Some circuit help for 1w leds

Projects General Guidance
1

Hi guys

I need some help with the circuit below under step 8: Circuit 5

I am using these part:
IRL540N Mosfet Q2 - http://www.vishay.com/docs/91300/91300.pdf
2N2222A Transistor Q1 - ST Microelectronics 9288 datasheet pdf

I would like to run 2 of these 1W leds in series with a single cell lipo.
http://shop.rabtron.co.za/datasheets/WW-P05IR1W.pdf

I would also like to rapidly turn them on and off 1 microsecond on and 100 microsecond off.

Now what I noticed is that when I turn them on at 1us the leds dont turn on and there is no voltage between the positive and the drain of Q2 , if I turn them on for 25us and more the voltage is there.

So I changed R1 from a 100k resistor to a 1K resistor and then the leds turned on and the voltage was there when switching them on for 1us.

So what would be the best resistor to use for R1 ?

I know the resistor protects the Arduino but they also state

  • R1 has high resistance, so that when Q1 starts turning on, it easily overpowers R1.

Thanks
Mathys

pwm.jpg
pwm.jpg620×570 52.4 KB

2

What method are you using to switch a pin on and off again in the space of 1 microsecond ?

3 
int led = 11;

void setup(){  
  pinMode(led, OUTPUT); 
}

void loop() {
  digitalWrite(led, HIGH);
  delayMicroseconds(1);             
  digitalWrite(led, LOW);
  delayMicroseconds(100);  
}
4

The attached circuit, is that the circuit you use ? I want to be 100% sure of that.
What voltage is available for the leds ?
What is the value of R3 ?
Which Arduino board do you use ?

About 1us is fast for a slow mosfet. The input capacitance of your mosfet is 2.2nF. That is a lot !
You could add a "mosfet driver" with a combined transistor and fet output (a gate-charge-blaster-chip).
Or you could use a fast mosfet or a normal transistor for Q2.

The value of R1 can be as low as 150 ohm for an Arduino Uno.

I suggest to use two transistors. Use modern high gain and fast transistors. The 2N2222 was used by dinosaurs.

What is your project ? Why do you need it to turn on for 1us ? To make photos ?
There is another option : The peak current is 1A. Perhaps you could make a circuit without current limiter, and use that maximum peak current. Use a stable voltage and a resistor to keep the pulse below 1A. I would use about 700mA, that is double the continuous value and well below 1A.

The delayMicroseconds(1) is not accurate. I suggest to temporarily turn the interrupts off and use the special function to delay a number of clock cycles. But that depends on which Arduino board you have.

5

Yes I am using the attached circuit.
Voltage for the leds can be a 3.7 lipo or a 2cell lipo which is around 7.4v.

I am using an Uno now but want to move to a attiny later.

I got to switch it fast to get rid of sun interference, I can push the 1us to 5us but I would rather not

I was just thinking now switching at that speed makes the led look dim which is a problem, could i not switch it with higher current to get it brighter, how will that effect the life of the led?

I have also found a driver that has a High Switching Frequency up to 1MHz, will that not work?

Thanks..........

6

You have to change something, that mosfet with that circuit is slow.

Do you have a link to the high switching driver ? Those drivers have often special high-speed mosfet outputs, that will work.

The life-time of a led is often a big mystery. Only well known manufacturers give details that are accurate. That's why I wrote using short pulses of 700mA which is double the normal current and still well below the peak of 1A. I think that is safe.

For diodes it is easier. That is often documented, and high peak currents will not reduce the life-time.
For leds, the brightness could be 50% after 2000 hours at the normal rated current (they were cheap :wink: ). I have UV leds that are even worse.

Can you tell what your project is about ?

7

Link to the driver: FemtoBuck LED Driver - COM-13716 - SparkFun Electronics

Its a pity I dont have a scope because I cant see the amp draw when switching so fast with my multimeter.

Mine are also cheap haha, if I could get 1000 hours out of the leds that would be good.

Ive attached a pic of the thing I am trying to copy because I cant afford to bye it due to import cost and currency conversion etc and my project needs are a little different.

The camera picks up infrared light and gives x y position to my arduino but the sun interferes so you have to modulate your source to differentiate between the 2.

So for the short pulses idea:
I would probably need a voltage regulator to keep my voltage stable any suggestions?
Then a resistor for the led.
And a fast mosfet or transistor any suggestions?

leds.jpg

8

That is not a fast on/off switching thing for leds. It uses an inductance to create an efficient current source. For the inductance it has a switching frequency, and that can be up to 1MHz. The control line to turn the led on and off does not turn the led on and off at 1MHz. I took a look at the graphs of the datasheet, but I don't understand the PWM dimming.

Now I understand the 940nm: they are IR leds :o The forward voltage of 1.6V already gave it away.

I would use a transistor. Most normal mosfets have a gate capacitance that slows it down.
A BC338 is 800mA. I can't find a fast and common NPN transistor of 1A with high gain at the moment, sorry.

What about a single Li-ion or lipo cell of 3.6V (3.0 to 4.2V), and only one led per transistor. Add more circuits parallel for more leds.
The datasheet does not have a graph for the forward voltage with higher currents. I will use the 1.6V, but that is completely wrong.
4.2 - 1.6 = 2.6V, at 1A that requires a resistor of 2.6 ohm. Let's use 3.3 ohm.
With 3.3 ohm, the current could be maximum 780 mA for a full battery and 400 mA for an empty battery. Since that current is only a short pulse, the led should be okay.

9

Ok thanks man got it, going to give everything another try.

I am just going to have to figure out the correct forward voltage because it going to be more than 1.6 I think or I will just get leds with better datasheets :slight_smile: