R8249 capacitor

[Jsdcs07]

Hi I am new and just after some help. I have got a Lima 50 Class and looking at installing the hornby R8249 decoder. I understand the best part of the installation, the red & black cable goes to the pick up and the orange & Grey goes to the motor connections.

 

Please correct if I am wrong. Now on the 50 Lima I have a capacitor inbetween the motor, do I leave the or remove the capacitor.

I am guid it say to remove it but I would like to know if I need to and why.

 

See the link.

 

http://www.rmweb.co.uk/dcc/Lima.htm

 

 

[Fishmanoz]

Remove it. The capacitor is there to suppress interference to radio and TV signals nearby for DC running. It can degrade DCC signal, and the 8249 has suppression anyway, so not needed for DCC.

[Gregd99]

if the capacitor is between the decoder and the track (red/black wires) it can degrade the dcc signal and cause all sorts of unexpected behaviour that will send you prematurely grey.

 

if the cap is between the decoder and the motor (orange/grey wires)

 

this is just a normal PWM control signal and I would think that there is no problem to leave it in place.

 

Happy to be corrected by others with more detailed knowledge.

[Rog RJ]

Gregd99 said:

if the capacitor is between the decoder and the track (red/black wires) it can degrade the dcc signal and cause all sorts of unexpected behaviour that will send you prematurely grey.

if the cap is between the decoder

and the motor (orange/grey wires) this is just a normal PWM control signal and I would think that there is no problem to leave it in place.

Happy to be corrected by others with more detailed knowledge.

There are some makes of decoder which

don't work very well if the capacitors are left on the motor (the capacitor can affect the BEMF feedback sensing) but Hornby decoders, in my experience, aren't affected at all. In the case of the OP, I would remove the capacitor as it's not needed when a decoder

is fitted.

[Jsdcs07]

Thank's for the help, an i am quite sure that as i upgrade the other loco's i will be asking more questions.

I am starting with the Lima 50 class as it was the cheapest that i have bought. If is goes wrong not to much of a costly mistake, LOL.

 

Next

 

on the list will be a Hornby Top link 58 class 58. but i will do the 50 first and see how i get on.

 

Another other advice would be appreciated.

[Michael_A]

The capacitor across the motor reduces RF inteference by supressing the spark fromm the brushes on the commutator as the motor armature rotates. So without it not only would you get RF interference, you would potentially burn brushes away more quickly,

 

and could cause burning on the armature.

 

However, as this suppression is effectively built into the DCC decoder, you can safely remove it, but if you do and for whatever reason in the future you remove the decoder, you should replace it.

 

I tend

 

to leave them in after fitting a decoder and only remove them if I notice any operating issues, especially at low speeds. (which out of 50 locos I have chipped is yet to happen)

 

Capacitors that are effectively straight across the track itself must always

 

be removed when using DCC.

[Jsdcs07]

Ok Thanks Michael,

 

so really just leave it in and if getting problems / interference then remove.

 

I could be being thick but

 

"Capacitors that are effectively straight across the track itself must always be removed when using DCC."

 

dont

 

get this, could you explain but i could be not reading this right.

 

thanks

[pidder]

If the capacitor is not in the way of refitting the model body, just disconnect one wire, make sure it's out of the way when you re-assemble...

[Michael_A]

Some locos have suppressor capacitors also across the pickups. These need to be remoeved as they are effectively straight across the track, whereas the capacitor across the motor is isolated from the track by the decoder.

[Fishmanoz]

With DC, pickups are connected to the motor, so a capacitor across one is also across the other, although I get the point as to where it might be mounted and where you do your disconnections to fit a decoder.

 

A more typical capacitor across the

 

track is the one fitted to a DC power track. It must be removed to use it for DCC. The reason is simple. Capacitors vary in "resistance" with frequency. At DC, they are open circuit, and at very high frequency they are a short circuit. DCC signals are quite

 

high in frequency, so a capacitor across the track effectively shorts the signal out.

[Jsdcs07]

Hi all, just an update.

 

I fitted the decoder to the lima 50 class on Saturday afternoon, wow that was easy. all hard wired in and look clean and tidy.

 

Took all of 30 min's to do. I left the white capacitors in-between the motor hook ups

 

and has not made any problems.

 

Then tested her. To me top speed was improved and also better shunting capability. then i started to play with the acceleration and deceleration setting, I can see that when i got a loco on its own i can set it to stop

 

feraly quickly but how ealsy i can change if got lots of weight behind i set the acceleration and deceleration to a longer time.

 

Over all i hapy with how the install went and the benfit to doing the DCC. will now be getting more for the other loco.