Model Forum / General / Railroads / November 2008

On 11/9/2008 11:28 AM Ray Haddad spake thus:

Not helpful, "mate":

1. That's the second of two articles, the first having the circuit
description, schematics, etc.:
http://www.siliconchip.com.au/cms/A_110837/article.html

2. It's a pay site, so we can't see anything without paying for it,
which I'm not about to do.

What the *f.ck* do you mean by that, "mate"?

On 11/9/2008 12:55 PM Robert Heller spake thus:

Thank you; 'zactly what I was looking for. You da man.

On 11/9/2008 12:55 PM Robert Heller spake thus:

Further follow-up: while this looks like an extremely useful circuit, it
isn't really a PWM source; the description of the circuit says:

  Q4 is the output transistor, and is mounted on a sizable heat sink to
  dissipate the substantial heat generated due to not running Q4 in a PWM
  switching mode.

So it appears to be running in "pseudo-PWM" mode. It's a good
compromise, as the circuit is very simple and the only downside is some
heat dissipation in the output stage.

However, the output waveforms look like this (refer to the article for
better illustrations):

Full throttle:

    __      __      __      __      __
   /  \    /  \    /  \    /  \    /  \
  /    \  /    \  /    \  /    \  /    \
 /      \/      \/      \/      \/      \
 |      ||      ||      ||      ||      |
-------------------------------------------

Medium throttle:

    __      __      __      __      __
   /  \    /  \    /  \    /  \    /  \
  /    \  /    \  /    \  /    \  /    \
  |    |  |    |  |    |  |    |  |    |
-------------------------------------------

Low throttle:

    __      __      __      __      __
   /  \    /  \    /  \    /  \    /  \
   |  |    |  |    |  |    |  |    |  |
-------------------------------------------

where true PWM waveforms would look more like this (idealized, of
course, taking into account that actual waveforms wouldn't be absolutely
rectangular):

Full throttle:

   ______ ______ ______ ______ ______ ______
   |    | |    | |    | |    | |    | |    |
   |    | |    | |    | |    | |    | |    |
   |    | |    | |    | |    | |    | |    |
   |    | |    | |    | |    | |    | |    |
-----------------------------------------------

Medium throtle:

   ____   ____   ____   ____   ____   ____
   |  |   |  |   |  |   |  |   |  |   |  |
   |  |   |  |   |  |   |  |   |  |   |  |
   |  |   |  |   |  |   |  |   |  |   |  |
   |  |   |  |   |  |   |  |   |  |   |  |
-----------------------------------------------

Low throttle:

   __     __     __     __     __     __
   ||     ||     ||     ||     ||     ||
   ||     ||     ||     ||     ||     ||
   ||     ||     ||     ||     ||     ||
-----------------------------------------------

I realize that a true PWM circuit would probably be more complicated
than the Cooler Crawler, requiring different wave-shaping parts, perhaps
using triacs instead of transistors.

My questions to you are:

1. Would it be wothwhile to build a true PWM power supply as opposed to
the Cooler Crawler implementation? How would true PWM perform in the
real world?

2. If the answer to (1) is "yes", do you have any true PWM circuit
schematics?

On 11/10/2008 8:02 PM Jason Davies spake thus:

Thanks for that.

Now I'm tempted to build both versions and "race" them against each other.

On 11/10/2008 8:02 PM Jason Davies spake thus:

Just did that latter thing, with a little motor pulled from a VCR for a
test. (If you see a VCR tossed out on the street, pick it up: at least 3
small DC motors inside.)

Works nicely, very smooth speed changes, except for one thing: the motor
makes an annoying buzzing sound. Oddly, the frequency doesn't change
much until you rev it up pretty fast, so it must be the PWM frequency,
or a subharmonic. I'll try tweaking it later (the author suggest a
capacitor substitution to raise the frequency). Wouldn't want a singing
or growling loco now, would we?

Gosh, it sure was simpler years ago when all one had to do to get pulse
power was to cut and insert a simple on /off toggle switch on the metal
full wave rectifier cross rod or whatever it was called...:)

Hi,

[...]

In my university lecture, I remember being told that you still get some
heat. The argument went like this - the change itself is not
instantaneous but takes a (very short) time. During that time, the
switch (e.g. MOSFET) does heat up some. If you switch it on and off fast
enough, it'll get cooking hot - compare computer CPUs. Applied to model
railroad this isn't significant, but I'd still leave a ventilation
opening, especially in very small boxes. A few small holes will be
sufficient.

Let me ask a different question.

My layout is analog DC, with 15 power blocks. All blocks can be switched
(throttle 1 - off - throttle 2) on the "live" wire and have a common
return. Thus I can drive two trains from two throttles. This works fine
with the throttles I have running off two different transformators
(because of the common return).

What I'm looking for is a circuit for a throttle that does not use DPDT
switches to reverse polarity so I can drive two trains off one
transformator. I understand that I need one with 2x 12V secundary and
attach the throttle itself to +- 12V with the common rail running on 0V.
But the circuit itself is still a mystery to me, as I want a few "extra"
features:

- a push-button to reverse train direction (only if speed ~ 0)
- a linear slider for train speed (stop at one end of the slider)
- a "stop" button
- Momentum (i.e. train starts slowly and stops slowly)
- momentum and top speed configurable via plug mechanism (different
settings for each train according to the plug used, also to set a "low
speed plug" for children ;-)
- some kind of PWM would be fine but is not necessary.

On my own, I can do most of the parts, but I still have not worked out a
schematic to do "all at once".

For example, I figured I can use a stereo potentiometer (linear sliding
type) with the "lower" ends connected to each other and the upper ends
connected to +12V and -12V individually. A relais (ugh) would select
forward or reverse direction. Using a resistor and capacitor I can
simulate momentum and with an additional resistor between the poti and
the +-12V line I can limit top speed (all three resistors put on a plug
I get the individual train settings). So now I have a low-power control
signal, how do I block the direction button if the train is moving? My
high-school electronics is a bit rusty :-( And how do I connect the
low-power signal to the PWM-circuit? ;-)

I guess, such a throttle would be of interest to some analog-DC
railroaders? You get some of the digital features on non-digital layouts
(e.g. loco-adaption). And - at least in my case, my layout is too small
for a digital system to have any benefit whatsoever (I can not operate
more than two trains anyway and some of my locos may prove difficult to
convert).

Thanks for your input!