Transistor Advice/Feasibility (Schematic Included)

Wanted to get everyone’s input before I began.

This project will use a electret microphone to read volume and a 3A 12V power source to power a system that will have around 60 LEDs, each drawing 20mA. I have the source powering both the Arduino Uno and the collector pin of each NPN transistor.

The base of each transistor will be powered by a different Arduino pin and will turn on according to the volume being read by the electret mic.

What transistors would be optimal for this design?

good ole 2n2222 will handle it, or a 2n4401

Any particular reason that you want to run the transistors emitter-follower?

It won't work. Because you have the load in the emitter then you can only get a maximum of 4.3V across the load, not the 12V from the supply.

Switch the circuit so the load is in the collector, in other words so that the transistor sinks the current.

The VF of the “60 LEDs” is not known.
If they’re just garden-variety LEDs, then it could “work”, but going the pass transistor route that way is kind of odd (inefficient.)

Grumpy_Mike: It won't work. Because you have the load in the emitter then you can only get a maximum of 4.3V across the load, not the 12V from the supply.

Switch the circuit so the load is in the collector, in other words so that the transistor sinks the current.

Not strictly true, it will work providing the resistor values which drive the LEDs are low enough....

If you insist on emitter follower, try a depletion mode MOSFET as a source follower : http://www.farnell.com/datasheets/46048.pdf

BSS139

Vt is -1 volt, so use power supply of 6 volts.

But better is : a common emitter 2n2222 as other have written.

Use transistors in the common emitter configuration as has already been said. 2N2222 is OK but BC337 is better (lower saturation voltage). Also, since you will be running the LEDs from a 12V supply, you should consider putting several LEDs in series in each string, with 1 series resistor per string. This will reduce not only the number of resistors you need but also the total current consumption.

Grumpy_Mike:
It won’t work.
Because you have the load in the emitter then you can only get a maximum of 4.3V across the load, not the 12V from the supply.

Switch the circuit so the load is in the collector, in other words so that the transistor sinks the current.

The LEDs pictured are 3Vf, so the emmiter would be able to drive the circuit

so why bother with a 12V supply, that transistor is just going to get warm.

bzuiss,

Here’s what I figure to be a better way (attached).
I’m just staying with your group of 6, in this example, but the circuit could be adapted to run many more.

doing6.JPG

You should really move them to the collector side of the transistors though, you're dissipating a fair amount of power...

Your power supply will thank you :)

Uln2803?

cjdelphi: Your power supply will thank you :)

Why would your power supply care if the power is dissipated in a transistor rather than in a resistor?

wasted heat = wasted energy right? (bigger demand on your power supply)

vs that of runaway pankake's solution, or is my assumption wrong?

wasted heat = wasted energy right?

yes

(bigger demand on your power supply)

no.

To get a specific current down the LED you can either change the resistor or the voltage driving it. The total power requirements from the supply is the same if the current is restricted by a resistor or a transistor / resistor combination.

Grumpy_Mike: so why bother with a 12V supply, that transistor is just going to get warm.

Based on your quote that's why... I was wondering how much dissipation was going on reducing the voltage down to 5v, but then I thought about it and yes it's going to be no different to the entire load being placed on the resistor instead.

But i then remember you saying the resistor getting warm, well if it does then so would the resistor..... (and normally resistors don't) hence the confused logic.

and normally resistors don't

Depends what you do with them. Resistors can run hotter than transistors with fewer reliability issues so if you have to dissipate heat it is a lot better doing it with a resistor than a semiconductor

[quote author=Runaway Pancake link=topic=181645.msg1347318#msg1347318 date=1375998214] bzuiss,

Here's what I figure to be a better way (attached). I'm just staying with your group of 6, in this example, but the circuit could be adapted to run many more. [/quote]

Thanks for the schematic Pancake, appreciate it.