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Transistors

Started by Uncle Steve May 9, 2013
Ok, this is a question that is not so battery-charger specific, but
is obviously related.

So here's a circuit fragment:

18V  +----+--------__^------/\/\/-----------+
          |  2n3055|        0R5             |
          |        |         5W             /
          |        |                        \  5R10W
          |        |                        /
          |_______|>                        \
          |       |_ 2n2906                 |
          |        |                        |
          |        |                        |
          |        |                        |
          +------__^ 2n2222                 |
                  |                         |
                  /                         |
               1M \                         |
                  /                         |
                  \                         | 
                  |                         |
                  |                         |
                  \ SW                      |
                   \                        |
                  |                         |
5V    +-----------+                         |
                                            |
GND   +-----------+-------------------------+

So this circuit shows ~38mA going in to the base of the 2n3055, and
~13.9V across the 5 ohm resistor.  It gets warm quickly.

If I change the 2n3055 for a TIP147 (PNP, beta 1k) and make the
obvious changes to the 2n2906 to make it work, there is ~80mA current
at the base of the TIP147, and ~14.1V across the 10W resistor.

WTF, over?

Needless to say I am happy that my $14.00 DVM has a transistor tester,
because with 10k feeding the 2n2222 the 2n2906 doesn't last very long
with a TIP147 in-circuit.  Of course, in that configuration the base
of a 2n3055 still shows 38mA.  These results suggest that the
base-emitter current of the 2n3055 cannot go any higher without a
larger power supply, but that is supposition that the 2n3055 somehow
limits the base current to some proportionate ratio to the collector-
emitter current available to it.  Is that in any way a reasonable
explanation for what is occurring here?



Regards,

Uncle Steve

-- 
There should be a special word in the English language to identify
people who create problems and then turn around and offer up their own
tailor-made bogus non-solutions designed to completely avoid the root
causes of the situation under consideration.  'Traitor' might be a
good choice, but lacks the requisite specificity.  One of the problems
with contemporary English is it lacks many such words that would
otherwise categorically identify certain kinds of person, place, or
thing -- making it difficult or impossible to think analytically about
such objects.  These shortcomings of the English lexicon are
representative of Orwellian linguistics at work in the real world.

"Uncle Steve"

> Of course, in that configuration the base > of a 2n3055 still shows 38mA. These results suggest that the > base-emitter current of the 2n3055 cannot go any higher without a > larger power supply, but that is supposition that the 2n3055 somehow > limits the base current to some proportionate ratio to the collector- > emitter current available to it. Is that in any way a reasonable > explanation for what is occurring here? >
** Yep. The B-E current is related to the C-E current by a ratio called "beta" or "Hfe". Beta is not however a fixed number, it varies over the range of possible collector currents. For a 2N3055 over the range of 0.1 amps to 3 amps, the number is fairly constant at about 50 to 100 - depending on the particular device. The value falls at lower and higher currents PLUS when the C-E voltage is very low. See figure 3 " DC Current Gain, 2N3055 NPN " http://www.onsemi.com/pub_link/Collateral/2N3055-D.PDF All the graphs are worth study, as this is how most power transistors behave. ... Phil
On Thu, 09 May 2013 20:59:01 -0400, Uncle Steve <stevet810@gmail.com>
wrote:

