Help understanding a schematic of a model train sound generator

[Andreas B.]

Hi all,

As the title explains, I have a schematic of the elecronics in a model-train:

http://imageshack.dk/imagesfree/69e47200.png

It's split in three sections like the original (on paper).

At my level of education, I'm not supposed to understand all of it, but because it is included as preparations for a test, it would be nice to roughly understand what happens in the different sections.

* My best guess is that the "voltage regulator" is some sort of on/off for the sound generator, when the train has enough voltage from the tracks to start.
* Is the sound generator two oscillators? Then what does D2 do?
* I recognize the last part as a class A-amp connected to an AB, with feedback(?).

Thanks,
Andreas.

 

[Chris]

Hi all,

As the title explains, I have a schematic of the elecronics in a model-train:

http://imageshack.dk/imagesfree/69e47200.png

It's split in three sections like the original (on paper).

At my level of education, I'm not supposed to understand all of it, but because it is included as preparations for a test, it would be nice to roughly understand what happens in the different sections.

* My best guess is that the "voltage regulator" is some sort of on/off for the sound generator, when the train has enough voltage from the tracks to start.
* Is the sound generator two oscillators? Then what does D2 do?
* I recognize the last part as a class A-amp connected to an AB, with feedback(?).

Thanks,
Andreas.

Hi, Andreas. You're on track in understanding the circuit action.

The bridge rectifier is there to prevent things from being hooked up
bass-ackwards and smoking everything.

The first segment (voltage regulator) is a rather poorly conceived
method of trying to get the transistor Q1 to turn on (making the input
to IC1, pin 1 low) whenever the input voltage from the bridge rectifier
exceeds about 6.9V. Since this is a classroom, you should understand
that zener diodes don't have a perfect V-I curve -- there is some
current if the voltage across the zener is below I(zk). That will
result in fairly quick turn-on of Q1 in the real world, far below 6.9V.

The signal from "voltage regulator gates on/off what you correctly
perceived to be an oscillator, and its output gates on/off another
oscillator. D2 is there to bypass the second 330K resistor, making the
node charge-up of the 220nF cap through only R4, where charge-down is
through R4 and R5. This should give IC1A ouput (pin 3) a 2/3 duty
cycle. It's not a very complicated trick, but it does work.

You're also pretty much right about the last segment of the circuit.

I hope this was of help. Good luck on your test.

Chris

 

[Andreas B.]

The first segment (voltage regulator) is a rather poorly conceived
method of trying to get the transistor Q1 to turn on (making the input
to IC1, pin 1 low) whenever the input voltage from the bridge rectifier
exceeds about 6.9V. Since this is a classroom, you should understand
that zener diodes don't have a perfect V-I curve -- there is some
current if the voltage across the zener is below I(zk). That will
result in fairly quick turn-on of Q1 in the real world, far below 6.9V.

6.9V? is that because of the 6.2V over the zener (D1), and the other 0.7V as Ube / Vbe voltage drop in Q1?
What is the purpose of C1 and R2? C1 should get charged very quickly, right? If not, I would have thought it was there to allow quick changes/spikes in the track voltage to pass.. or something.

I hope this was of help. Good luck on your test.

It sure was! Thanks!!


Andreas B.

 

[Phil Allison]

"Andreas B."

6.9V? is that because of the 6.2V over the zener (D1), and the other 0.7V
as Ube / Vbe voltage drop in Q1?

** Yep - plus allow some drop across the 4.7 kohms.

What is the purpose of C1 and R2?

** The voltage on the rails could raw, rectified AC rather than smooth DC -
analysing the schematic is impossible without knowing important details like
this.

C2 likely stores charge for long enough to keep the voltage supply to the
zener approximately constant.

R2 discharges the C2 down to zero volts when the rail voltage is zero.

D2 affects the waveform of the first oscillator stage - makes it
asymmetrical in time.




........ Phil

 

[Chris]

"Andreas B."


** Yep - plus allow some drop across the 4.7 kohms.




** The voltage on the rails could raw, rectified AC rather than smooth DC -
analysing the schematic is impossible without knowing important details like
this.

C2 likely stores charge for long enough to keep the voltage supply to the
zener approximately constant.

R2 discharges the C2 down to zero volts when the rail voltage is zero.

D2 affects the waveform of the first oscillator stage - makes it
asymmetrical in time.




....... Phil

Thanks for the catch, Phil. You're right, of course -- many
controllers just provide raw, rectified AC, which would make this
circuit a non-functioning mess for other reasons, too.

I hope the OP got enough out of the circuit explanation to help him
with his test.

Thanks again for the spot
Chris

 

[Andreas B.]

** The voltage on the rails could raw, rectified AC rather than smooth DC -
analysing the schematic is impossible without knowing important details like
this.

Sorry, but I seem to have overlooked that the voltage on the track is 0-15V DC (another sheet).

C2 likely stores charge for long enough to keep the voltage supply to the
zener approximately constant.

Then C2 must have another purpose than smoothing the supply voltage for the zener(?)

R2 discharges the C2 down to zero volts when the rail voltage is zero.

Still applies I guess..

Thanks for having a look at it!

Andreas B.

 

[Bob]

Sorry, but I seem to have overlooked that the voltage on the track is
0-15V DC (another sheet).

I don't see the first section as being a voltage regulator. It's a
voltage-dependant switch. The threshold would be about 8.3V (two BR1diodes,
one Vbe, plus the zener).

The the first oscillator (should we call it a modulator?) will be gated off
whenever Q1 turns on. The second oscillator section will always run, but
will be gated (modulated) by the first oscillator as long as Q1 is off.

The sound should change when the rails hit about 8.3V. Have you built one of
these? Is this what happens?

Bob

 

[Andreas B.]

I don't see the first section as being a voltage regulator. It's a
voltage-dependant switch. The threshold would be about 8.3V (two BR1diodes,
one Vbe, plus the zener).

Agreed, a bit confusing at first.

The sound should change when the rails hit about 8.3V. Have you built one of
these? Is this what happens?

Nope, it's a schematic from the preparations to a test. But as far as I can understand (now), that is what will happen too.

Thanks Bob,

Andreas B.

 

[Phil Allison]

"Andreas B."

Sorry, but I seem to have overlooked that the voltage on the track is
0-15V DC (another sheet).

** That does not mean the voltage is pure DC.

It is standard practice to quote the AVERAGE value of rectified AC as a DC
voltage.

Model trains normally run from a raw, rectified AC or a pulse width
modulated DC supply - both of which work better than pure DC for
controlling the speed.




........ Phil