Transistors

So basically I'm approaching this problem backwards. I guess it's time to step back and re-read some of the tutorial materials I didn't understand the first time around.

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.

[snip]

Trying to zero in on the object of your endeavor... it sounds like you want a trickle charger, BUT, when a 20W load is added, you want this "regulator" to support that load, IF you have 18V available... is that your target function? ...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

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Well, it has to trickle charge the battery if it is fully charged, but otherwise must also support the load device if it is turned on, which it will be most of the time.

Stated more generally, the charger has so much capacity, 3A in this case, with the transformer I am currently using. So, the power available to charge the battery is whatever isn't being used by the load device. I realize I may not need PWM to control this thing after all. The idea I would came before I understood exactly how lead-acid batteries behave during the charge cycle. This is why I haven't bothered using the microcontroller in-circuit yet.

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.

--
In the data sheets for your parts, notice that the values for Vce(sat) 
and Vbe(sat) are given when Hfe = 10. 

What that means is that if you need about (say) 100mA of collector 
current for your load, then 10% of that (10mA) needs to be forced 
through the b-e junction in order to force the transistor into 
saturation. 

Looking at a typical circuit using an NPN with a beta of (say) 100 to 
300 at an Ic of about 100mA: 


+5>-+--------+ 
    |        | 
   [Rb]    [50R] 
    |        | 
    O-> |    | 
        |    C 
        O--B  NPN 
             E 
             | 
GND>---------+ 

Assuming a Vce(sat) of 0.3V, 

           Vs - Vce(sat)     5V - 0.3V  
     Ic = --------------- = ----------- = 0.094 amperes  
                Rl              50R 

Then, to force 10 percent of that through ther b-e junction we figure 
out the value of Rb like this: 


           Vs - Vbe(sat)        5V - 0.8V 
     Rb = --------------- = --------------- = 446.8 ohms 
             0.1 * Ic         0.1 * 0.094A 


420 ohms and 470 ohms are the closest 5% values and either would work 
perfectly well since the natural beta of the transistor is much 
greater than 10.

Naaaah! Lose the micro. Micros are only applied to analog functions by people who don't understand analog >:-}

What you need is a buck switcher charging you battery. Biggest problem (for you) is tracking the battery temperature to avoid overcharging.

I'm wondering if it might be possible to modify an off-the-shelf alternator regulator to control a buck switcher... I'll pursue the concept. ...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.

The biggest problem that I see in that drawing is that you've got the collector of a PNP connected to a supply voltage that's higher than it's emitter voltage. So the CB junction is forward biased and the BE junction is reverse biased. So the 2N2906 is going to work in reverse -- the collector will work (poorly) as an emitter, and the emitter will work (poorly) as a collector. The current gain will be around 1 or less.

The other biggest problem that I see is that you don't have nearly enough current-limiting resistors in there.

If you turn the 2N2906 around then when the 2N2222 pulls current from its base it'll deliver LOTS of current to the 2N3055 base. In fact, you'll generally get too much current all around -- the 2N2222 will pull current out of the 2N2906 that's limited only by the trickle of base current into the 2N2222 and the 2N2222's beta, and the 2N2906 will put current into the base of the 2N3055 that's limited only by it's base current and beta. Everything will be hugely device- and temperature dependent, so you'll constantly be fighting the thing not working or burning up.

You really want a resistor into the base of each transistor. Do that, and you can significantly reduce the 1M resistance to the 2N2222, speed up switching, and not burn out transistors.

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com

I expect it's rather difficult to build an analog circuit to send "powerok" at 38400bps to a serial port.

PCM still isn't out of the question. The way battery voltage rises, it might be feasible to use PCM if the input supply is variable, and have the thing to a proper job independent of the voltage/current capacity of the transformer. Who knows, perhaps next week a 36V-10A switching power supply will fall from the sky and I'll want to use it to charge the battery.

Mind you, I want to charge this battery fairly slowly to minimize hydrogen-gas production as it will live indoors. Pulse charging lead-acid may also be good to reduce sulfation of the plates, but I don't know yet how much.

I wonder if I can make an inductor out of lacquered motor-winding wire.

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.

In reality the PNP is connected the right way.

Well that's my thought too, but...

10k or 1M, the current into the base of the 3055 is never more than 38mA. Combined beta of the two small transistors is 180000, so Vb-e of the 3055 sort-of ought to be 90mA. If I used a TIP122 it probably would be, but then I'd really only need 3mA to turn it on. I suppose this mean I could use a 2.2M resistor on the base of the 2n2222.

I have five 2n3055 parts so it would probably be better to actually use them, not that they're all that expensive in the first place.

