Hornby Elite fails to switch peco point motors

Hi Fazer and welcome,

Most CDU's are designed to be supplied with an AC supply generally of a magnitude in order of 16 to 24 volts. This voltage is then rectified by an 'on board' rectifier before it charges the capacitor via the transistor. As the rectification of the AC supply is on board it doesn't matter one iota which way around the supply is connected as the rectifier will sort all that out for you. If you supply the CDU with a DC voltage however it will be necessary to make sure the supply is connected the right way around else the on board rectifier will stop the DC current from reaching the capacitor. The CDU may still 'acquire' some power from the DC supply as it's odds on that the DC is not pure and will contain some AC voltage ripple which the on board rectifier will allow to pass to the capacitor - hence the reduced power you are experiencing. Simple solution to prove if this is what's happening in your the case is to simply swap the wires from the DC supply (the Hornby unit) and try again. No damage will be suffered by the CDU if you get it wrong, it just means you points won't work very well.

Steve

QUOTE (Gofer @ 24 Apr 2007, 10:34) <{POST_SNAPBACK}>Hi Fazer and welcome,

Most CDU's are designed to be supplied with an AC supply generally of a magnitude in order of 16 to 24 volts. This voltage is then rectified by an 'on board' rectifier before it charges the capacitor via the transistor. As the rectification of the AC supply is on board it doesn't matter one iota which way around the supply is connected as the rectifier will sort all that out for you. If you supply the CDU with a DC voltage however it will be necessary to make sure the supply is connected the right way around else the on board rectifier will stop the DC current from reaching the capacitor. The CDU may still 'acquire' some power from the DC supply as it's odds on that the DC is not pure and will contain some AC voltage ripple which the on board rectifier will allow to pass to the capacitor - hence the reduced power you are experiencing. Simple solution to prove if this is what's happening in your the case is to simply swap the wires from the DC supply (the Hornby unit) and try again. No damage will be suffered by the CDU if you get it wrong, it just means you points won't work very well.

Steve
If it has an on-board bridge rectifier then the DC polarity doesn't matter. It will go one way or the other through the rectifier. Swapping the DC supply will make no difference whatsoever.

Since it's a CDU, the current capability of the supply isn't too important unless the CDU is trying to recharge the capacitor too quickly after firing.

The biggest difference is the voltage. The 16V AC from the gaugemaster, when rectified, will peak at over 20V DC, even allowing for voltage drop in the rectifier. The 16V DC from the Hornby controller will be 14 - 15V after the rectifier.

The solution is to stick with the gaugemaster controller or buy a dedicated AC or DC supply from somewhere like Rapid http://www.rapidonline.com/ or Maplin. It depends on the exact nature of the CDU so I would stick to an AC supply since that is proven to work.

Andrew

Wow - amazing responses - thanks to you all.

Just to get to the bottom of some of the replies I tried the following :-

1 - swapped the 16v ac output from the gaugemaster controller - points switched correctly irrespective of orientation of the wires.

2 - swapped the dc output from the elite - it attempted to switch the points in one orientation (but actually failed to provide enough power to switch most of them) and was completely silent in the other orientation.

It seems a waste to tie up a dual gaugemaster controller just for its ac supply and I want to use it on a different layout so I will look to purchase a stand alone ac supply just for the points. The CDU is a standard gaugemaster model (I'm afraid I don;t have any more specs than that without opening up the case) - would it be worth sticking with 16v supply or moving to 20v or 24v?

Thanks

Fazer

QUOTE (fazer @ 24 Apr 2007, 22:00) <{POST_SNAPBACK}>Just to get to the bottom of some of the replies I tried the following :-

1 - swapped the 16v ac output from the gaugemaster controller - points switched correctly irrespective of orientation of the wires.

2 - swapped the dc output from the elite - it attempted to switch the points in one orientation (but actually failed to provide enough power to switch most of them) and was completely silent in the other orientation.

