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Author Topic: difference between a tip120 and mosfet  (Read 1012 times)
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hello,

i just want to strobe an array of around 360 little LEDs all hooked together. They turn on and strobe fine with a 12V, 5amp wall wort. I've been using a TIP120 to strobe them from my arduino. Is this okay? Should I be using a mosfet? What's the difference really?

thanks!
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That depends on the resistor you're using to the tip120.... is the tip120 a transistor? base collector emitter? vs a fet?

a fet has a high impedance, and only takes a small amount of voltage to switch on, where as a tip120 relies on current and without a resistor you could be damaging your pin out on the arduino. with a fet you'd need a pull down or the small capacitance keeps the led's on...
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TIP120 is two NPN transistors arranged so that 1 transistor drives the 2nd transistor.
http://www.fairchildsemi.com/ds/TI/TIP122.pdf
NPNs need current to turn them on - 1st transistor is used to drive higher current into the 2nd.
Voltage drop across the 2nd transistor collector-emitter is pretty high, 2 to 4V depending on current.
N-channel MOSFET is driven by voltage level instead, needs very little current.
Voltage drop across drain-source is dependent on the Rds of the device and the current flow.
Rds can be very low, like 0.030 ohm, so for 360 LEDs, arranged in 90 parallel strings of 4 LEDs in series, drawing 20mA/string, so 90 * 0.02 = 1.8A,
the voltage drop across the MOSFET would be 1.8A * 0.03ohn = 0.054V
http://aosmd.com/res/data_sheets/AOI516.pdf
This might allow strings of 5 LEDs from 12V even, reducing your current draw even more - 360/5 = 72 * 0.02 = 1.44A.

Power dissipated = Current * Voltage
So for TIP120, 1.8A * 2V = 3.6W, gonna need heatsinking.
For a MOSFET, 1.8 * 0.054 = 0.09W,  heat sink likely not needed.

MOSFET, want to  switch voltage levels quickly, so go from high resistance (off) state to very low resistance (on) state quickly, and not burn up the part going thru the medium resistance region slowly and dissipating more power.
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Oh gosh perfect! Thanks so much! That really helps! The tip120 was getting hot! I'll get a MOSFET
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i use tip31 transistors with a small heatsink and pwm out to control the brightness (analogWrite) of my high power LED's...
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Vce Saturation (Max) 1.2V @ Ib = 375mA, Ic = 3A

1.2V, could mean the difference between strings of 4 and strings of 5 with 12V source.
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Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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