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Topic: Transistors and Resistors (Read 1 time) previous topic - next topic

Finnius

#10
Feb 16, 2013, 02:13 am Last Edit: Feb 16, 2013, 02:22 am by Finnius Reason: 1
I used this calculator to calculate resistor values for transistors:
http://kaizerpowerelectronics.dk/calculators/transistor-base-resistor-calculator/

I thought forward voltage was not operating voltage – just the volts that drops once it’s through the LED? I could run a 1.4.1.7v drop led on 5v no?

On another note I could use 50x 5mm LEDs like these:
Forward Voltage (V) : 1.2-1.4V.
Forward Current (mA): 100mA.

Once again they have 1.2-1.4 forward voltage – could I run them from 5v? If so would the Arduino have enough milliamps to run 50 of them in series and what resistor should I have? Looks to me like they would use 5A.
So i guess i am stuck with the 350-500ma star LED!

Any ideas to get it going - no one use those cheap LED drivers for something like this??


SirNickity

If all you're doing is switching, use a MOSFET and skip the math.  A 220ohm resistor between the pin and gate (analogous to a BJT's base) helps prevent goof-ups and over-current from capacitance, but with a FET, the value of that resistor is not critical at all.  Anything from 10R to 10k would probably work.

Henry_Best


I used this calculator to calculate resistor values for transistors:
http://kaizerpowerelectronics.dk/calculators/transistor-base-resistor-calculator/

I thought forward voltage was not operating voltage - just the volts that drops once it's through the LED? I could run a 1.4.1.7v drop led on 5v no?


With a series resistor to drop the voltage, yes.
Quote

On another note I could use 50x 5mm LEDs like these:
Forward Voltage (V) : 1.2-1.4V.
Forward Current (mA): 100mA.


50 in series would need 50 * 1.2V = 60V @100mA  (no resistors) to light up.
50 in parallel would need 50 * 100mA = 5Amps @ 1.2V to light up. You'd also need a resistor on each one so that it can be run from 5V.

Neither can be supplied from any Arduino directly. Each Arduino pin can only supply (max) 20mA @ 5V.

Quote


Once again they have 1.2-1.4 forward voltage - could I run them from 5v? If so would the Arduino have enough milliamps to run 50 of them in series and what resistor should I have? Looks to me like they would use 5A.


See above.

Finnius

#13
Feb 16, 2013, 06:45 am Last Edit: Feb 16, 2013, 09:12 am by Finnius Reason: 1
Thanks for the help guys.

I am sorted out for the 2n2222 and some 270ohm resistors that would only need 2.75mA to saturate from the arduino.

I still need to find a way to run the 1W LED. So i gather a MOSFET is a good way to do this? I have found the MOSFET IRF3205/IRF3710.
Datasheets (respectively):
http://www.taydaelectronics.com/datasheets/A-104.pdf
http://www.taydaelectronics.com/datasheets/A-105.pdf

The 3205 should be fine. Although can anyone tell me from the graphs in the datasheets how much current at 5v is required to saturate for a 350ma collector for the LED?

SirNickity said that a 220 ohm base resistor on a FET would be fine - is this for my 350/500ma LED?? Whats so good about a MOSFET - do they have low saturation values?

On a side note - i thought arduino could supply 40ma through the output pins not 20ma as Henry_Best said? And up to an Amp (although i wouldn't) out of the 5v pin?

Also what resistor should i have between the 5v arduino output and the led to stop it drawing more than 350ma? That is the real question for me!!!
My best idea at the moment is to run the LED using a 2W 10ohm resistor - will this limit it to somewhere between 350-500ma (recommended)?
But i still need a transistor to turn it off and on... :(

Finnius

So thanks to your guys help my latest solution is thus:

2n2222 transistors with 270ohm base resistors - requires 2.75ma to saturate at 220ma load

TIP120 transistors with 2500 ohm base resistors - requires 0.25ma to saturate at 500ma load

2W 10ohm resistor for the 1w LED at 5V <500ma running from the 5v arduino pin

Questions:

Will it work?

Is 0.25ma to saturate too low a value for the TIP120, it is only small cos of lots of gain?

Thanks so much for the help

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