Re: MRC Ampack upgrade: Attn electronics froods


<pawlowsk002@xxxxxxxxxx> wrote in message
news:1191617827.558621.255350@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Greg Procter wrote:
I'd put a ceramic capacitor across the voltage supply as close to the
transistor as possible and a reverse oriented diode across the output
(before the reversing switch. Also, if your output voltage doesn't go
fully to zero then a diode (or two) in series with the output.

Add a few more components and you can have a momentum controller.


GP1:

I assume the capacitor would be a filter, or am I wrong about that?

Ceramic capacitors have a low enough capaitance that it would have no
noticeable affect on the sinusoidal pulses, but it would greatly reduce
(filter) any high frequency components. I assume something in the 0.01 uf
to 0.1 uf range, voltage rating greater than the output measured in normal
mode X 1.414. Actually, I can't imaging any high frequency stuff coming
back from the layout, but it couldn't hurt, possibly to surpress any
oscillations from the high gain darlington. On the other hand, I wouldn't
make a special trip to buy one. It probably isn't going to be needed.

You could have used just a regular power transistor, with lower value R2 and
R3. It would have reduced the heat in the transistor from about 1 watt per
amp to about 0.3 watts per amp.

I have a power pack with pulse mode but I never looked at the output on a
scope. I thought they used some kind of pulse width modulation, where the
pulse amplitude was always at max voltage, and they went from sliver thin at
the almost zero setting and got wider and wider as the knob was turned up,
until they disappeared at the 100% full voltage point.

With your proposed circuit arrangement, you can run it in the normal full
wave rectifier mode with 120 sinusoidal pulses per second, or the half wave
rectifier 60 sinusoidal pulses per second mode, which I expect will result
in a slower top end loco velocity, but if the loco is running and you switch
from 60 PPS mode to 120 PPS normal mode, I predict there will be a
noticeable sudden increase in speed, if that's objectionable to you.

Also, since the transistor is electrically "hot", you'll need to use an
insulator under it, or somehow insulate the heat sink from ground. And
don't forget the silicone thermal grease under the darlington.

Let us know how it works!


I'm a little uncertain about filtering, because I don't want to smooth
out my output current too much. I know this can cause more heating,
but that's not something my motors and I have much trouble with.
Our problems are more along the lines of "Oh ***, the Tyco
Plymouth at the end of that smoke trail wasn't powered down, it
was stalled."

Or is the cap for RFI purposes?

What would the diode be for? I'm trying to use this as a learning
experience, as well as a practical controller.

The diode drop is a good plan. I may have to use that with my
KF pack, whose selenium cells are getting replaced with silicon
diodes, because I don't intend to run BLI K4s </snide>. It depends
what voltage drop was allowed for; I'll have to stick a meter on it.

The momentum idea is interesting. I have some old MR articles
which cover simple transistor throttle circuits (good old LHW
and his ability to put basic info, four practical circuits, AND
a plug for his True Action Throttle in three pages of dense
text and small drawings) and I may use it eventually once I
understand the basic principles involved better. I find that a
little experimentation helps me do that. Study a little, think
a little, try a little, repeat as necessary.

Cordially yours:
Gerard P.
President, a box of track and a gappy table.



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