decoder question

I have been zooming around the net looking at stuff for DCC and spending almost all of my time looking at the 'big bits' like boosters, command stations etc, and only just took a look at the all important 'little bit'....decoders.... What does it mean when it says '1 Amp decoder' or '1.25Amp decoder'? Is this how much current that lil sucker is going to take? So if I have a 5amp booster, I can only run 4-5 decoder equipped locos?

 

The decoder amp rating is the maximum the decoder can take, not what the engine draws. You base your amperage needs by what the engines draw.

 

Ahh, great. Thanks Rick.

OK..hold on...that sorta raises another question..... Is the the max it CAN draw, of the max it WILL. ie, if your boosting power down the layout at 5 amps, and only one train is running and you short, can you let the smoke out of the decoder?

 

Usually the decoders list 2 values such as 1.5/2.0 where the first value is the maximum continuous power and the second value is the maximum momentary power.

For example, the Digitrax DH163 is listed as a 1.0/1.5 amp decoder. So, if your locomotive draws less than 1 amp in normal operation and your locomotive draws less than 1.5 amps with the motor stalled (seized), then you can use this decoder.

David

 

So I can gather from that that a loco isnt gonna draw an awful lot, even in a short situation (shortest path = track to track so mr decoder keeps his smoke in), correct?

I only intend running 4 locos, max, even on a large layout (I only have so many hands), so I dont see 5 amps being a problem for current, even with really inefficient locos.

 

At risk of being obvious, can I just clarify about current and 'shorts'.

Boosters don't really 'push' current in the sense of "boosting power down the layout at 5 amps". They push voltage and the current that flows is a factor of the voltage and the resistance of the device(s) down the line. 5 Amp booster just means it can supply a maximum of 5A, and probably has a cutout that will operate somewhere around that level.

If you short the rails together, the resistance is low and a big current will go through it. With all, or most, makes of booster it will instantly shut itself off due to the built in protection. The short circuit current does not pass through the decoder, so there should not be any smoke effects there.

If you get a short inside the engine, between wires to the motor or lights (or if you have a loco where the frame connects one side of the motor (eeuurggh ) and it touches the track) then you will probably smoke the decoder. If it was your lucky day the current drawn might be enough for the booster to shutdown before the decoder fried, but I wouldn't expect that in most cases. This is why you should take great care with how and where the wires go when installing the little beasts.

 

The quickest and easiest way to get a feel for engines and power is to look at a number of MR reviews. You will find the "stall" speed is the highest demand of amps.

You will find modern engines draw a lot less current than older engines.

I grabbed the nearest MR to me and in it is a review of a Life Like HO GP20 diesel. At scale miles per hour, at 66 MPH the current draw is .15 of an amp. Its stalled power draw is .71 amp.

 

Bewdy... I can see that even I am gonna struggle to let the smoke out of anything with that explanation, as long as I keep all my wires the hell away from each other

I think the decoder would be the least of my problems if i had an live frame touch a rail.... The loco would become a nice (permanently) stationary scenery object as well.

OK...the fear factor is diminishing rapidly...time to go fry some stuff....

 

You really don't have to worry about that either. As Mike said above, if you have a short circuit then the booster should shut down before damage occurs. There are some exceptions to this:

1. The booster will not shut down if the power supply to the booster is putting out less than the rated capacity of the booster. This is why you shouldn't use old power packs. If you have a 5 amp capacity booster, you hook a 2 amp power pack up to it, and you get a short, the booster will not shut down and you risk melting something.

2. If there is two much resistance between the booster and the location of the short, the booster may not see the short. A lot of people install automobile tail lights in series with the feeds to the frog of turnouts. If a short occurs the light will heat up (get brighter) and increase the resistance in the circuit preventing a booster shut down. Other sources of increased resistance include bad solder joints, to few power feed attachments to the track, etc.

To make sure that your booster's automatic shut down circuit is working properly, you can perform a "quarter" test. (Does Oz have quarters?) Go around the layout using a quarter and short the track. The booster should immediately shut down. If it doesn't then find the problem.

David

 

ahh...so theres another valid argument for running multiple power connections to the track.... to cut down resistance and force a booster shutdown.

Nope, no quarters here, but a 10 cent piece will do the job (no, we dont call em dimes!).

My objective, at this stage, is to run fully matched Lenz gear throughout, so if I end up with junk, theres a model railroad shop thats gonna get a visit from a VERY disgruntled customer :>) Should all hang together fine though, now I know this stuff... Thanks a lot guys!