>Ok, this is a question that is not so battery-charger specific, but >is obviously related. > >So here's a circuit fragment: > >18V +----+--------__^------/\/\/-----------+ > | 2n3055| 0R5 | > | | 5W / > | | \ 5R10W > | | / > |_______|> \ > | |_ 2n2906 | > | | | > | | | > | | | > +------__^ 2n2222 | > | | > / | > 1M \ | > / | > \ | > | | > | | > \ SW | > \ | > | | >5V +-----------+ | > | >GND +-----------+-------------------------+ > >So this circuit shows ~38mA going in to the base of the 2n3055, and >~13.9V across the 5 ohm resistor. It gets warm quickly. > >If I change the 2n3055 for a TIP147 (PNP, beta 1k) and make the >obvious changes to the 2n2906 to make it work, there is ~80mA current >at the base of the TIP147, and ~14.1V across the 10W resistor. > >WTF, over? > >Needless to say I am happy that my $14.00 DVM has a transistor tester, >because with 10k feeding the 2n2222 the 2n2906 doesn't last very long >with a TIP147 in-circuit. Of course, in that configuration the base >of a 2n3055 still shows 38mA. These results suggest that the >base-emitter current of the 2n3055 cannot go any higher without a >larger power supply, but that is supposition that the 2n3055 somehow >limits the base current to some proportionate ratio to the collector- >emitter current available to it. Is that in any way a reasonable >explanation for what is occurring here? > > > >Regards, > >Uncle Steve
Draw it up real purty, so you can tell PNP's from NPN's and can see that current can only flow OUT of the NPN emitter and INTO a PNP emitter... and DO THE MATH ;-) ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On Thu, 09 May 2013 20:59:01 -0400, Uncle Steve wrote:

