I am HOPING someone will be able to help me in here?
I am trying to power 4 x 1W LED's on four different pins using an external 3.5v supply, through a 2N5551 Transistor. I have trawled the net for information on what current limiting resistor I should use. Some say you don't need a resistor between the output pin and the Transistor Base ass there is no more than 400mA. But if the Arduino is powered via an external PSU, there is potentially 900mA.
If I understand correctly, this resistor is needed to limit current to the transistor to prevent damage. I see 470 ohm and 1K resistors recommended.
If I power the Arduino with a 3.5v PSU, the the LED's from the same source, then what value resistor would I need to protect the base?
Apologies if this is a STUPID question, but I'm learning as fast as I can.
Hi.
I'm afraid you got lost because you were told wrong information =(
Most Arduino boards run internally at 5V. You can not use 3.5V power supply to make an Arduino board run.
There are a few special Arduino boards that do run at 3.5V.
You can not use a 3.5V power supply for leds. I'm sorry, but that won't work. Or is it a special power supply for leds ?
The 1N5551 is ment to be used at a range of 10mA to 100mA.
When a led requires about 300mA, the gain of that transistor could be as low as 5. A gain of 5 is not enough, the Arduino output pin still has to supply too much current.
The BC337 is better for this, perhaps also the 2N2222 is possible, but the BC337 has a higher gain with high currents.
With the BC337 and a 5V Arduino board, a resistor of 470 ohm to 1k is indeed a good choice from Arduino output to base of transistor.
But instead of transistors, you could try a logic level mosfet.
An Arduino output pin can be used for about 20mA (it can do a little more, but 20mA is a nice number). That is enough for a common 3mm or 5mm led.
I have a 5v 2500mA PSU which I want to power the Arduino in the project and the LED's.
I was going to feed the main PCB with 5v from this PSU and also power the Arduino from there.
For the LED's, I would run a lead from from Digital Pin 3 (for example) , to a 470 ohm resistor, then to the Base of the transistor. The Collector will take a feed from the 5v rail on the main board and the Emitter to LED 1 via a 10 ohm resistor. This will be repeated a further 3 times to power 4 different LED's in total (each on a different pin and each on it's own transistor set-up as per the above) so I can stagger/control the LED's lighting up.
Thank you very much for your input guys - I really hope I have explained this well this time, I'm conscious of wasting peoples time on here.
You do not drive power LEDs like that.
You use a constant current drive. There are lots of designs but they all involve at least two transistors. I assume you want to dim them as well.
You use a constant current drive. There are lots of designs but they all involve at least two transistors. I assume you want to dim them as well.
I'm not wanting to dim them, just switch them on and off. The idea is for under lighting for my kitchen cupboards, when a PIR detects movement, the Arduino will switch on the LED's for 10 minutes, then switch them off again.
You use a constant current drive. There are lots of designs but they all involve at least two transistors
Are you saying the Transistors I've bought are not suitable and the 10 ohm resistors are not required?
jonny2014:
I'm not wanting to dim them, just switch them on and off. The idea is for under lighting for my kitchen cupboards, when a PIR detects movement, the Arduino will switch on the LED's for 10 minutes, then switch them off again.
The PIR module does that.
What do you want the Arduino to do?
{If the delay control on the PIR does not switch the output on for long enough - and I would be surprised if it did not - the thing you want is a CMOS 555.}
The PIR will identify movement, and a Light Sensor breakout board will identify room brightness. If it's suitably dark, and the PIR identifies movement, the Arduino will switch LED 1, LED 2, LED 3 and LED 4 on (500ms apart) in that order to give a staggered lighting effect.
The Arduino will then delay for 600,000 ms (10 mins) before setting reversing the effect on the LED's and setting the LED pin values to LOW, turning the lights off. Should the same criteria (above) me met, they will switch on again.
Actually, it sounds to me as if you do want to dim these progressively illuminated LEDs for best effect and using PWM with an Arduino is a good way of doing that. And you do not even need to use the PWM functions of the Arduino (Pro Mini would be most suitable), you can do it in software.
The circuit Mike cites - which I have been pointing out frequently with some embarrassment because it is a bit scruffy as diagrams go, not my work though - is what you need (with appropriate transistors - BC337 has been suggested) for the LEDs noted and is perfectly suited to being controlled for on/ off switching and PWM by an Arduino.
While they indicate for the LEDs an operating voltage of 3.5V, that does not mean to suggest in any way that you can operate them with a 3.5V supply, it means that when you supply them with the specified current of 350 mA each, then they will have approximately 3.5V across them; you need to provide significantly more than 3.5V in order to control them and in fact, the suggested current control circuit requires about an extra volt to operate so you do need 4.5 or in practice, 5V to power them.
Perhaps you should step back for a overall look at your goal of lighting your 1W LED and what that requires.
I could not find your LED but here is a link to what I think is a similar part 1 LED eBay
The specification table shows the forward voltage is in the range of 3.0 to 3.2 Volts when powered with 350 milliamps.
This means if you connected the LED directly to a variable supply with a ammeter in the circuit you would:
adjust the voltage so the LED was drawing 350ma. The LED will operate fine.
Now that is is powered, the LED will now start to heat up so the voltage across the LED gets lower. This causes the 350 ma to increase. So you turn the knob on the power supply down until the current is back at the rated 350 ma.
Your actions of controlling the voltage so the LED always has 350 ma is called "constant current control." It is what this LED requires when powering from low voltages.
If in the above scenario you accidentally increased the voltage much above 3.2V the LED will overheat and likely fail.
Conversely if you dropped the voltage much below 3.0 v the LED would get dimmer quickly.
This cannot be accomplished with one transistor.
Sorry but this electronic stuff is full of "details" that need to be addressed.
BTW:
Even with the required 350 ma these LED's get warm and must be connected to a heat sink of some sort else they will over heat. This does not have anything to do with the overheating I mentioned above but in normal operation this LED generates more heat that it can get rid of sitting in open air.
