View Full Version : Uln2803a & pwm
08-10-2005, 05:45 AM
Quick question, if I were using a ULN2803A would I be able to perform PWM based dimming on lights run from the ULN2803A?
08-10-2005, 06:17 AM
Depends on how much current your lights draw and how many channels you are using at one time.· With a duty cycle of 10% the 2803 can handle about 350ma per channel.
Do you have a Stamp Tester yet?
08-10-2005, 06:27 AM
you should better buy the motor mind from parallax, which handles pwm perfectly. The BS2 can do PWM but it takes too much time.
Manuel C. Reinhard
08-10-2005, 06:24 PM
I want to use it to light individual LED's on a 4x4 matrix with different brightness levels, one at a time of course, the only way I se to do this is using pwm, is there another way to regulate/specify the brightness of a pin? I need to use the ULN becuase my led's draw 350ma each...
08-10-2005, 07:29 PM
What kind of LED draws 350ma??????????
Do you have a Stamp Tester yet?
08-10-2005, 11:00 PM
A 1 watt luxeon star :)
08-10-2005, 11:01 PM
Sid, it is likely a typo, but there are LEDs which require that much current. I have a couple Luxeon Star LEDs which take 350mA, they are 1 watt in power and will give you a blind spot if you look at them with the lights off (and mine are the side emitter lens, the standard and batwing lenses are even brighter), with the lights on they are just painful to look at.
<edit> well its not a typo since hes driving the same LED http://forums.parallax.com/images/smilies/smile.gif , I have done switched mode brightness control using the driver·circuit they sell, simply use a transistor to switch the supply going to the driver, not the output of the driver, because the driver is designed for constant current and can cause inductive like behaviour which isn't good for the Luxeon </edit>
Post Edited (Paul Baker) : 8/10/2005 3:07:33 PM GMT
08-10-2005, 11:09 PM
··· I doubt it's a typo....· I have on my bench right now, a 4 watt white LED module with integral heat sink, and it draws 720mA at 12 VDC.· WOW, does it put out a serious amount of light!
08-10-2005, 11:18 PM
BTW, they have expanded thier selection of drivers since I purchased mine. I have the micropuck, but the thing doesn't like to drive the LED at low brightness (it wasn't really designed for fast switching), if you want much more control get the boostpuck, since it was designed for such a purpose, but it requires a minimum of 7.5V whereas the micropuck can be run off of a couple batteries.
08-11-2005, 10:39 AM
As an experiment, I have connected a 1 watt Luxeon to a ULN2003 using a pot to control the base (as in the schematic on page 266 of WAM).· Instead of two 100k resistors in parallel to limit current into the base I used a single 10k resistor.· Vdd was regulated to 4.92V with a LM2940.· The Luxeon drew about 330mA at maximum.· The brightness was adjustable but not in a linear way.· There was a threshold where the brightness increased dramatically with a small turn of the pot that was difficult to control.· Up to that threshold it worked pretty well.·
I did not create a curve of base voltage vs brightness but it seems to me that it could be controlled with PWM.· Somewhere in the Luxeon literature it says that 1kHz is the·minimum frequency for proper PWM control.· I have not been able to find any other references on Google·for PWM·control of Luxeons.
I have also used the micropuck but I didn't try to control brightness.· I think they are too expensive so I haven't bothered.
Sid, how did you determine the 350mA at 10% duty cycle?
08-11-2005, 03:16 PM
If I have adequate current and voltage being fed to the ULN2003, why would I need the puck/driver? Perhaps the easiest way to control the brightness is to limit the voltage going to the LUXEON thus causing it to glow dimmer, is that possible using the ULN2003? Can I regulate the voltage per channel?
It's all connected using a simple 4x4 matrix, 0-3 on the anode, 4-7 on the cathode....so low 0 and high 3 would cause the 3rd led on the 1st column to light...any ideas would be greatly appreciated!
Thanks for the help!
08-11-2005, 03:20 PM
oh, and I don't care if the brightness increases/decreases in a non-linear/smooth fashion, I only want it to light at a certain brightness and then turn off, I don't plan on dimming it brighter or darker gradually (although the title of the post indicated that .... sorry :)