Re: MRC Ampack upgrade: Attn electronics froods

dgw wrote:

"Greg Procter" <procter@xxxxxxxxxx> wrote in message
news:4707D173.AECFB884@xxxxxxxxxxxxx
pawlowsk002@xxxxxxxxxx wrote:

Greg Procter wrote stuff about diodes and capacitors.

David Starr wrote:
That circuit will work, and the emitter follower TIP120 will maintain a
constant voltage at the emitter, no matter how much juice the motor
draws. The theory of emitter followers is simple. Given a constant
base voltage, the emitter voltage will be 0.7 volts (one diode drop)
less than that base. The transistor will flow whatever current is
necessary down from the collector to keep the emitter voltage one diode
drop below the base voltage. Given a constant voltage out of the
emitter, the motor will maintain a constant speed even if the
mechanical
load varies from sticky side rods, wheels out of round, grades,
whatever. It's a good improvement and will make loco's run smoother
than a plain rheostat will.
If you have 1 amp going thru the transistor, base current might be
as
bad as 2% of that, I.e. a gain of 50. Large signal gain of power
transistors is lower than the small signal gain. To stabilize the
circuit you want current going thru the pot to be large compared to the
base current, so that the base current draw doesn't alter the voltage
at
the pot wiper. I'd make the pot more like 500 ohms than 5K, that's
only
a 1/4 watt of power in the pot, a 1/2 watt pot will run cool enough and
the standing current in the pot will be 24 mA, large compared to the
estimated 2 mA base current. Then I'd make the base resistor about 100
ohms. The only purpose of the base resistor is to surpress oscillation
in the emitter follower, and 100 ohms is enough to do that job. With
only 100 ohms, the 2 mA base current will only drop 0.2 volts across
the
base resistor. With a 5K base resistor, 2 mA base current drops 10
volts, nearly your entire supply voltage, across the base resistor.
Build this circuit and get it running before getting into momentum
throttles. A momentum throttle needs a lot more gain then you get with
a single transistor. The fundamental theory of a momentum throttle is
a
large capacitor which changes of throttle setting charge or discharge
slowly. For this to work, the power amplifier cannot draw much current
from the capacitor, which means a high gain amplifier, either several
transistor stages or an opamp. It's a more complex circuit which has a
lot more ways of going wrong. I'd recommend the fairly straight
forward
circuit you have for a first try. Do the fancier stuff after you get
something simple working.

GP1:

The diode idea seems good. I presume it can be fairly small, since it
doesn't handle full output current?

A signal diode will do, or an IN400x which is the cheapest I know of.

Gerald and Greg - Greg, you talked about adding a diode in series with the
output IF the voltage doesn't go all the way to zero when the pot is turned
all the way down. I agree with you.....the diode would fix or reduce a
problem like that.

However, the voltage will go all the way down when the pot is turned all the
way down. In fact, the voltage will go to zero when the wiper of the pot
gets down to about 1.4 volts, so, there is no need for a diode in series
with the output.

It's a good theory, and is almost certainly true with straight
emitter/follower circuits but can go wrong once you start adding
momentum etc.

If you use one to stop some phantom reverse voltage coming
from the track, the diode must be speced to handle the full current plus
some. a 1N400X is not beefy enough.

Sure, but I've been doing it for ... umm ... forty years and never had
that diode fail.

Also, if you add a diode on the
output, the output voltage will now drop to zero when the pot wiper gets
down to 2.1 volts. So if you want the output voltage to get to zero when
the wiper voltage gets to the end of its travel, you will need to add three
diodes in series between the bottom of the pot and ground, instead of the
two I suggested in a precious post.

Fair enough.
I've often played with series and parallel resistors and diodes to give
the pot non-lineal characteristics.
Giving the 0-6 volt range to about 60-65% of the pot arc and 6-12 volts
40-35% arc works for me.



I will also consider the RFI
filter.
I suppose the disc cap should be small, so as to shunt high
frequencies
without smoothing off my pulses too much...this is going to take some
figuring out.

Just solder a little one in from the junk box.
:-)


DS:

I may have drawn my sketch in a somewhat misleading way, because
Q1 isn't a single transistor, but a Darlingon arrangement, so I
probably
should have used some sort of IC symbol.

I knew what you meant. the normal Darlington represertation has two
interconnected internal transistor bits inside the circle.

I looked up the specs to
get the gain of 1000, which is at 3v collector-emitter and 3A current,
but I'm going to test the circuit first. I think I'll hook up some
wirewound
resistors across the output and check voltage under load, and then
perhaps try it with a motor, and if the voltage seems to drop too much
I'll reduce the resistor values and try again.

One thing I'm confused about - isn't the base resistor also there to
limit base current? 100 ohms would do that, of course. I used the
large resistor because

The base resistor is there to limit base current.
Advance the pot to full voltage without the base resistor in circuit and
a short circuit load and there's full voltage (= infinite amps) between
base and emitter, not a good look!
Ditto if you put the momentum capacitor in there without a base
resistor.


The circuit worked on the breadboard, with no load. This was a
very pleasant surprise.

Life's like that, it sets you up to expect success and then it runs you
over with a steam roller!
;-)


All the advice is very much welcome. At this point I know enough
to get into trouble. I'm trying to learn enough to get out before
proceeding farther in

The components aren't expensive, so as long as you don't set the house
on fire or rub your nose with the soldering iron it's all gain.

-- already, I'm doing better than last time I
tried something like this, which was an attempt to build Peter
Thorne's similar circuit in an old MR. It didn't work, but I couldn't
figure out why, so I'm trying to avoid blind copying this time.

Which one was that? I built most of them way back and learned enough to
figure out when it was my fault.
;-)


Greg.P.
.

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