> Ok, this is a question that is not so battery-charger specific, but is > obviously related. > > So here's a circuit fragment: > > 18V +----+--------__^------/\/\/-----------+ > | 2n3055| 0R5 | > | | 5W / > | | \ 5R10W > | | / > |_______|> \ > | |_ 2n2906 | > | | | > | | | > | | | > +------__^ 2n2222 | > | | > / | > 1M \ | > / | > \ | > | | > | | > \ SW | > \ | > | | > 5V +-----------+ | > | > GND +-----------+-------------------------+ > > So this circuit shows ~38mA going in to the base of the 2n3055, and > ~13.9V across the 5 ohm resistor. It gets warm quickly. > > If I change the 2n3055 for a TIP147 (PNP, beta 1k) and make the obvious > changes to the 2n2906 to make it work, there is ~80mA current at the > base of the TIP147, and ~14.1V across the 10W resistor. > > WTF, over? > > Needless to say I am happy that my $14.00 DVM has a transistor tester, > because with 10k feeding the 2n2222 the 2n2906 doesn't last very long > with a TIP147 in-circuit. Of course, in that configuration the base of > a 2n3055 still shows 38mA. These results suggest that the base-emitter > current of the 2n3055 cannot go any higher without a larger power > supply, but that is supposition that the 2n3055 somehow limits the base > current to some proportionate ratio to the collector- emitter current > available to it. Is that in any way a reasonable explanation for what > is occurring here?
OK. First, the drawing came through mangled. I hope I reconstituted it correctly. Second, the way you have it drawn, the 2N2222 isn't going to act like a transistor. If you _really_ did it as drawn, the BE junction of the 2N2222 will act like a zener diode at around 6V, because that's just what little transistors do when you challenge their BE junctions with too much voltage. Third, you've drawn the 2N2906 as an NPN, but my references says it's a PNP. Frankly, I don't see how this circuit can do _anything_, which leads me to believe that either it is not drawn as built, or there's some more zener breakdown or other oddball things happening. Like JT said: you've got to put current _into_ the base of an NPN, and pull current _out_ of the base of a PNP for them to act properly. You don't seem to be making that happen. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
On 2013-05-10, Uncle Steve <stevet810@gmail.com> wrote:
> Ok, this is a question that is not so battery-charger specific, but > is obviously related.
if you want to use a microcontroller you have to design the low side switch part so that current flows out of the 5V node, not into it. so dont do this:
> | | > | | > \ SW | > \ | > | | > 5V +-----------+ | > | > GND +-----------+-------------------------+
do either this | |/ 5V--[10K]--| |>| | | \ sw \ | ------------+---------- or this: sw | / |/ 5V--/ --[10K]--| |>| | | ------------+------- -- &#9858;&#9859; 100% natural
On 2013-05-10, Uncle Steve <stevet810@gmail.com> wrote:
> Ok, this is a question that is not so battery-charger specific, but > is obviously related.
If you want to use a microcontroller you have to design the low side switch part so that current flows out of the 5V node, not into it. so dont do this:
> | | > | | > \ SW | > \ | > | | > 5V +-----------+ | > | > GND +-----------+-------------------------+
do either this | |/ 5V--[10K]--| |>| | | \ sw \ | ------------+---------- or this: sw | / |/ 5V--/ --[10K]--| |>| | | ------------+------- if you wat to be sure put in a diode from the switch output to +5 , a microcontroller will have that internally. it also has a diode from ground. like this: | | |/ 5V--+--[10K]--| | |>| | | `----|<-----+ | .---->|-----+ | | | \ sw | \ | | +-----------+---------- or this: | | / |/ 5V-+-/ -+-[10K]--| | | |>| `-|<-+-|<-. | | | --------+-----+------- the followig has no chance of working, because once you put the protection diodes in it becomes obvious that the current from the high side switch components will just flow through the top diode | .--|<---+ | | | | | \ SW | | \ | | | | 5V +---+-------+---|<---. | | | GND +--------------------+----------------+ infact the oscillations and other wierd behavior you have seen may be due to your 5V supply being back fed. -- &#9858;&#9859; 100% natural --- news://freenews.netfront.net/ - complaints: news@netfront.net ---
On Fri, May 10, 2013 at 11:29:31AM +1000, Phil Allison wrote:
> > "Uncle Steve" > > > Of course, in that configuration the base > > of a 2n3055 still shows 38mA. These results suggest that the > > base-emitter current of the 2n3055 cannot go any higher without a > > larger power supply, but that is supposition that the 2n3055 somehow > > limits the base current to some proportionate ratio to the collector- > > emitter current available to it. Is that in any way a reasonable > > explanation for what is occurring here? > > > > ** Yep. > > The B-E current is related to the C-E current by a ratio called "beta" or > "Hfe". Beta is not however a fixed number, it varies over the range of > possible collector currents. > > For a 2N3055 over the range of 0.1 amps to 3 amps, the number is fairly > constant at about 50 to 100 - depending on the particular device. The value > falls at lower and higher currents PLUS when the C-E voltage is very low. > > See figure 3 " DC Current Gain, 2N3055 NPN " > > http://www.onsemi.com/pub_link/Collateral/2N3055-D.PDF > > All the graphs are worth study, as this is how most power transistors > behave.
