Digitrax PM42 Short Detection Question

[Vonzack]

Hi,

 

I'm involved in building a large N Gauge show layout run with a Digitrax DCC system and a few weeks ago, we had a go at running the layout as if it was at a show.

 

To simulate show running, we filled the Fiddle Yard (14 lanes) and we noticed that the normally accurate short detection we get from our PM42 stopped working and that it was now the Command Station that detected any problems. This is where my understanding of how a PM42 works is a little weak.

 

How exactly do they detect a short?  Do they trip when the current draw in a Power District goes above the value set (the default 3A in our case) or do they detect a spike of value above the current draw in the PD.

 

From observation it seems to be the later as with the layout loaded with trains, it's quite possible that there was insufficient current to reach the trip value set for the PM42, but enough for the CS to trip.

 

If somebody can clarify this for me that would be great. Either way, it looks like we need to add a booster ;-)

 

Cheers, Mark.

[Oldddudders]

My recollection is that the PM42 is a bit old-fashioned, and uses relay technology to cut the current. Other protection using solid-state devices is available - but not from Digitrax. This page might help. http://www.tonystrains.com/products/type_powerprotect.htm

 

If you close OPSwitch #18 on the DCS 100 you will increase the time delay before the short-circuit protection cuts in from 1/8th second to 1/2 second, which may give the PM42 time to act first.

[Vonzack]

Hi Oldddudders,

 

It does indeed use relays, or at least mine does.

 

I'll check out the link, but normally the short protection works exactly as expected, with the PM42 triggering first. Its just when we had severe load on the layout that it stopped and the command station triggered first. When we noticed it, we'd been doing some re-wiring, so I was convinced that we'd messed something up while doing the work. But when we tested the short protection with locos off the layout (coin test), everything was back to normal again with the PM42 working happily. It's got to be something to do with the load and spikes caused by shorts.

[roundhouse]

I did use my PM42 on my HO layout with numerous sonud equiped locos. It ccouldnt handle the large current draw especailly with the current inrush from all the sound equipped locos. I now use PSX circuit breakers. However they will still trip out occassionally and not come back on with 10 or  locos in the one section. I proably need to play with the setting.s

 

The PM42 is now used on the N scale and performs better but if the fiddleyard shorts then the whole system cuts out due to there being 20 or so locos in the same powered section. Its fine id one or two locos are on the other sceninc or fiddleyard cure sections. I think the only way to overcome this is to use another PM42 and split up the fiddleyard or change it altogether.

 

ian