Make a DC Controller: Part 1
Entry posted by Platform 1 in Electrics
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Circuit Design
As described in "A Test Track in N - 4: electrics" I started with an emf-sensing feedback design by Jonathan Scott (www.scottpages.net/UniversalTrainController.html second circuit from top). It was intended that the controller would be powered from AC, DC or battery to enable my N test track to be used 'anytime anywhere'.
Construction
I made a few small changes to Mr Scott's original circuit:
- C3 to 3uF to reduce inertia on a small layout
- add 1000uF reservoir/smoothing capacitor across the supply rails (I'm never happy running op-amps from raw unsmoothed DC even if they will work that way)
- use a 3-pole direction switch with the 3rd pole switching a red LED to warn of reverse before the knob is turned.
Having gathered all the parts together, the big question was, "Will it all fit in there?".
As this is only a small, one-off circuit, I decided against making a printed circuit board. So a piece of copper SRBP stripboard (aka Veroboard) was cut to fit in one of the slots helpfully provided by the box manufacturer. Then components were placed around the op-amp IC according to the circuit diagram - a few wire links were required to hop between strips. Everything fitted except for the bridge rectifier which was bolted to the box base. Very careful measurement was necessary to ensure that no components would foul the speed and direction controls mounted on the lid of the box.
Then it was a case of cabling the various bits together: rectifier to board, board to switch, speed control and LEDs, switch to output. All wiring was checked twice before proceeding...
Testing
A 12v battery was connected, quick voltmeter check on the output, place a loco on my test plank, and... movement! But only a crawl, even with the knob up to maximum. Luckily I have access to an oscilloscope - this revealed several interesting things that I probably should have realised 
before testing:
- the output waveform never achieves better than a 50:50 mark:space ratio
- hardly any back-emf in the 'off' pulses
- the op-amp peak output is ~1.2v less than the supply rail (data sheet confirms it!)
- voltage drops occur across the bridge rectifier (1.4v), D3 and Q1 (1.3v)
Together, this meant the loco only ever received 8v peak and 4v average - no wonder it moved slowly! So I tried a 15v power pack instead of the battery. Things improved - top speed was faster, with great 'creep' control too, but it just didn't feel right.
A trawl on t'internet revealed feedback controllers tend not to to work that well with many small motors such as those used in some N gauge locos. Indeed, some tend to get very hot and in a few cases burning out. This wasn't looking good... 
In a flash of inspiration, I tried a 00 loco on my test plank (it has two tracks). Wonder of wonders - it ran great! Superb creep control, smooth acceleration, reasonable top speed. So this box works after all. Curious, I checked waveforms with the 'scope - sharp 12v pulses, plenty of back-emf in between, and generally 'alive'.
Conclusion
So yes, a result, but not quite as intended. After considerable pondering, I've decided to keep this controller box for other uses, but make a simpler no-frills DC one for my N test track. Reasoning is two-fold:
1) it doesn't meet my original requirements for multiple power sources (12v battery in particular), and
2) it works really well with 00 locos so it'd be a shame to waste it.
The next exciting instalment - part 2 - will cover the simpler DC Controller.
Postscript
In case it may be helpful to someone, here's what a typical waveform looks like (it varies with loco - I guess every motor is different):
Having done further tests, some 'DCC Ready' locos perform less well, with jerkiness at low speed and flickery headlights. But my old Lima pancake motors seem to perform quite well, though sounding raspy. Interesting!
Edited by Platform 1
Restore pics
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