I dunno. I'd be happier if I could just burn out transistors a little more slowly so I could see it happening and stop it before they are completely destroyed.

I have a 2n2906 that works in-circuit, but doesn't register at all on my cheap transistor tester. The good ones show 120Hfe.

I guess it's just part of that "learn by destroying" thing.

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.

Not if you have it the way it's pictured!

The Vbe of the 3055 can't be 90mA, because Vbe denotes voltage.

Unless the 3055 is getting close to saturation you should be flowing plenty of base current out of that 2906, and get way more than 38mA into the base of the 3055.

So something's just not right.

Try connecting a 1K resistor to the base of the 2906 and manually switching it to ground. You should get tons-o-current into the base of the 3055 _assuming_ that there's enough voltage drop across the 2906 for it to work.

If that doesn't lead to joy, back up and try connecting the base of the

3055 to +18V with a 2.2 to 10 ohm resistor -- that should turn the 3055 on good and hard. If it doesn't, don't mess around with anything else until you get that figured out.

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com

Ignore Jim's ranting. He's just a superannuated old geezer. Everyone knows that analog circuits are only applied to analog functions by people who don't understand microprocessors.

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com

What picture?

Base-emitter current is what I meant.

I'll measure it again. 37mA with one 2n3055 and 39mA with another. I have a TIP122, and unlike the PNP TIP147, it shows .5mA when used as a drop-in replacement for the 2n3055. So it seems that the base current is being limited to what is required to saturate the power transistor

44mA. 10 ohms: 38mA

At this point the circuit is pretty minimal so it's difficult to say whether there could be a component defect. Ignoring the LED/2.2k pair on the output, that leaves a 10 ohm resister, the 2n3055, and a 5 ohm,

10 watt resistor for a load -- plus some wire.

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.

_your_ picture. The emitter of the 2N2906 should be connected to +18V, the collector should connect to the emitter of the 2N3055. You have the emitter and the collector swapped.

Hah. OK, 10 ohms implies 380mV. That, plus the VBE of the 2N3055 is a bit above 1V. You probably have something similar with the 2N2906, where the VCB of the 2N3055 drops, dropping the VCE of the 2N2906, which makes it go into saturation and reduces base current to the 2N3055 -- particularly if the 2N2906 is backwards.

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com

Yes, it's electrons moving around that gets the work done. But in the end it's usually easier to just think of current as a magic fluid that goes in the direction indicated.

I'm not sure what you mean by "accurate sense of what these things do". If you mean transistors, yes, the subject is both deep and wide. You can do most anything with one of two models, though:

Model 1: there's a fixed drop from base to emitter of 0.65V, the collector current equals beta * base current, and odd things happen in saturation.

Model 2: There's a diode from base to emitter, and a diode from base to collector. Both either point to or away from the base (that's the meaning of the arrow on the emitter lead). There's a current source in parallel with the collector. When current flows through the base-emitter diode, the current source in parallel with the collector flows almost the same amount of current (beta / (beta - 1), in fact, also known as alpha in the old literature).

With that model you not only predict beta, but you can predict a lot of behavior around saturation because that base-collector diode gets forward- biased in saturation and starts to flow its own current.

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com

--- Since the power transistor is wired as an emitter follower, you'll never be able to run it into saturation, but it'll probably be good enough for your application.

The best you'll be able to do is to get the emitter voltage equal to the supply voltage minus the sum of Vbe of the power transistor and Vce(sat) of the PNP.

If you like, try the circuit I posted for you; it just works. :-)

-- JF

The 2N2906 is orientated incorrectly. The emitter should be on the

18Volts and 2N3055 collector and of course the collector to the base of the 2N3055.

As for the base drive to the 2N2906 from the 2N2222 collector, you should be using a resistor, otherwise you'll let the blue smoke out.

Jamie

Sno-o-o-o-ort ;-) ...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.

hmm, your newsreader claims to be "nn" some sort of *nix prolly.

Assuming you're using X and something with an X86 processor install "wine" and "xchm" then having downloaded the installer do

wine swcadiii.exe

after the installer ends do

wine 'C:\Program\ Files/LTC/SwCADIII/scad3.exe'

to launch it.

Many off-the-shelf regulated chargers will do that reasonably well. Jim's the expert on lead-acid battery charging.

that bit is probably going to need to be custom, or overpriced.

--
?? 100% natural 

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but it doesn't even come close to simulating a microcontroller output.

it's part of the static protection circuitry typical of microcontrollers in your circuit it does nothing. It's there for completeness. if I left it out there would be four of five people telling me. These diodes are pretty wimpy, it's usualy best that they never conduct any current.

--
?? 100% natural 

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--
Wow, an echo claiming to be source?