I see the significance of these curves a little better now, and it's probably somewhat unreasonable to compare a BJT with a Darlington. The interesting thing is that the behaviour of the BJT seems to be more 'reasonable' in this application since it protects the previous stage. The next step for me is to figure out how to predict these factors to reduce the amount of fooling around I am still doing. Nevertheless, there is nothing like making measurements to show what is in fact going on with these devices. Regards, Uncle Steve -- There should be a special word in the English language to identify people who create problems and then turn around and offer up their own tailor-made bogus non-solutions designed to completely avoid the root causes of the situation under consideration. 'Traitor' might be a good choice, but lacks the requisite specificity. One of the problems with contemporary English is it lacks many such words that would otherwise categorically identify certain kinds of person, place, or thing -- making it difficult or impossible to think analytically about such objects. These shortcomings of the English lexicon are representative of Orwellian linguistics at work in the real world.
On Thu, May 09, 2013 at 07:11:43PM -0700, Jim Thompson wrote:
> On Thu, 09 May 2013 20:59:01 -0400, Uncle Steve <stevet810@gmail.com> > wrote: > > >Ok, this is a question that is not so battery-charger specific, but > >is obviously related. > > > >So here's a circuit fragment: > > > >18V +----+--------__^------/\/\/--------+ > > | 2n3055| 0R5 | > > | | 5W / > > | | \ 5R10W > > | | / > > | | \ > > +------__v 2n2906 | > > | | > > | | > > 2n2222 +---| | > > | |>----+ | > > | | | > > / | | > > 1M \ | | > > / | | > > \ | | > > | | | > > | | | > > \ SW | | > > \ | | > > | | | > >5V +-----+ | | > > | | > >GND +---------------+------------------+ > > > >So this circuit shows ~38mA going in to the base of the 2n3055, and > >~13.9V across the 5 ohm resistor. It gets warm quickly. > > > >If I change the 2n3055 for a TIP147 (PNP, beta 1k) and make the > >obvious changes to the 2n2906 to make it work, there is ~80mA current > >at the base of the TIP147, and ~14.1V across the 10W resistor. > > > >WTF, over? > > > >Needless to say I am happy that my $14.00 DVM has a transistor tester, > >because with 10k feeding the 2n2222 the 2n2906 doesn't last very long > >with a TIP147 in-circuit. Of course, in that configuration the base > >of a 2n3055 still shows 38mA. These results suggest that the > >base-emitter current of the 2n3055 cannot go any higher without a > >larger power supply, but that is supposition that the 2n3055 somehow > >limits the base current to some proportionate ratio to the collector- > >emitter current available to it. Is that in any way a reasonable > >explanation for what is occurring here? > > > Draw it up real purty, so you can tell PNP's from NPN's and can see > that current can only flow OUT of the NPN emitter and INTO a PNP > emitter... and DO THE MATH ;-)
Sorry, I was tired and really mangled the schematic. Fixed above. Regards, Uncle Steve -- There should be a special word in the English language to identify people who create problems and then turn around and offer up their own tailor-made bogus non-solutions designed to completely avoid the root causes of the situation under consideration. 'Traitor' might be a good choice, but lacks the requisite specificity. One of the problems with contemporary English is it lacks many such words that would otherwise categorically identify certain kinds of person, place, or thing -- making it difficult or impossible to think analytically about such objects. These shortcomings of the English lexicon are representative of Orwellian linguistics at work in the real world.
"Uncle Steve"
> > Nevertheless, > there is nothing like making measurements to show what is in fact > going on with these devices.
** Give this man a Kewpie doll. ... Phil
On Thu, May 09, 2013 at 10:57:58PM -0500, Tim Wescott wrote:
> > OK. First, the drawing came through mangled. I hope I reconstituted it > correctly.
No, this is actually it: 18V +----+--------__^------/\/\/--------+ | 2n3055| 0R5 | | | 5W / | | \ 5R10W | | / | | \ +------__v 2n2906 | | | | | 2n2222 +---| | | |>----+ | | | | / | | 1M \ | | / | | \ | | | | | | | | \ SW | | \ | | | | | 5V +-----+ | | | | GND +---------------+------------------+
> Second, the way you have it drawn, the 2N2222 isn't going to act like a > transistor. If you _really_ did it as drawn, the BE junction of the > 2N2222 will act like a zener diode at around 6V, because that's just what > little transistors do when you challenge their BE junctions with too much > voltage. > > Third, you've drawn the 2N2906 as an NPN, but my references says it's a > PNP. > > Frankly, I don't see how this circuit can do _anything_, which leads me > to believe that either it is not drawn as built, or there's some more > zener breakdown or other oddball things happening. > > Like JT said: you've got to put current _into_ the base of an NPN, and > pull current _out_ of the base of a PNP for them to act properly. You > don't seem to be making that happen.
My brain isn't trained to recognize the symbols and circuits with any fluency yet, so when I did the original drawing while tired, it didn't register. Apologies for the confusion. When I make silly errors like that when writing code it's also vastly amusing. Regards, Uncle Steve -- There should be a special word in the English language to identify people who create problems and then turn around and offer up their own tailor-made bogus non-solutions designed to completely avoid the root causes of the situation under consideration. 'Traitor' might be a good choice, but lacks the requisite specificity. One of the problems with contemporary English is it lacks many such words that would otherwise categorically identify certain kinds of person, place, or thing -- making it difficult or impossible to think analytically about such objects. These shortcomings of the English lexicon are representative of Orwellian linguistics at work in the real world.