hi,
This is a bit of a noobish problem but I've about half finished my first proper arduino based gadget and I want to make sure I don't burn anything out. I'm trying to drive an array of LEDs from an ATMEGA328P, 1 bank of white lights that can be used as a flashlight or emergency beacon, and 1 bank of IR transmitters for using the IRremote library. (It's a bit of a multitool).
The problem is I'm not sure how best to drive them, as each LED draws 100mA, making 300mA per bank. I was planning on using a BC517 transistor on each pin, as shown here:
This circuit seems incomplete, and I don't know if the BC517 is fast enough for IR communication. It seems like there would need to be a pull-down resistor between each base pin and ground, is that needed? What about a series resistor between the arduino pin and the base pin? Any help or advice would be much appreciated, I've been reading bits and pieces of theory but it just feels like I need to get a foothold on this.
It seems like there would need to be a pull-down resistor between each base pin and ground, is that needed?
Yes, 1kohm for example (for fast switching)
What about a series resistor between the arduino pin and the base pin?
Yes, 1kohm (for fast response, it is a darlington)
You must put a resistor into the collectors as well (in series with diodes).
BC517 is fast enough for normal ~38kHz IR toying. The collector max is 500mA, so it may work..
PS: Be aware the BC517 darlington has >1Volt collector-emitter saturation voltage - that means for you the fully saturated (on, fulll 300mA current) its power dissipation will be >0.3Watt.
Thanks everyone for the fast response. I take it then if I add a 1k resistor between the arduino and each base pin, a 1k resistor from each base pin to ground, and an appropriate resistor (selected by an LED resistor calculator) between each diode bank and it's corresponding collector pin, those resistors should control the current that passes through the LEDs and transistors? or is something else needed for current limiting?
Most of the resistors I have are 1/4 watt, I take it higher wattage resistors would be needed for the LEDs, but would 1/4 watt be sufficient for connecting to the base pins?
pito:
PS: Be aware the BC517 darlington has >1Volt collector-emitter saturation voltage - that means for you the fully saturated (on, fulll 300mA current) its power dissipation will be >0.3Watt.
Would that cause overheating problems? because the board is likely to be packaged in quite a tight space. I have some less powerfull LEDs that I might use. Anyway, thanks again.
caramellcube:
Thanks everyone for the fast response. I take it then if I add a 1k resistor between the arduino and each base pin, a 1k resistor from each base pin to ground, and an appropriate resistor (selected by an LED resistor calculator) between each diode bank and it's corresponding collector pin, those resistors should control the current that passes through the LEDs and transistors?
Correct.
caramellcube:
would 1/4 watt be sufficient for connecting to the base pins?
Yes, all you're doing is restricting the current that can come out of the Arduino pin to under 40mA. 1k achieves that while still allowing enough to turn on the transistor.
caramellcube:
pito:
PS: Be aware the BC517 darlington has >1Volt collector-emitter saturation voltage - that means for you the fully saturated (on, fulll 300mA current) its power dissipation will be >0.3Watt.
Would that cause overheating problems? because the board is likely to be packaged in quite a tight space.
The white LEDs might be better off with a MOSFET. A logic-level MOSFET with low Rds will hardly heat up at all (all the heat will be dissipated in the resistors).
The IR transmitter is probably better off with the BC517 - the on/off transitions will be faster/cleaner and usage will be infrequent enough to not worry about heat. (Is a darlington really necessary? A BC337 might be a better choice...)
I too think that a BC337 would be a better choice, with a base resistor of 220 or 330 ohms. The BC517 doesn't have any internal base-emitter resistors, so it might be slow to switch off.
