What type of capacitor for track feed

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Cowcatcher
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Joined: Fri Aug 21, 2015 10:17 pm

What type of capacitor for track feed

Postby Cowcatcher » Sat Aug 05, 2017 1:13 pm

Hi everyone
I read that a ceramic disc capacitor of 0.1uF is used for track feed power cables to help with suppression for tv interference etc.
I didn't have a ceramic to hand, so used a polypropylene type, it works fine, but should I replace it as soon as I get some ceramic types?
Do ceramic discs have some advantage that is not obvious to me?

Thank you for any help

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Bufferstop
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Joined: Thu Mar 11, 2010 12:06 pm
Location: Bottom end of N. Warks line

Re: What type of capacitor for track feed

Postby Bufferstop » Sat Aug 05, 2017 1:50 pm

Not sure of the specifics of polypropylene caps, they weren't around in my days They are probably the modern equivalent of the aluminium foil and waxy paper that were still in use in the sixties and seventies. It's a matter of being reversible, the best capacitance to volume ratio is an electrolytic capacitor but these are definitely non-reversible. We used to wire them across the terminals in the back of a light switch which was on but isolated at the main. They survived the power being turned on but when the victim tried to switch it off they took the full 240v and went off like a fire cracker. 'elf'n'safety spoilsports put a stop to that. Disk ceramics are fine so I'd swap to them sometime at your leisure. You'd be better off fitting them across the wires going into the motor. Fitting them to the track at the power clip isn't much use if you have more than a couple of feet of track in either direction, as it's the length of rail between sparking wheel and capacitor that acts as an aerial for wheel to rail. The RF investigation team would recommend caps across the rails as a last resort and would then put them every couple of feet. The one on the motor is supposed to take care of the sparking at the brushes. The two best RFI killers are the quality of the aerial and the distance between track and downlead, The effectiveness of distance actually goes up by its square.
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Cowcatcher
Posts: 20
Joined: Fri Aug 21, 2015 10:17 pm

Re: What type of capacitor for track feed

Postby Cowcatcher » Sat Aug 05, 2017 4:07 pm

Thanks for the reply, Bufferstop. I will do a bit of experimenting

BorderShed
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Joined: Mon Nov 07, 2016 12:16 am
Location: SW. England

Re: What type of capacitor for track feed

Postby BorderShed » Sun Aug 06, 2017 1:34 am

Cowcatcher wrote:Do ceramic discs have some advantage that is not obvious to me?

I think Ceramic Disks have the advantage for suppression specs of lower self inductance

Cowcatcher
Posts: 20
Joined: Fri Aug 21, 2015 10:17 pm

Re: What type of capacitor for track feed

Postby Cowcatcher » Thu Aug 24, 2017 9:59 pm

Thanks, Bordershed.


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