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Topic: how to calculate resistor to a base of a 2n2222? (Read 11815 times) previous topic - next topic

cabecinhas

Hi, michael_x thank you for your answer!

I just did what you said and worked but the 2n222 started to get REALLY overheated.. I can't even touch it.

I was searching on internet in a while and found the Tip120 that I believe can handle easily 400mah. and is hfe 1000.

I made a simple design that I would like you to see if I'm understanding what you are telling me..

Could you take a look?


MarkT

Bear in mind darlingtons don't saturate, since the collector is held at about 0.8 to 1.5V
above the emitter due to the configuration.  So you'll probably get about 1/2 a watt of
heat from it.  At least it has a metal tab for a heatsink.

2N2222 isn't going to handle 400mA continuous comfortably without a lot of current into
the base, perhaps 60mA, to bring the Vsat down.  You could try driving it again with a
base resistor of 150 ohms, giving about 30mA base drive (the most that's safe for an
Arduino).  330 ohms is a bit too high.  It will still run hot, its a small package.
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

cabecinhas

Hi, Mark. What If i change and use a bd135 instead?

MarkT

Look at its datasheet for the saturation characteristics?  Is it better or worse?
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

polymorph

Please try not to put your response inside the quotes. It makes it difficult or impossible to figure out what you are saying.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

cabecinhas

hi. I think its better because the it handles 1,5A and do the job easily.

And the gain is about 25-250. What exactly does this gain mean? I can count on 25 or 250?

michael_x

BD135 has a bigger case, but you should have a rather high IB, too.
In the datasheet you see saturation at IC = 10* IB,
In Fig 2. with a IC = 500mA and IB = 25mA (only) there's a VCE of typical 0.2V -->  0.1W heat  
And there's the line VCE(sat)  @(IC = 500 mA, IB = 50 mA) with a max 0.5V ( 0.25W )

As for the TIP120, Rb = 9k is Max. I'd never calculate by hFE. ( Neither the 1000 here, nor the 25 ..250 for BD135 )
You don't want to use them as amplifiers but as switches.
( Rb = 1k @5V signal is more typical for a Darlington as switch )

Quote
I just did what you said and worked but the 2n222 started to get REALLY overheated

How hot does your 5 Ohm resistor get, BTW ?
Res   400mA * 5 Ohm = 2 V -> 0.8W
Led    400mA * 3V = 1.2 W
________________
Total  400mA * 5V = 2 W

polymorph

If it says the gain is 25-250, what do you think? Would you count on gain being 250 if it can be anywhere between 25 and 250?

For saturation, overdrive the transistor.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

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