Either saturation or cut-off
in [Embedded]
|
From: Tomás Ó hÉilidhe on 19 Apr 2008 07:57 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!
From: Martin Griffith on 19 Apr 2008 08:14 On Sat, 19 Apr 2008 04:57:23 -0700 (PDT), in comp.arch.embedded Tom�s � h�ilidhe <toe(a)lavabit.com> wrote: > >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! 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
From: Tim Wescott on 19 Apr 2008 11:24 On Sat, 19 Apr 2008 04:57:23 -0700, Tomás Ó hÉilidhe wrote: > 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! 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
From: Robert Adsett on 19 Apr 2008 12:39 In article <0dad419e-a5e3-40db-9edf-7df7984a406e@ 26g2000hsk.googlegroups.com>, Tom�s � h�ilidhe says... > > 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. <snip> > > If any of my reasoning is a bit wishy-washy then please point it out > to me! Or, alternately use a logic level FET and you don't need to worry about hfe Robert ** Posted from http://www.teranews.com **
|
Pages: 1 Prev: PCL over USB Next: Seiko Men's Diamond Le Grand Sport Watch #SGEA45 - Replica Watch Fake |