On Fri, May 10, 2013 at 01:27:21PM -0400, ehsjr wrote:
> On 5/8/2013 9:49 PM, Uncle Steve wrote:
> >On Wed, May 08, 2013 at 03:56:44PM -0700, Jim Thompson wrote:
> >>On Wed, 08 May 2013 17:55:36 -0400, Uncle Steve <stevet810@gmail.com>
> >>wrote:
> >>
> >>>On Wed, May 08, 2013 at 02:19:05PM -0700, Jim Thompson wrote:
> >>>>On Wed, 08 May 2013 10:36:21 -0400, Uncle Steve <stevet810@gmail.com>
> >>>>wrote:
> >>>>
> >>>>>On Wed, May 08, 2013 at 04:44:25PM +0800, Rheilly Phoull wrote:
> >>>>>>On 07/05/13 10:57, Jim Thompson wrote:
> >>>>[snip]
> >>>>>>>
> >>>>>>>How about something like this...
> >>>>>>>
> >>>>>>> http://www.analog-innovations.com/SED/ChargerForUncleSteve.pdf
> >>>>>>>
> >>>>>>> ...Jim Thompson
> >>>>>>>
> >>>>>>If Uncle is using a micro to control it, why not just control a mosfet
> >>>>>>with the PWM output ???
> >>>>>
> >>>>>I have the app notes for the LM338 and such, which show simple charger
> >>>>>circuits like that, but the fact of the matter is I have a couple of
> >>>>>3055s and this project is as much about learning some basics as it is
> >>>>>about charging a battery. For my requirements it is easier to use
> >>>>>what I have on hand than try to select among parts that I don't really
> >>>>>understand. If there were a fundamental reason why the 3055 is bad
> >>>>>for this application I might consider a mosfet, but I don't see why I
> >>>>>should bother.
> >>>>>
> >>>>>Regards,
> >>>>>
> >>>>>Uncle Steve
> >>>>
> >>>>The 3055 has a minimum beta of 20 at 4 Amps, which you spoke of at one
> >>>>point in this thread, which means the base current that must be
> >>>>supplied is 200mA... not a big deal EXCEPT the driving device will see
> >>>>considerable dissipation.
> >>>
> >>>I'm starting to appreciate these factors. Yet, after removing the
> >>>4004 diode as superfluous and changing the .5 ohm sense resistor to
> >>>.1 ohms, I'm seeing 3A output which is pretty much the capacity of the
> >>>power supply. The 3055 runs cooler as well; not sure what that means.
> >>>
> >>>Now I have to use an op-amp comparator to measure the current to get
> >>>decent resolution.
> >>>
> >>>>You ought to go buy the TO-3 version of the LM317 for the circuit I
> >>>>suggested and avoid the magic smoke ;-)
> >>>
> >>>The literature doesn't specify the switching frequency, rise time,
> >>>etc. for that part.
> >>>
> >>>
> >>>Regards,
> >>>
> >>>Uncle Steve
> >>
> >>I'm curious why you need to "switch" it? Isn't your only concern the
> >>_average_ current, as determined by the PWM?
> >>
> >>All you need is a low-pass R/C between the micro and the base of Q1
> >>(as noted in the text box on the schematic).
> >>
> >>I'll run a simulation of that and repost later (under this same link).
> >>
> >> http://www.analog-innovations.com/SED/ChargerForUncleSteve.pdf
> >
> >You're probably correct, except the rating of the LM317 is half of
> >what I need.
>
> I would urge you to follow JT's advice. Build his circuit.
>
> It is easy to add a pass element to get higher current later on.
> Here's an example diagram to give you an idea how it's done:
>
>
> MJ2955
> Vin ---+----> ------------------+-----------> Vout
> | e\ /c |
> [3R] --- |
> | | ----- |
> +------+---Vin|LM317|Vout---+-------+
> | ----- | |
> | Adj [240R] |
> | | | |
> | +----------+ |
> | | |
> [.22uF] / [.1uF]
> | \ |
> | 5K /<---+ |
> | \ | |
> | / | |
> | | | |
> Gnd ----------+---------+----+-------------+
Actually, I recalled seeing this circuit in the app notes, but only
after I replied.
> Current drawn from the 317 causes a voltage drop across the
> 3 ohm resistor. When that voltage drop is below about .6 volts,
> the 2955 conducts no e-c current. When the voltage drop rises
> to about .6 volts the 2955 begins to conduct e-c current. The
> more current drawn by the load, the greater the e-c current.
> The e-c current passes around (not through) the 317 so the 2955
> is called a pass element or pass transistor.
>
> You can also do it with your 3055, but you need an inverting
> stage to its base. See "High Current Adjustable Regulator"
> diagram in the datasheet for the 317 for an example of using
> an NPN pass element. They use 3 195's in that diagram, but
> you need only the single 3055.
That's the one.
> >It's an interesting idea to use PWM to control a voltage
> >regulator, but it's a completely different approach from what I
> >intended to build.
>
> In reading your various posts it seems clear to me that you are
> expending a lot of effort and have a lot of interest in this, which
> is all to the good. But I also see that you are speeding by some
> points where you could benefit by working to understand them,
> rather than passing them by - which is what prompted me to reply.
That's the understatement of the week. The problem is there are too
many points, here and in the various bits of literature I've read, to
study in any reasonable length of time.
> >Perhaps it will be worthwhile to refactor your
> >design and substitute a LM338, which has the requisite capacity.
>
> No perhaps about it, it is definitely worthwhile. The earlier
> part of my reply suggests how you can do it with parts on
> hand, then later on add a pass element, or, as you mention,
> substitute a 338.
>
> >For
> >now I think I will fool around with the existing arrangement.
> >
> >Even at 3A output, the 3055 doesn't appear to be saturated as Vce is
> >only .9V. I picked up a couple of BC517 which have a beta of 30000
> >and 1A current capacity. Perhaps that will be more effective than the
> >BC557 in the pseudo-Sziklai arrangement.
> >
> >
>
> Well, you may get it to work, but you won't learn why
> it does, or why the previous attempts didn't. It is
> my impression that you are leap frogging some of the
> basics. Sometimes throwing bigger/better/stronger parts
> at a problem is not in your best interests.
I'm more interested in using the different parts to see how they
affect the whole thing. I think I am at the stage where it is
becoming practical to do that sort of experimentation.
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.