWM - an N-channel at the bottom of the chain should work.
 
Wow the lm3409 is a little pricey. Well I guess I need to just bit the bullet and get my arduino atmega 2560 figured out then make pwm controlled driver's for my leds .
I'm shooting for indivigual control of each led there will be 8 20 watt multichips plus sump lights. Well thanks for the help.

Funky
 
In terms of driver cost per watt of output, the LM3409 is pretty reasonable. Shop around, price for the IC itself varies considerably. You'll make up the cost of building a few more efficient drivers in a reasonable amount of time compared to spending less on a less efficient driver up front.

Individual control of eight LEDs means you have to have 8 drivers. If you could fudge that and run 4 channels of 2 LEDs each your driver cost and overall efficiency would both improve. 20w is a pretty "small" batch of LEDs in terms of improving efficiency or driver cost. Larger batches are generally better for both metrics regardless of the specific driver you pick.
 
Well thanks for the links but I need at least 2 amp load. I would feel better having something rated at least 5 amps.
After doing a little more Googling I run across this http://www.instructables.com/id/Cir...tep8/a-little-micro-makes-all-the-difference/
I'm going to be using arduino Mega for pwm signal. I built the circuit #5 on that page it seems to work fine I'm using a 555 timer pwm signal to test with. Q1 I'm using 2n222a
And Q2 I'm using irf510 power mosfet R3 is a 1 ohm rite now .
http://www.reefcentral.com/forums/album.php?albumid=6881&pictureid=47435

Does this look like a better circuit? Thanks

Funky
 
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You should not run that array at 2A max current as it will shorten its life significantly. Very poor choice IMHO as you cannot control the color and the $/lumen is very high. You can get 6 Cree XT-E LEDs that will blow that array away. But I guess everyone has their preferences. Good luck!
 
There will be 8 total rite now and may 2 more depending on the color . 2- 14k chips 4- super actinic hybrid chips and 2- 420nm chips.
I'm looking for a clean and compact fixture not a 4 foot heat sink over my tank but thanks for the suggestion thou.

Funky
 
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