Hi,
I am trying to power 5*50 (250)LED. I don't need to have control over every single LED. I only need to fade in and out 5 chunks of 50 LED.
What do I need to power them on and be able to control them?
5 MOSFET?
How do I calculate how much Power I need to power them?
thanks
you need to know how much current each LED uses and multiply by 50 to get the power need for each group and the of course multiply by 5 to get the total.
As a rule of thumb, a standard LED use around 20 mA, but this may be much more for some type of LED's
How do I calculate how much Power I need to power them?
Power is simply current times voltage but I suspect you are not asking how much power you need but rather how much current you need.
MikMo's advice is good for parallel wired LEDs. However, you have an option of wiring the 5 LEDs in series, in that case all the current will flow through each diode and you don't have to multiply the total current by 5, but you will need a higher voltage to drive them.
It is not quite clear how many groups you have, is it 5 groups of 50 LEDs each or 50 groups of 5 LEDs?
While the current is the same that actual way you would control it is different. For 50 groups you will need some sort of hardware, for 5 groups you could conceivably use a FET for each group but that brings the problem of a large current.
At 20mA you have 1A of current to switch if they are in parallel (and 50 current limiting resistors)
I hope you are not trying to drive high power LEDs.
thanks for the replies.
@ Grumpy_Mike: I'm using 5 groups of 50 3mm LEDs.
I made a little sketch of my circuit. Would this work? What do I need to change?
Would this work?
No
What do I need to change?
Assuming the FETs are wired up right (I don't know the physical drain, source, gate pins)
You would need-
thanks again. I updated the sketch. Could you take a look at it?
5V 5A would be enough?
The capacitors should go across the power supply not in line with it.
That is the capacitors go from the common LED resistors to ground. One capacitor on each group.
You haven't told us what the voltage of your LEDs is. But let's assume it's about 2V or so.
What I would do is get a 12V regulated power supply (because they're relatively common and cheap), and wire each group of 50 LEDs as 10 strings of 5. Each string will need a current-limiting resistor, the value of which needs to be calculated based on the LED and power supply voltages. Then you wire the strings in parallel, and you have the equivalent of one giant 12V 200mA LED.
You'll need 5 drivers to run your 5 "megaLEDs". A simple and cheap way to do that is to use a ULN2003: a single chip will drive all of them. There should be examples in the Playground of how to wire one up. If not, a forum search will turn up postings from people who've used them to control all kinds of things.
The maximum current draw with all LEDs on will be just about 1 Amp. I'd look for a 1.5 or 2A power supply, because they work better and last longer when they're not running at their maximum rating. If you can't find a 1.5A (or greater) one cheap, settle for one that's good for 1.2A.
If your LEDs need more than 2V each, you can get a power supply with a higher voltage: there are lots of laptop supplies out there in the 15-20V range at good prices, and they're usually rated for more than 2A. You'll need one with an output that's about 5 x the LED voltage, plus about 2V because the ULN2003 outputs don't go all the way to ground when they're low. If you have, say, 3V LEDs and a hot deal on an 18V power supply, you can just increase the value of the current-limiting resistor a bit to dissipate the extra Volt as heat.
It's hard to get cheaper or easier than that. But be aware that running 5 200mA strings of LEDs is right at the recommended limits of the ULN2003. If you expect to do more LEDs, you should look into FETs.
Ran
5 200mA strings of LEDs is right at the recommended limits of the ULN2003.
5 * .2 = 1A
Seeing you can only switch about 600mA from this chip due to power dissipation factors I would say it is way over the limit.
http://www.thebox.myzen.co.uk/Tutorial/Power_Examples.html
Why am I beginning to think some teacher somewhere handed their students an Arduino and a bag of 100 LEDS and said "make something"?
Seeing you can only switch about 600mA from this chip due to power dissipation factors I would say it is way over the limit.
And you would be wrong.
Einstein once said "Everything should be made as simple as possible, but not simpler". Your simplifying assumptions about the ULN2003 make an okay first approximation for conservative design, but are no substitute for examining the specifics of particular cases.
If you look at the datasheet, the Vce(sat) at 200mA is around 1.1V, not 2V. Which means that, with 5 outputs on, the chip will be dissipating the very 1.1W (roughly) that you used to pick your 600mA limit. Which is why the plot of peak current vs duty cycle shows a limit of about 200mA each for 5 active outputs at 100% duty cycle.
If you feel the need to pick nits, you could make a positive contribution to the discussion by pointing out the relative wisdom (or lack thereof) of pushing the design limits, and the value of looking at the actual behavior of the system (in this case, the range of duty cycles of the LED strings) in determining how close to the edge you're running.
But presenting rough rules of thumb as though they were hard-and-fast laws is counterproductive.
Ran
thank you for all the great replies. the number of LEDs I use for this project changed a little. I am now using 35 for each group. I built a mock up version of 35 LEDs today using the LU2905 and it was working great.
Instead of using 15 V I will use 10V. the Leds need 2.0-2.5V.
here is the most recent circuit.
I will use 10V. the Leds need 2.0-2.5V.
If the spec says "2.0-2.5V", you probably go with a 12V supply, and 68 Ohm current limiting resistors. That'll give you about 11V at 15mA, which gives you good performance for the typical 2.2V LEDs, while keeping the current from going completely crazy if you happen to get some from a 2.0V batch. If you want to be safe, use 100 Ohm resistors, which will limit the current to 20mA worst case, but will cost you a little (but probably tolerable) amount of brightness for LEDs from batches with higher thresholds.
If you want to squeeze maximum brightness out of the LEDs, you can test them, and pick the resistors to match the batch you wind up with.
Ran