Connecting DMUs

[Potrail2378]

I have converted my class 142 to DCC, with cab, saloon and directional lights. Each unit has a motor and a decoder (set to the same address), they work well together.

Problem is - every so often there is a bit of saloon light flicker. Can I open up the units and connect two wires (one for each rail) to each unit, so that there are eight wheels picking up current for both units, rather than four wheels picking up power for one unit? I can't really see an issue, but would like to confirm there will be no problems.

Thanks in advance.

[96RAF]

As long as your link wires are simply connecting same side wheels to same side wheels then there will be no problem as it is no different to a loco and tender lash-up, except you have two decoders feeding from the common link, which again is no problem if you consider the wheels pick-ups as an extension of the rails, which all your decoders listen to.

[Potrail2378]

Thanks, 96RAF. I didn't think there would be any issues, but better to ask and be sure.

[Potrail2378]

Hi 96RAF, just to let you know that I connected the pickups from each unit, no problems.

However, when I switched the Hornby Select on, I got the 'short circuit' notification. I then switched off the Select, and back on again, then it worked fine.

I have fitted 2,200 microfarad, 25v caps to each decoder.

But upstairs in the loft, on my layout with the Hornby Elite I have had no problems whatsoever.

Could this be something to do with back EMF? I don't really know much about that, it's something that I have read about in the group.

[Chrissaf]

Just based purely on analysis of your written words. I would deduce that the 'inrush' charging currents for your two 2,200µF capacitors [presumably decoder stay alives?] overwhelmed the 1 amp supply of the Select when added to all the other things that the 1 amp supply has to power as well. Capacitors when completely discharged, very briefly look electrically like a 'short circuit' until a charge is built up within them. You then rebooted the Select, by which time the decoder capacitors had already had a partial charge in them, thus reducing the 'inrush' charging current down to a level that the Select power supply could handle. The Elite had no issues because it has a 4 amp power supply. I doubt very much that 'Back EMF' would have anything to do with it.

[96RAF]

Normally stay-alives have a resistor in circuit to control the in-rush Chris spoke about.

Are you using commercial units (if so which make) or DIY ones (if so to which circuit plan).

[Chrissaf]

The additional in-rush charging current protection resistor Rob mentions is connected thus:

The diode blocks the 'in-rush' charging current, forcing the charge current via the resistor. The resistor thus throttles the current to allow the capacitor to charge up more sedately so as not to overwhelm the power supply. However, when the charge on the capacitor is needed, the output current from the capacitor can now pass easily through the diode and is not restricted in any way. The diode just adds a very small 0.6 voltage drop loss. The positive / negative orientations of the capacitor and diode are critical and must follow those in the schematic. Any generic Silicon diode will do. Say for example one in the 1N4000 series.

[Potrail2378]

Hi, and sorry for the late reply - I never seem to get notifications from the forum.

I have soldered a 2,200 microfarad, 16v cap to each decoder via a red and black length of decoder wire. No diodes or resistors fitted. I soldered them to the bridge rectifier of the decoder.

Apart from the available room, I should be able to fit a diode/resistor in series.

[Chrissaf]

Hi, and sorry for the late reply - I never seem to get notifications from the forum.

 

 

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