Transistors

"Uncle Steve"

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 "

All the graphs are worth study, as this is how most power transistors behave.

... Phil

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

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

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.

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:

do either this

| |/ 5V--[10K]--| |>| | | \ sw \ | ------------+----------

or this: sw | / |/ 5V--/ --[10K]--| |>| | | ------------+-------

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:

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]--| | | |>| `-|

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

Sorry, I was tired and really mangled the schematic. Fixed above.

Regards,

Uncle Steve

"Uncle Steve"

... Phil

No, this is actually it:

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

When I actually hook this up to a microcontroller the 5V will be the output from the pin designated for PWM, assuming I stick with that strategy. As I said earlier, the switch is there to simulate a microcontroller.

Not sure what the purpose of the second diode is there. Doesn't look like it can do much of anything.

It's hard to say. When I was using a motor as a load there were wierd things happening all over the place, but I was also making stupid mistakes and damaging small signal transistors in non-obvious ways.

Regards,

Uncle Steve

I've always wanted a Kewpie doll.

It's interesting that all of the literature and web-pages I've read so far have failed to impart an accurate sense of what these things do. I don't know whether that is because the learning curve is so steep, or whether the terminology is truly confusing to the uninitiated. I think it might be easier to understand electronics if circuit diagrams represented electron flow and electron charge potential more obviously. After all it's the electrons moving around that gets work done, correct?

Regards,

Uncle Steve

PNP collector must be more negative than its emitter... swap emitter and collector on the 2N2906.

But you're still in "fry" land :-(

WHAT is the OBJECT of your endeavor? Trickle charging a back-up battery? ...Jim Thompson

Nope, that is your main conceptual error.

...Jim Thompson

I wish I could run that fancy ASCII schematic CAD tool, but I don't use Windows.

I wouldn't characterize 3A as a trickle, but in essence I hope to end up with a battery charger that will charge the battery while it is under a moderate 20W+ load. I have bit-banged serial out on the microcontroller, so it will log status to the load device that way, and inform it when mains power fails.

It is a learning exercise, but I hope to end up with something vaguely resembling a UPS when all is said and done.

Regards,

Uncle Steve

Except in PNPs. ;)

Seriously, once current starts to be an intuitive concept for you, you'll rarely need to think about electrons at all.

Cheers

Phil Hobbs

Because the TIP147 is PNP and operates completely differently. Is the issue here that the 3055 is 'fully on' with 5 uA driving the

Perhaps my frustration with arrangements other than the Sziklai Pair is due to an irrational desire to decouple the 5v circuit from the 18V section as much as possible. Other than in a darlington arrangement, I am still having trouble making 2 2n2222 stages do what I want and the frustration of things not working as I think they should is really annoying.

But at pennies per transistor at least it's not expensive to fiddle.

Regards,

Uncle Steve