Either saturation or cut-off

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I think I have a sound understanding of how to use a transistor to
"get more current" from a microcontroller pin, but I'd just like to
run my reasoning past you to see if any of it's flawed.

Let's say I have a micrcontroller pin that can supply 25 mA, but I
want to use it to power a lightbulb which will draw 250 mA.

I'll use a transistor as a "switch" to achieve this.
When the "switch" is on, I want there to be a short circuit from the
collector to the emitter.
When the "switch" is off, I want there to be a broken circuit from the
collector to the emitter.

I want to set the circuit up so that the transistor is always either
in cut-off mode or in saturation mode -- I don't want to be in the
"active region" at all.

* So I get a transistor, and I connect the microcontroller pin to a
resistor that goes to the base of the transistor.

* I connect the emitter of the transistor directly to ground.

* I connect the collector of the resistor to the lightbulb which goes
directly to Vcc.

Now I just need to pick the resistor value that goes into the base of
the transistor:

* I get the Beta of the transistor (which for the TIP121 device is
1000)
* I decide on a maximum current that will flow into the collector.
(I'll pick 260 mA in the case of my lightbulb).
* Now I divide the collector current by Beta to figure out what base
current I need to put the transistor in saturation. (260mA / 1000 ==
260uA)
* Now I consider the voltage applied by the micrcontroller pin, which
is 5 V. From this 5 V, I subtract the Vbe voltage drop of the
transistor. (Which for the TIP121 device is 1.4 volts).
* So now I know that I need a resistor that will allow at least 260
microamperes to flow when there's 3.6 volts applied to it.

R = V  /  I
   = 3.6 / 260 / 10e-6
   = 14 kilohms

So, am I right in thinking that the maximum value for my base resistor
is 14 kOhms, and that I'd be more than safe to use a 12k resistor?

If any of my reasoning is a bit wishy-washy then please point it out
to me!

Re: Either saturation or cut-off
On Sat, 19 Apr 2008 04:57:23 -0700 (PDT), in comp.arch.embedded Toms

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I generally use 10K, so that you always switch the transitor on hard

Get the book from
http://www.tte-systems.com/pttes.php

covers all that


martin

Re: Either saturation or cut-off

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Make sure that you are using the H_FE figure for the transistor in
saturation, over the extremes of temperature.  I usually go for somewhere
between two and ten times the apparently necessary base current, just to
make sure.

Note that a bipolar transistor requires base current, and that a
Darlington has a quite significant C-E drop.  You may find that a logic-
level MOSFET is easier to apply.

--
Tim Wescott
Control systems and communications consulting
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Re: Either saturation or cut-off
In article <0dad419e-a5e3-40db-9edf-7df7984a406e@
26g2000hsk.googlegroups.com>, Toms hilidhe says...
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<snip>
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Or, alternately use a logic level FET and you don't need to worry about
hfe

Robert


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