[theflyingspanner]

can anyone help
i have a digitrax db150 pm42 and a bdl168 fitted on my dcc system i have power from one power supply on the pm42 to the track and another supply to the cobalts
i and running train controller bronze everything works fine BUT if i get a short on the track some of the cobalts operate when the short is released and i cannot work it out
any ideas please

 

[Richard Johnson]

*** More information please. The PM42 is not my favourite overload device because it uses relays which are hellishly slow compared to electronic overloads and prone to voltage spiking the track... but it generally works OK if set up properly.

* Which model Cobalt.
* How do they "Operate".

* Do they shuffle slightly when the power cycles or do they actually change?

If you have a PM42 then the whole system should not be shutting down with a short, just the affected output of the PM42.

If the layout is larger, ideally you should have a power bus for the track and another for the accessories.

This also needs differing trip currents on each power bus. The accessory bus can be left set to about system power total but the track bus outputs should be set lower - to say 1/2 to 2/3 of the accessory bus trip current. That way track shorts do not shut the system down

Richard

 

[theflyingspanner]

hello richard and thanks for the reply

i have 24 digital cobalts and 4 anologue
all the digital cobalts will shuffle at power on then settle down i have resistors fitted to the digital cobalts

i have the cobalts on one power district from the pm42 and two tracks from two other power districks of the pm42 and a one spare
it doesn't matter which one of my track shorts all the digital cobalts will change a direction and keep changing until the short is cleared

i have even tried putting the cobalts wired directly to the track bus and its still them same

i also have two smd 84 to operate peco point motors but this short thing does not affect them in any way its only the digital cobalts that are affected

 

[Richard Johnson]

*** Hi

these are I am sure the original Cobalt Digital, They are working normally / properly. They have a short "shuffle" at power on to self test and reseat blades.

What is happening is that your PM42 is constantly trying to re-start power with the short still in place, so power is constantly going on and off. This makes the shuffle repeat (it is not a full change)

This is really a PM42 issue as it does not happen with any other overload units on the market. You can change the operation / settings of the PM42 so it doesn't re-try so often if possible.

If you cannot, then make sure that.

(1) all PM42 outputs are set at below the overall DCC system trip current.
(2) The accessory power bus and track bus really are separated on different outputs. This means that frog switching on the Digital Cobalts uses the second switch not the direct frog connection.
(3) Make sure the track current is set lower than the accessory power bus trip current so a short on track does not shut down the booster. IE if system is 5 amps, then accessory bus at say 3.5 amps and track at say 1.5 ~ 2 amps.

Its good to do this anyway as the PM42 is an old design that has really destructive voltage peaks each time it tries to energise - they are not good for loco or accessory decoders!

Richard

 

[theflyingspanner]

5 of the cults do actually change direction which causes derailments.
i have set the pm42 so i have 5 amps for the cobalts and 1.5 amps for the track but the problem is still there with the 5 cobalts
for some reason the others are not affected.