I would like to light 32 powerful leds (10W) one after the other,
(Ideally they would be on for 40 milliseconds each, on a loop that would repeat 64 times).
I found different threads and websites talking about how to do it, but my knowledge in electronics is far from advanced and I prefer to ask for reviews before burning my components by mistake.
I bought a 10W power supply that was recommended on the website where I bought the 10w leds.
According to the website the led need 0,3A and approximately 33volts.
Problem is the power supply delivers 62 volts !
I guess it s a bad idea to let the led for too long on such high voltage.
I will use a UDN2981A to distribute the high voltage to the leds. (Apparently it can take up to 50 volts)
To lower the voltage entering the UDN2981A, I thought that I could add a led in series before it. Is this a wrong idea? When I connect two leds in series, I measure a voltage of 27,5V on each led.
Something I don t understand also is why adding some resistors between the two ULN2803 ?
Where to plug them ? Is what I draw correct ? How to calculate their resistance ?
Please post links to the specifications of the leds and the power supply.
I suspect your ideas of using max7219, udn2981 and uln2803 will not be suitable. Max7219 is a multiplexing led driver and probably won't let you achieve your 40ms flashes for that reason.
I would suggest using 4x 74hc595 driving 32 logic level n-channel MOSFETs. But need to see those specifications to be sure.
Another possibility could be 4x tpic6a595 chips. However, their limit of 350mA is only just higher than the 330mA current for your LEDs, which feels uncomfortable. But if it is ok, you would not need the individual MOSFETs, since they are built in to the chips. EDIT: Ah, these chips have a 50V maximum and your PSU is higher. On the other hand, the recommended PSU has a much higher voltage than your LEDs, so maybe the recommendation is based on connecting 2 LEDs in series. Need to see those data sheets!
PS. I moved your topic to a more suitable forum section. The section you posted in is for completed projects, not for help/questions.
The power supply is a constant-current type, which is appropriate for this type of led. However, it is not clear if the PSU will work with your 32-led arrangement. Each time the Arduino switches between the LEDs, this will cause a short interruption to the current. Those interruptions may prevent the PSU from working correctly. I am not saying that it will not work. I am saying that I cannot be sure that it will work.
I intend to record with a camera at 25 frames per second an object lightened with those 32 leds.
That is why I aimed 40 ms per light.
I could multiply that amount by any number if it allows the PSU to work properly. Would that help ?
If instead of 40 ms, I chose 200 ms, would that make a big difference ?
That PSU was designed for continuous operation, so interruptions at any rate might cause it to behave strangely. Only an engineer who designed it or is very familiar with it's internal circuit might know for sure. The data sheet might indicate how it would perform, but maybe not. Probably the only way to know will be to try it. If you choose to do that, be very careful. Monitor the PSU, LEDs, voltages and currents, ideally with an oscilloscope, and do not leave the circuit unattended during testing, in case something gets hot and could be in danger of causing a fire.
This is a constant current PSU, and I think the product page indicated that it did not support dimming. Perhaps a PSU which did support dimming given a PWM input signal would be more likely to work correctly, I'm not sure.
This will be tricky and it might require some "engineering" or you might need a constant-current driver for each LED. And that driver needs to switch "fast". ...Of course, 40ms is actually slow for electronics as long as the driver isn't "trying" to maintain continuous constant current when you're switching it on & off.
That's OK. Constant current power supplies work oppositely from a regular constant-voltage supply. With a regular power supply the current depends on the load. With a constant-current power supply the voltage depends on the load. With no load the power supply is trying to "push current" through and infinite resistance and that would require infinite voltage, so the voltage just goes as high as it can within the constraints of the circuit design.
And if the constant current supply is well designed you can short the output, the voltage falls to (nearly) zero and the current keeps happily flowing. And since it's putting-out nearly zero power, if it's well designed it will consume very little power while shorted!
[quote]I intend to record with a camera at 25 frames per second an object lightened with those 32 leds.[/quote]I assume it needs to be synchronized with the video? How are you going to do that?
I did a few tests and couldn t manage to make the led blink. I don t have an oscilloscope and I am not good enough in electronics to really understand what is going on.
So I will try the solution of PaulRB, using 4 * 74hc595 and 32 logic level n-channel MOSFETs.
Could you please tell me which MOSFETs to buy considering that the current is 300mA and the voltage around 35V ?
The arduino duemilanove can output 12 signals is that correct ?
Do I need any resistors in such circuit ?
For the wires to connect the leds I will use a 0,14 mm² cable, will that be enough ? (The max distance of the cable I need is 2 meters).
To answer the question about the synchronisation of 25imgs/sec, I will edit the video afterward and extract the image sequence I need. If 25fps is too fast and no synchronised, I will slow down the blinking of the led by a factor of 3 or 5. Then extract one image every 3 or 5 frames, depending how the data looks like.
I didn t think about placing the shift registers in series.
The board will be used only to trigger the 32 leds. So 4 registers connected to it will be good enough, hence 4 registers * 3 pins = 12 pins.
Could you please tell me which MOSFETs to buy considering that the current is 300mA and the voltage around 35V ?
The most important thing is to get a "logic level" MOSFET because most standard MOSFETs require more than 5V on the gate to turn them fully-on.
Beyond that almost any "power MOSFETs can easily handle 350mA and 33V so you can just choose something available and affordable. Don't cut the current rating too close... Give yourself plenty of safety margin.
...Because the constant-current power supply will spike-up (in voltage) with no load, I'd turn the next one on a millisecond or so before turning the 1st on off.
...And for the same reason, if there is a time when no LEDs are on you might want to have a "dummy load" (a resistor) with it's own channel.
Okay, so I finally received all the components. It took way longer than excpected
I now have four 74HC595 and 32 MOSFETs IRLZ44N
capacitor 100uF 50V
Here is the connection I am going to do.
Power supply is a constant-current type. The current is 300mA. Leds need 10W.
Will that work ? Can I connect the ground of the PSU with the ground of the arduino directly ?
Can I remove the bottom led noted "A" (I first thought using it to lower the voltage)
I am a bit scared of destroying the arduino so I prefer to ask for confirmation before doing it. Thank you.
You have the MOSFETs incorrectly wired, as shown they are switching the positive side of the supply, they need to switch the negative. What is the LED 'A' for? Seems a strange thing to place there. The LED PSUs are constant current, they will only be 60V (or whatever) when there is no load on them. When loaded they will adjust to the LED voltage. As the LED drivers are constant current they should be labelled as such, it makes no sense to label them '60V' when they are only 60V unloaded. If they are rated for 300mA then label them '300mA'.
You need one LED driver PSU per LED lamp, although if you can guarantee only 1 LED lamp will be on at any one time then probably 1 LED driver for all of them will be OK.
As others have commented LED constant current PSUs are not intended to be used this way, however I have successfully done so with PWM. I would consider such use to be experimental and might not work, might produce clouds of nasty smelling smoke, or might work perfectly.
