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Either saturation or cut-off

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!

Reply to
Tomás Ó hÉilidhe
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I generally use 10K, so that you always switch the transitor on hard

Get the book from

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covers all that

martin

Reply to
Martin Griffith

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
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
Reply to
Tim Wescott

Or, alternately use a logic level FET and you don't need to worry about hfe

Robert

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Reply to
Robert Adsett

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