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At last it's complete (apart possibly for adding buffers)!
Decided on the MERG kits in the end as I was able to buy these when attending the Missenden weekend.
Adding meters to the set-up was delayed by my ordering one incorrectly and having to wait for a new one: all mounted in a box with the following set up:
- input either 12V DC, or 16V AC and MERG rectifier (DPDT switch)
- voltage regulator (MERG);
- voltmeter (0-15V);
- ammeter (0-500mA) range seem OK at the moment but I could swap it if needed;
- DPDT switch with centre off for direction control;
- DPDT switch to feed either the rolling road rollers or to the tracks and an external output for layout testing;
- DPDT switch to choose between feeding rolling road & tracks from the internal controller or from an external one (eg DCC)
- external output and input for an external controller
The DCC Concepts rollers can, of course, be placed on the tracks as intended and pick up their power that way but having seen a better test system at an exhibition I decided to power each roller separately. I tapped each roller unit 10BA and attached tags and wired them to plugs that fit the back of the test track. Half the units have the wiring on one side, and half on the other, so that it is still possible to place the units as close together as intended by the design. I tapped the units after assembling them, but it would have been better to do them first.
A small piece of 10thou plastic attached to the bottom of the roller carriers with bluetack insulates the rollers from the track and also lifts them clear of the chairs - as has been reported elsewhere, I found the spacer unit is a little deep and hits the chairs of (my) bullhead track. I can now switch individual roller sets on and off which I think will be a great help in identifying any pick-ups that are not functioning well. My initial set up is for 4 axles but there's room in the box, and the basic wiring, to add more (up to a 4-6-0 with 6 wheel tender).
So far I've only tested one P4 loco which I felt ran acceptably. On the rollers two things became apparent: firstly when running forwards one set of rollers rocked back an forth so I guessed at a problem with a wheel being not really concentric, however this did not happen in reverse. (Such behaviour was not visible when the loco runs on track.) Close examination now suggests that the cause is a bit of slop in the horn guides so I've learnt that I'll need to take more care with my next build. The second discovery was that the ammeter needle did flicker, sometimes at random, but also at times regularly in time with the wheel revolution - not the expected indication of something sticking, but poor current collection: the current dropped at times during each revolution. (The loco had not run for 2-3 years and was not cleaned before testing).
One surprise I had was to discover a small N gauge loco drawing very nearly 500mA whereas my P4 one takes around 120mA (at full power on the test unit).
I expect to make much use of this set up, so very many thanks for all the suggestions made.