It seems a waste to tie up a dual gaugemaster controller just for its ac supply and I want to use it on a different layout so I will look to purchase a stand alone ac supply just for the points. The CDU is a standard gaugemaster model (I'm afraid I don;t have any more specs than that without opening up the case) - would it be worth sticking with 16v supply or moving to 20v or 24v?

Thanks

Fazer

Fazer, something is wrong here. As previously discussed, no difference should be noticed when swapping the Gaugemaster wires (16vac) but the
results you report with the wire orientations coming from the Hornby Elite beg some questions. Firstly, and as expected, swapping the wires made the difference of the point motors either doing something or doing nothing. What is disturbing is that in the wire orientation which caused some reaction from the point motors that reaction was poor. Have you checked that the voltage coming from the Hornby Elite is actually what it says it is? I can see no reason why the Hornby unit shouldn't operate your points, it is presumably an output provided for the operation of accessories.

As regards the voltage required by the CDU, personally I'd opt for 24vac.

Steve

QUOTE (Gofer @ 24 Apr 2007, 12:56) <{POST_SNAPBACK}>I think you may be assuming full wave rectification in which case I agree swapping the input would have no effect. However, as far as I'm aware CDU's are generally only fitted with a single diode (half wave rectifier) in which case swapping the inputs would most definitely make a difference.

This also means of course that the DC from the Hornby controller will should actually achieve a greater output from the CDU as practically all the 16vdc will reach the capacitor whereas the half wave rectified 16vac will be much reduced being as the circuit is only 'seeing' one half of the peak AC voltage.

I don't agree with this. For capacitor charging the peak voltage is the important one, and as SPROGman says this is over 20V for the AC signal and only 16V for the DC signal. Peak-to-peak voltage is not relevant because the rectification removes the negative half of the waveform. Full wave rather than half wave rectification reduces the charging time a bit but will actually charge to a lower voltage because the rectifier has two diodes in series and therefore a greater voltage drop.

Furthermore the energy in the capacitor is proportional to the square of the voltage so going from 16V to 20V arguably makes your CDU more than 50% more effective. The higher the input voltage the better in this respect, provided the peak voltage stays below the capacitor's rating and the pulse doesn't get powerful enough to damage something.

Hi,

I am using a combination of Gaugemaster controlled 16VAC powered Switch Units working with Peco PL10 and PL10W points. I also have 2 busses powered from ZTC550 Power Boosters (In separate power districts - 1 for a Fleischmann Turntable and Shed Area, 1 for a triangle effect extension area.) which have a combination of Lenz150 Accessory Decoders and Team Digital SMD82 Switch Machine Drivers.

From this experience I have found that I needed extra power supplies for the Lenz Units, which still need a fair amount of cabling, as did the AC control of the points, although I make use of ribbon cable; where the SMD82's are concerned I need only the main bus supplying command to the track and that these units which have 8 outputs, which can effectively control 8 pairs of point and also provide for routing. I have yet to sort out routing, but that will come with time. Interestingly these SMD82's can handle points a fair distance from where they are located and that they are happy to be located on their own spur off the main bus at a considerable distance, say 10 - 15 feet from the power booster. Additionally they control stall motors (tortoise) and are configurable with Loco CV's. The SMD82 has an integral CDU. The Lenz Units are also cabled up to these separate track busses.

The SMD82's, therefore, only need local cabling to the point motors. There is plentiful advice in Team Digital's literature re cabling, but in the main their SMD82 works well with almost any cabling/wiring to the point motors. That being said, given time, I will improve this wiring.I configure the SMD82 by the ZTC511 controller, by programming on the main, although I isolate the track bus when I am programming this unit.

SMD82's bought from www.digitrains.co.uk. Team Digital web is www.teamdigital1.com.

I have no connection with Digitrains/Team Digital, other than that of a customer.

Hope this is of some use in planning. Quite how it works with the track bus emanating from the Elite , I can't say, but if the Amperage is adequate, I would think that there shouldn't be a problem. BTW ZTC550's output 5A, and are supplied with 15.5 VDC from a bench PSU.

Regards,

Hugh Williams