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No - not the type of passenger train with a driving trailer!

 

I was thinking about amplifiers (otherwise known as class B or class AB) and train controllers, and wondered if anyone had ever put a push-pull output stage on a model railway controller.

 

My thinking was - if the output is symmetrical about ground to +/-12V, then it removes the need for the traditional DPDT polarity reversing switch between the power output and track, and makes common return easy with multiple controllers powered from the same source.

 

The slight downside- your power source has to provide smooth +12V and -12V (and a bit) rails instead of a single 12V rail.

 

To be honest, I've only seen them used in the past for audio and RF where the signal is approximately equally divided between +ve and -ve, and hence both output transistors see approximately the same power. Are there any problems when driving DC and one transistor will be driven continuously and dissipating power while the other is off? (drift possibly as they warm at different rates?)

 

 

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So 50Hz as the frequency, presumably mains derived. Then drive it all one way to produce a sort of DC, actually almost identical to rectified AC with no smoothing. But why ? Seems an awful lot of effort to achieve......what ?

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11 minutes ago, cliff park said:

to achieve......what ?


The same function as a very simple hardware switch.

 

Oh, and you might well need to have a hardware switch to select between poles of the amplifier output, or if not a software interface ...... there has to be something for the operator to prod, pull, swipe, poke, shout at etc to select polarity.

 

So to me, it would seem to add ‘component count’ to no useful purpose.

 

Unless I’m missing a trick somewhere.

 

 

Edited by Nearholmer

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The control knob would have a centre off like the Morley controllers.

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So, unless Morley are already using this, they do as the makers of ‘centre-off’ controllers have done since the dawn of time, which is to integrate the DPDT function with the control knob.

 

What additional functionality does the push-pull amp provide in this context?

 

(as an aside, a lot of people don’t like centre-off controllers)

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I wouldn't be the first to try to control motors like this... 

 

after a bit of googling I found:

 

https://www.depotbassam.com/2012/02/analog-dc-motor-driver-push-pull-power.html

 

although as a bench-top experiment rather than for moving model trains. 

 

H-drives are a kind of push-pull configuration but with PWM usually used with the transistors driven hard on or hard off to act as switches which minimises the power dissipated in each transistor. With a class B or AB amplifier the transistors are not driven into saturation, so are working as amplifiers rather than switches, which means that a lot of power can be dumped in the transistors (or for a DC controller, one transistor or the other depending on the polarity) - I suppose a hybrid might be possible a push-pull configuration driven hard with +ve or -ve pulses to create a bipolar PWM output without the same power losses in the transistors. 

 

The principle achievements would be:

a single control knob - reverse one way, forward the other (as per the Morley as David mentioned) 

a no-fuss common return without any need to isolate the supplies of each control channel to prevent shorts. 

 

Edit:

Andrew: Thanks for that example - I thought I most likely hadn't had an original idea! 

 

Nearholmer: as someone who grew up with H&M controllers I'm quite happy with a centre-off controller. 

 

Edited by sharris

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50 minutes ago, sharris said:

no-fuss common return without any need to isolate the supplies of each control channel to prevent shorts. 


For large/complex layouts that might constitute a ‘sell’.

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1 hour ago, sharris said:

I suppose a hybrid might be possible a push-pull configuration driven hard with +ve or -ve pulses to create a bipolar PWM output without the same power losses in the transistors. 

 

In audio land this would be a class D, followed by a low pass filter. DCC decoders, for example, use a high enough PWM frequency that the motor inductance forms the LPF and the motr effectively sees a DC voltage..

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3 hours ago, Nearholmer said:

So, unless Morley are already using this, they do as the makers of ‘centre-off’ controllers have done since the dawn of time, which is to integrate the DPDT function with the control knob.

 

What additional functionality does the push-pull amp provide in this context?

 

(as an aside, a lot of people don’t like centre-off controllers)

Morley don't integrate the DPDT with the control knob.   It is just a potentiometer with a knob on it and does not have an off position, only a centre click.  The Morley has a change over switch to change from the potentiometer on the controller to a DIN plug on the casing into which you can insert an extension lead to a second potentiometer on a hand held, or in my case to one of several hand helds. 

I Keep getting spell check telling me Potentiometer wrongly, how do you spell it?

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Potentiometer is correct, to add to dictionary right click on word and select 'add to dictionary', it will never fail again. Works well with frequently used names that keep getting flagged.

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15 hours ago, cliff park said:

Potentiometer is correct, to add to dictionary right click on word and select 'add to dictionary', it will never fail again.

 

I had a colleague who was always making spelling mistakes, "but the computer spell checks it" he protested. He than said he was dyslexic and had added his "correct" spellings to the dictionary :)

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About the late 60's or early 70's a magazine, Electronics Australia did a project for a train controller 

This controller used a 2N3055 & MJ2955 for the output transistors & a (+)12V,0V,(-)12V power supply

IIRC the circuit went something like this. The electronics to control the output transistors used a dozen or so parts (transistors resistors & capacitors)

There was overload protection & simulated inertia

Over the years I built upwards or 10 of these & they were in use until I completed the change to DCC(5-10yrs ago)

 

1429151222_ctcontroler.png.14d6db0144666fe44135300225c5b600.png

 

I've shown the pot in 2 positions & the voltmeters reflect the voltage at the 2 locations, The actual reading on the output voltmeter (on the lower drawing) may be a bit less than shown due to voltage drop across the transistor

 

John

 

 

 

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On 17/01/2020 at 04:40, sharris said:

No - not the type of passenger train with a driving trailer!

 

I was thinking about amplifiers (otherwise known as class B or class AB) and train controllers, and wondered if anyone had ever put a push-pull output stage on a model railway controller.

 

My thinking was - if the output is symmetrical about ground to +/-12V, then it removes the need for the traditional DPDT polarity reversing switch between the power output and track, and makes common return easy with multiple controllers powered from the same source.

 

The slight downside- your power source has to provide smooth +12V and -12V (and a bit) rails instead of a single 12V rail.

 

To be honest, I've only seen them used in the past for audio and RF where the signal is approximately equally divided between +ve and -ve, and hence both output transistors see approximately the same power. Are there any problems when driving DC and one transistor will be driven continuously and dissipating power while the other is off? (drift possibly as they warm at different rates?)

 

 

 

Yes you can. And because you have +/-12V you can use op-amps for all the "clever" bits like current limiting and fold-back. If you want to be even fancier you can even sense the motor current and compensate for the voltage drop in the motor's internal resistance to keep the motor running at constant speed regardless of the load.

 

I did make a controller like that a few* years ago but I need modernize the design a bit.

 

 

*Come to think of it that was before our first was born and she'll be 43 on Friday.

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